It was the SDIO that built DC/X and flew it many times. General Graham, Max Hunter, and I talked the head of SDIO (VP Dan Quayle in his capacity as Chairman of the National Space Council) into building the DC/X. It flew straight up, moved sideways, and landed on a tail of fire just as God and Robert Heinlein intended rockets to do.

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Aerobraking is a spaceflight maneuver that reduces the high point of an elliptical orbit (apoapsis) by flying the vehicle through the atmosphere at the low point of the orbit (periapsis). The resulting drag slows the spacecraft. Aerobraking is used when a spacecraft requires a low orbit after arriving at a body with an atmosphere, and it requires less fuel than does the direct use of a rocket engine.

Method

When an interplanetary vehicle arrives at its destination, it must change its velocity to remain in the vicinity of that body. When a low, near-circular orbit around a body with substantial gravity (as is required for many scientific studies) is needed, the total required velocity changes can be on the order of several kilometers per second. If done by direct propulsion, the rocket equation dictates that a large fraction of the spacecraft mass must be fuel. This in turn means the spacecraft is limited to a relatively small science payload and/or the use of a very large and expensive launcher. Provided the target body has an atmosphere, aerobraking can be used to reduce fuel requirements. The use of a relatively small burn allows the spacecraft to be captured into a very elongated elliptic orbit. Aerobraking is then used to circularize the orbit. If the atmosphere is thick enough, a single pass through it can be sufficient to slow a spacecraft as needed. However, aerobraking is typically done with many orbital passes through a higher altitude, and therefore thinner region of the atmosphere. This is done to reduce the effect of frictional heating, and because unpredictable turbulence effects, atmospheric composition, and temperature make it difficult to accurately predict the decrease in speed that will result from any single pass. When aerobraking is done in this way, there is sufficient time after each pass to measure the change in velocity and make any necessary corrections for the next pass. Achieving the final orbit using this method takes a long time (e.g., over six months when arriving at Mars), and may require several hundred passes through the atmosphere of the planet or moon. After the last aerobraking pass, the spacecraft must be given more kinetic energy via rocket engines in order to raise the periapsis above the atmosphere.

The kinetic energy dissipated by aerobraking is converted to heat, meaning that a spacecraft using the technique needs to be capable of dissipating this heat. The spacecraft must also have sufficient surface area and structural strength to produce and survive the required drag, but the temperatures and pressures associated with aerobraking are not as severe as those of atmospheric reentry or aerocapture. Simulations of the Mars Reconnaissance Orbiter aerobraking use a force limit of 0.35 N per square meter with a spacecraft cross section of about 37 m², equate to a maximum drag force of about 7.4 N, and a maximum expected temperature as 340 °F (170 °C). The force density (i.e. pressure), roughly 0.2 N (0.04 lbf) per square meter, that was exerted on the Mars Observer during aerobraking is comparable to the aerodynamic resistance of moving at 0.6 m/s (2.16 kph, or 1.34 mph) at sea level on Earth, approximately the amount experienced when walking slowly.

Related methods

Aerocapture is a related but more extreme method in which no initial orbit-injection burn is performed. Instead, the spacecraft plunges deeply into the atmosphere without an initial insertion burn, and emerges from this single pass in the atmosphere with an apoapsis near that of the desired orbit. Several small correction burns are then used to raise the periapsis and perform final adjustments. This method was originally planned for the Mars Odyssey orbiter, but the significant design impacts proved too costly.

Another related technique is that of aerogravity assist, in which the spacecraft flies through the upper atmosphere and utilises aerodynamic lift instead of drag at the point of closest approach. If correctly oriented, this can increase the deflection angle above that of a pure gravity assist, resulting in a larger delta-v.

Aerobraking in fiction

In Robert A. Heinlein's 1948 novel Space Cadet, aerobraking is used to save fuel while slowing the spacecraft Aes Triplex for an unplanned extended mission and landing on Venus, during a transit from the Asteroid Belt to Earth.

In the fourth episode of Stargate Universe, the Ancient ship Destiny suffers an almost complete loss of power and must use aerobraking to change course. The episode ends in a cliffhanger with Destiny headed directly toward a star.

The spacecraft Cosmonaut Alexey Leonov in Arthur C. Clarke's novel 2010: Odyssey Two and its film adaptation uses aerobraking in the upper layers of Jupiter's atmosphere to establish itself at the L₁ Lagrangian point of the Jupiter – Io system.

In the 2004 TV series Space Odyssey: Voyage to the Planets the crew of the international spacecraft Pegasus perform an aerobraking manoeuvre in Jupiter's upper atmosphere to slow them down enough to enter Jovian orbit.

In the space simulation sandbox game Kerbal Space Program, this is a common method of reducing a craft's orbital speed. It is sometimes humorously referred to as "aerobreaking", because the high drag sometimes causes large crafts to split in several parts.

In the 2014 film Interstellar, astronaut pilot Cooper uses aerobraking to save fuel and slow the spacecraft Ranger upon exiting the wormhole to arrive in orbit above the first planet.

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Aerocapture is an orbital transfer maneuver used to reduce the velocity of a spacecraft from a hyperbolic trajectory to an elliptical orbit around the targeted celestial body.

Aerocapture uses a planet’s or moon’s atmosphere to accomplish a quick, near-propellantless orbit capture to place a space vehicle in its science orbit (most science orbits require a near circular orbit around the celestial body). The aerocapture maneuver starts as the spacecraft enters the atmosphere of the target body from an approach trajectory. The aerodynamic drag generated by the dense atmosphere slows the craft. After the spacecraft slows enough to capture into orbit, it exits the atmosphere and executes a small motor firing to circularize the orbit. This nearly fuel-free method of deceleration could significantly reduce the mass of an interplanetary spacecraft. Less spacecraft mass allows for more science instrumentation to be added to the mission or allows for a smaller and less-expensive spacecraft, and potentially a smaller, less-expensive launch vehicle.However, this approach requires significant thermal protection and precision closed-loop guidance during the maneuver. This level of control authority requires either the production of significant lift, or relatively large attitude control thrusters.

Benefits of aerocapture

NASA technologists are developing ways to place robotic space vehicles into long-duration scientific orbits around distant Solar System destinations without the need for the heavy fuel loads that have historically limited vehicle performance, mission duration, and mass available for science payloads.

A study showed that using aerocapture over the next best method (propellant burn and aerobraking) would allow for a significant increase in scientific payload for missions ranging from Venus (79% increase) to Titan (280% increase) and Neptune (832% increase). Additionally, the study showed that using aerocapture technology could enable scientifically useful missions to Jupiter and Saturn.

Aerocapture technology has also been evaluated for use in manned Mars missions and found to offer significant mass benefits. For this application, however, the trajectory must be constrained to avoid excessive deceleration loads on the crew. Although there are similar constraints on trajectories for robotic missions, the human limits are typically more stringent, especially in light of the effects of prolonged microgravity on acceleration tolerances.

Aerocapture spacecraft designs

The aerocapture maneuver can be accomplished with three basic types of systems. The spacecraft can be enclosed by a structure covered with thermal protection material also known as the rigid aeroshell design. Similarly another option is for the vehicle to deploy an aerocapture device, such as an inflatable heat shield, known as the inflatable aeroshell design. The third major design option is of an inflatable, trailing ballute—a combination balloon and parachute made of thin, durable material towed behind the vehicle after deployment in the vacuum of space.

Blunt body, rigid aeroshell design

The blunt body, rigid aeroshell system encases a spacecraft in a protective shell. This shell acts as an aerodynamic surface, providing lift and drag, and provides protection from the intense heating experienced during high-speed atmospheric flight. Once the spacecraft is captured into orbit, the aeroshell is jettisoned.

NASA has used blunt aeroshell systems in the past for atmospheric entry missions. The most recent example is the Mars Exploration Rovers, Spirit and Opportunity, which launched in June and July 2003, and landed on the Martian surface in January 2004. Another example is the Apollo Command Module. The module was used for six unmanned space flights from February 1966 to April 1968 and eleven manned missions from Apollo 7 in October 1968 through the final manned Apollo 17 lunar mission in December 1972. Because of its extensive heritage, the aeroshell system design is well understood. Adaptation of the aeroshell from atmospheric entry to aerocapture requires mission-specific customization of the thermal protection material to accommodate the different heating environments of aerocapture. Also, higher-temperature adhesives and lightweight, high temperature structures are desired to minimize the mass of the aerocapture system.

Inflatable aeroshell design

The inflatable aeroshell design looks much like the aeroshell or blunt body design. The inflatable aeroshell is often referred to as a hybrid system, with a rigid nosepiece and an inflated, attached decelerator to increase the drag area. Just prior to entering the atmosphere, the inflatable aeroshell extends from a rigid nose-cap and provides a larger surface area to slow the spacecraft down. Made of thin-film material and reinforced with a ceramic cloth, the inflatable aeroshell design could offer many of the same advantages and functionality as trailing ballute designs. While not as large as the trailing ballute, the inflatable aeroshell is roughly three times larger than the rigid aeroshell system and performs the aerocapture maneuver higher in the atmosphere, reducing heating loads. Because the system is inflatable, the spacecraft is not enclosed during launch and cruise, which allows more flexibility during spacecraft design and operations.

Trailing ballute design

One of the primary inflatable deceleration technologies is a trailing ballute configuration. The design features a toroidal, or donut-shaped, decelerator, made of a lightweight, thin-film material. The ballute is much larger than the spacecraft and is towed behind the craft, much like a parachute, to slow the vehicle down. The “trailing” design also allows for easy detachment after the aerocapture maneuver is complete. The trailing ballute design has performance advantages over the rigid aeroshell design, such as not constraining the spacecraft size and shape, and subjecting the vehicle to much lower aerodynamic and thermal loads. Because the trailing ballute is much larger than the spacecraft, aerocapture occurs high in the atmosphere where much less heat is generated. The ballute incurs most of the aerodynamic forces and heat, allowing the use of minimal thermal protection around the spacecraft. One of the primary advantages of the ballute configuration is mass. Where the rigid aeroshell may account for 30–40% of the mass of a spacecraft, the ballute mass fraction could be as little as 8–12%, saving mass for more science payload.

In practice

Aerocapture has not yet been tried on a planetary mission, but the re-entry skip by Zond 6 and Zond 7 upon lunar return were aerocapture maneuvers, since they turned a hyperbolic orbit into an elliptical orbit. On these missions, since there was no attempt to raise the perigee after the aerocapture, the resulting orbit still intersected the atmosphere, and re-entry occurred at the next perigee.

Aerocapture was originally planned for the Mars Odyssey orbiter, but later changed to aerobraking for reasons of cost and commonality with other missions.

Aerocapture has been proposed and analyzed for arrival at Saturn's moon Titan.

In fiction

Aerocapture within fiction can be read in Arthur C. Clarke's novel 2010: Odyssey Two, in which two spacecraft (one Russian, one Chinese) both use aerocapture in Jupiter's atmosphere to shed their excess velocity and position themselves for exploring Jupiter's satellites. This can be seen as a special effect in the movie version in which only a Russian spacecraft undergoes aerocapture (in the film incorrectly called aerobraking).

Players of the video game Kerbal Space Program often employ aerocapture when exploring the satellites of Jool (a gas giant that serves as the game's analogue to Jupiter).

Related methods

Aerocapture is part of a unique family of "aeroassist" technologies being developed by NASA for science missions to any planetary body with an appreciable atmosphere. These destinations could include Mars, Venus and Saturn's moon Titan, along with the outer planets.

Aerobraking is another aeroassist maneuver that shares some similarities but also some key marked differences with Aerocapture. Aerobraking also uses the celestial body’s atmosphere to slow the aircraft down into the desired orbit however unlike aerobraking aerocapture only uses one pass through the atmosphere to reduce its velocity while aerobraking takes on the order of 100 to 400 passes to achieve the desired velocity reduction.

Aerocapture:

• Rapid process (hours to days)

• Descent into a relatively dense mid-atmosphere

• Requires a heavy heat shield due to rapid deceleration resulting in high g-forces

Aerobraking:

• Gradual process (weeks to months)

• Descent into sparse outer atmosphere

• Small reductions in spacecraft velocity per pass thus no additional mass for a heat shield is necessary

One of the main advantages of using an aerocapture technique over that of an aerobraking technique is that it enables mission concepts for human spaceflight due to the rapid process of transitioning to the desired orbit.

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An aerogravity assist, or AGA, is a theoretical spacecraft maneuver designed to change velocity when arriving at a body with an atmosphere. A pure gravity assist uses only the gravity of a body to change the direction of the spacecraft trajectory. The change in direction is limited by the mass of the body, and how closely it can be approached. An aerogravity assist uses a closer approach to the planet, dipping into the atmosphere, so the spacecraft can also use aerodynamic lift with upside-down wings to augment gravity and further curve the trajectory. This enables the spacecraft to deflect through a larger angle, resulting in a higher delta-v (change in velocity). This in turn allows a shorter travel time, a larger payload fraction of the spacecraft, or a smaller spacecraft for a given payload.

The related techniques of aerocapture, aerobraking, and atmospheric entry also attempt to use the body's atmosphere to help reduce propulsion requirements. In an aerogravity assist, however, the goal is not to use the atmosphere to slow the spacecraft down, but instead use it to achieve a larger change in direction.

Aerogravity assists, though proposed in theory, have not yet been used in practice.

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Boost-glide trajectories are a class of spacecraft guidance and reentry trajectories that extend the range of suborbital spaceplanes and reentry vehicles by employing aerodynamic lift in the high upper atmosphere. In most examples, boost-glide roughly doubles the range over the purely ballistic trajectory. In others, a series of skips allows range to be further extended, and leads to the alternate terms skip-glide and skip reentry.

The concept was first seriously studied as a way to extend the range of ballistic missiles, but has not been used operationally in this form. The underlying aerodynamic concepts have been used to produce maneuverable reentry vehicles, or MARV, to increase the accuracy of some missiles. More recently the traditional form with an extended gliding phase has been considered as a way to reach targets while flying below their radar coverage.

The concept has also been used to extend the reentry time for vehicles returning to Earth from the Moon, who would otherwise have to shed a large amount of velocity in a short time and thereby suffer very high heating loads. The Apollo Command Module used what is essentially a one-skip reentry (or partial skip), as did the Soviet Zond and Chinese Chang'e 5-T1. More complex multi-skip reentry is proposed for newer vehicles like the Orion spacecraft.

History

Early concepts

The earliest known use of the boost-glide concept for truly long-range use dates to the 1941 Silbervogel proposal by Eugen Sänger for a rocket powered bomber able to attack New York City from bases in Germany and then fly on for landing somewhere in the Pacific Ocean held by the Empire of Japan. The idea would be to use the vehicle's wings to generate lift and pull up into a new ballistic trajectory, exiting the atmosphere again and giving the vehicle time to cool off between the skips. It was later demonstrated that the heating load during the skips was much higher than initially calculated, and would have melted the spacecraft.

In 1943, the A-9 work was dusted off again, this time under the name A-4b. It has been suggested this was either because it was now based on an otherwise unmodified A-4, or because the A-4 program had "national priority" by this time, and placing the development under the A-4 name guaranteed funding. A-4b used swept wings in order to extend the range of the V2 enough to allow attacks on UK cities in The Midlands or to reach London from areas deeper within Germany. The A-9 was originally similar, but later featured long ogive delta shaped wings instead of the more conventional swept ones. This design was adapted as a manned upper stage for the A-9/A-10 intercontinental missile, which would glide from a point over the Atlantic with just enough range to bomb New York before the pilot bailed out.

Post-war development

In the immediate post-war era, Soviet rocket engineer Alexey Isayev found a copy of an updated August 1944 report on the Silbervogel concept. He had the paper translated to Russian, and it eventually came to the attention of Joseph Stalin who was intensely interested in the concept of an antipodal bomber. In 1946, he sent his son Vasily Stalin and scientist Grigori Tokaty, who had also worked on winged rockets before the war, to visit Sänger and Irene Bredt in Paris and attempt to convince them to join a new effort in the Soviet Union. Sänger and Bredt turned down the invitation.

In November 1946, the Soviets formed the NII-1 design bureau under Mstislav Keldysh to develop their own version without Sänger and Bredt. Their early work convinced them to convert from a rocket powered hypersonic skip-glide concept to a ramjet powered supersonic cruise missile, not unlike the Navaho being developed in the United States during the same period. Development continued for a time as the Keldysh bomber, but improvements in conventional ballistic missiles ultimately rendered the project unnecessary.

In the United States, the skip-glide concept was advocated by many of the German scientists who moved there, primarily Walter Dornberger and Krafft Ehricke at Bell Aircraft. In 1952, Bell proposed a bomber concept that was essentially a vertical launch version of Silbervogel known as Bomi. This led to a number of follow-on concepts during the 1950s, including Robo, Hywards, Brass Bell, and ultimately the Boeing X-20 Dyna-Soar. Earlier designs were generally bombers, while later models were aimed at reconnaissance or other roles. Dornberger and Ehricke also collaborated on a 1955 Popular Science article pitching the idea for airliner use.

The introduction of successful intercontinental ballistic missiles (ICBMs) in the offensive role ended any interest in the skip-glide bomber concepts, as did the reconnaissance satellite for the spyplane roles. The X-20 space fighter saw continued interest through the 1960s, but was ultimately the victim of budget cuts; after another review in March 1963, Robert McNamara canceled the program in December, noting that after $400 million had been spent they still had no mission for it to fulfill.

Russia in March 2018 unveiled hypersonic glide vehicle Avangard.

Reentry vehicle use

The technique was used by the Soviet Zond series of circumlunar spacecraft, which used one skip before landing. In this case a true skip was required in order to allow the spacecraft to reach the higher-latitude landing areas. Zond 6, Zond 7 and Zond 8 made successful skip entries, although Zond 5 did not. The Chang'e 5-T1, which flew mission profiles similar to Zond, also used this technique.

The Apollo Command Module used a skip-like concept to lower the heating loads on the vehicle by extending the re-entry time, but the spacecraft did not leave the atmosphere again and there has been considerable debate whether this makes it a true skip profile. NASA referred to it simply as "lifting entry". A true multi-skip profile was considered as part of the Apollo Skip Guidance concept, but this was not used on any manned flights. The concept continues to appear on more modern vehicles like the Orion spacecraft, using onboard computers.

Flight mechanics

Using simplified equations of motion and assuming that during the atmospheric flight both drag and lift forces will be much larger than the gravity force acting on the vehicle, the following analytical relations for a skip reentry flight can be derived:

Where gamma is the flight path angle relative to the local horizontal, the subscript E indicates the conditions at the start of the entry and the subscript F indicates the conditions at the end of the entry flight.

The velocity V before and after the entry can be derived to relate as follows:

Where L/D equals the lift to drag ratio of the vehicle.

This mostly true story is about the Space Shuttle payload bay doors and what to do if they won’t shut properly. And how we forgot about that emergency in our rush to achieve other mission objectives.

Some months ago, I visited Atlantis at the Kennedy Space Center visitor center. Superbly displayed, it is the only shuttle vehicle with the payload bay doors open. In the picture you can see some of the rube-goldberg mechanism that operates the doors critical latches to ensure the doors don’t spring open during aerodynamic flight. A set of push rods and hinges at the sill open and close the doors. A long set of push rod/bell crank mechanisms located in the front and rear edges of the doors operate the latch mechanisms. On the bulkhead just aft of the crew cabin you can see the black cylindrical knobs which the latch mechanisms grab onto to secure the door in place. More latches and their mechanisms ran down the centerline of the doors. If this sounds complicated, it is.

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Early in the shuttle days there was a huge concern about what to do if the doors did not close and latch properly at the end of the mission. The doors had to be open during most of the orbit stay time for cooling and to allow satellite deployments or other objectives. But during re-entry and the atmospheric part of the flight, the doors had to be firmly shut and latched down. If they were to spring open and rip off, the vehicle would become uncontrollable and catastrophe would ensue. Flight rules prohibited deorbit with any more than one set of latches not closed; and in that case the aerodynamic maneuvers were to be severely restricted.

Of course, the power to the motors that drove all this mechanism were redundant but that did nothing for a physical jam. During STS-3 a thermal engineering test caused the doors to become banana shaped and prevented – for a short time – closing the doors. This was done on purpose to see what the limits of the spacecraft were. We found out. And never did it again.

Long before STS-1 flew, a set of procedures for a spacewalking crewmember to deal with potential problems was developed and practiced. I got my opportunity to take the class; use the tube cutters to cut a pushrod that had jammed, put clamps around unclosed latches to hold doors tight, and more. There was a whole set of tools flown on every shuttle flight to deal with this contingency and every crew got at least one practice session on how to deal with it.

But the trick is that a space suited crewmember must get to the doors from the inside. Normally this is not a problem; with an empty or near empty payload bay the EV crewmember just translates to the worksite; latches everything down, returns to the airlock door and ingresses. Oh, and quickly gets out of the EVA suit and into the Launch/Reentry Suit and straps down because deorbit must occur shortly after the doors are closed and the radiators can no longer cool the ship.

If a payload blocked the path from the EVA worksite to the airlock, early in the program they were always jettisonable. Not a problem.

Despite the complicated design, the payload bay door and latch mechanisms worked perfectly on every mission. Over time, the concerns about having to deal with a failure faded away, even though the procedure and tools were on every flight.

During the evolution of the shuttle design, the European Space Agency built a laboratory to fit in the shuttle payload bay. You can see the SpaceLab module on display right behind Discovery at the Smithsonian Udvar-Hazy center in Virginia. Before the ISS, this was a facility to do micro-gravity research for up to three weeks in space.

But there is a trick that the designers missed.

If the mechanism on one of the aft payload bay door latches failed, and an EVA crewmember had to go back there to secure the doors down, there was no way for that crew member to squeeze past the SpaceLab module to get to the airlock with the payload bay doors closed. Jettisoning the SpaceLab was not a task that an EVA crewmember could do. The way home was going to be blocked.

Oops.

It may have been the first SpaceLab mission when we discovered this, but my recollection is that it was uncovered later, during a simulated mission. Those darn Sim Sups! They always made us work problems that were unrealistic! Except that most of the time they weren’t.

What to do? No more SpaceLab flights? Unacceptable.

Review the risk? Great history of reliability, low likelihood of needing to do the EVA. Case closed? Not exactly.

What if?

Here is the crazy resolution. If the aft latches had to be secured on a SpaceLab flight, then the crewmember would just stay back there.

There was a good place to strap down at the bottom of the payload bay aft of the SpaceLab module. It would be a short wait from finishing up with the doors until the deorbit burn, probably no more than an hour and a half. From deorbit burn to the ground was about an hour and in normal flight the g loading was light, nothing more than 1.5g. The EMU had plenty of consumables to work for the necessary time, but there are a couple of sticky things to resolve:

1. The EMU uses a water sublimator to keep the crewmember cool. As the payload bay repressurizes during entry, the sublimator will quit. If the crewmember had selected full cool and chilled down as much as he/she could stand, it would probably be OK.
2. Getting out of the spacesuit: after landing, getting out of the helmet and gloves is not a problem. Getting out of the hard upper torso by oneself is a chore but probably doable in 1 g if one is not in a hurry. The lower pressure garment (pants) would not be a problem.
3. Getting the crewmember out of the payload bay, well that is a problem. Remember the doors are latched shut and clamps have been applied to keep them shut. Surely the ground crew could figure something out . . . given several hours . . . .

So that is the story. Accept the risk because we think it is low; have a screwy contingency procedure ready if we’re wrong.

But that is not the way you really want to fly in space.

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      On 2 December 1988, Space Shuttle Orbiter Atlantis lifted off the pad at Kennedy Space Center on a classified Department of Defense mission. 85 seconds after launch, some ablative insulation fell off the right-hand solid rocket booster and struck the orbiter. The crew reported white material on the windshield at multiple points during ascent.
     Once on orbit, the crew used Atlantis’ Canadarm to observe the right wing of the orbiter, and were shocked to discover that large portions of the heat tiles were damaged, but with the low resolution camera on the Canadarm, they couldn’t see the full extent of the damage.

     “I will never forget, we hung the arm over the right wing, we panned it to the location and took a look and I said to myself, ‘we are going to die,’” recalled legendary shuttle commander Robert ‘Hoot’ Gibson. “There was so much damage. I looked at that stuff and I said, 'oh, holy smokes, this looks horrible, this looks awful.’“
     Due to the classified nature of the mission, the crew couldn’t use the standard method sending images and video back down to Mission Control, so they were forced to use slow, encrypted transmissions. NASA engineers received poor quality images, leading them to believe that the damage was just lights and shadows. The crew was told that the damage to the orbiter was no worse than any other mission.

     Understandably, the crew (particularly Commander Gibson) was furious. Gibson, assuming that his life was in danger, decided that if instruments showed that the orbiter were actually braking up on reentry, he’d have ‘at least 60 seconds to tell Mission Control what he thought of their analysis.’
     The day before reentry, Gibson and the crew spent time sightseeing, with Gibson saying that there was ‘no reason to die all tensed up.’ Thankfully, the orbiter actually reentered that atmosphere and successfully landed at Edwards Air Force Base on 6 December 1988, after 4 days, 9 hours, 5 minutes, and 37 seconds. It wasn’t until the orbiter came to a stop that NASA, and the crew of STS-27, realized the extent of the damage.

     Over 700 tiles were damaged, with one tile missing entirely. The tile was missing over the steel plate that an antenna was mounted on, so a complete burn-through didn’t happen. It was an extremely close call for Atlantis and her crew.

     A very similar event would happen during STS-107′s 16 January 2003 launch, when a piece of the External Tank’s foam dislodged and struck the Space Shuttle Orbiter Columbia. The crew, however, were not so lucky, and the orbiter broke up during reentry. Rick Husband, William McCool, David Brown, Kalpana Chawla, Michael Anderson, Laurel Clark, and Ilan Ramon perished aboard Columbia.

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I don’t think any of us really trusted the Nerva-K under our landing craft.

Think it through. For long trips in space, you use an ion jet giving low thrust over long periods of time. The ion motor on our own craft had been decades in use.

Where gravity is materially lower than Earth’s, you land on dependable chemical rockets. For landings on Earth and Venus, you use heat shields and the braking power of the atmosphere. For landing on the gas giants—but who would want to?

The Nerva-class fission rockets are used only for takeoff from Earth, where thrust and efficiency count. Responsiveness and maneuverability count for too much during a powered landing. And a heavy planet will always have an atmosphere for braking.

Pluto didn’t.

For Pluto, the chemical jets to take us down and bring us back up were too heavy to carry all that way. We needed a highly maneuverable Nerva-type atomic rocket motor using hydrogen for reaction mass.

And we had it. But we didn’t trust it.

(ed note: though as it turns out, Mercury has about six times the surface gravity of Pluto, and Mercury also does not have an atmosphere for braking. When this story was written it was thought that Pluto had a much higher gravity)

...From the outside there was no evidence of damage or repair. Part of the heat shield hung below the cutter's nose like a great shovel blade, exposing the control room blister, windows, and the snout of the cutter's main armament: a laser cannon...

...The starboard air lock had been reconnected to the embassy ship. They left by the port side. Lenin's boat crew had already rigged lines from the auxiliary vessel to the cutter. The boat was almost a twin for MacArthur's cutter, a flat-topped lifting body with a shovel-blade reentry shield hanging below the nose...

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     In the hours immediately before encounter, Floyd saw little of captain or navigator. The Orlovs scarcely left the bridge, as they continually checked the approach orbit and made minute refinements to Leonov's course. The ship was now on the critical path that would just graze the outer atmosphere; if it went too high, frictional braking would not be sufficient to slow it down, and it would go racing out of the Solar System, beyond all possibility of rescue. If it went too low, it would burn up like a meteor. Between the two extremes lay little margin for error.

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     And it was too late for second thoughts. From far, far away came the first faint whisper of sound, like the wailing of some lost soul. At the same moment, the ship gave a barely perceptible jerk; the cocoon began to swing around and its suspension tightened. After weeks of weightlessness, gravity was returning.
     Within seconds, the faint wail had risen to a steady roar, and the cocoon had become an overloaded hammock. This is not such a good idea, Floyd thought to himself, already it was difficult to breathe. The deceleration was only a part of the problem: Zenia was clutching him as a drowning person is supposed to clutch the proverbial straw.

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     The ship was now rocking noticeably, like a small boat in a choppy sea. Was that normal? wondered Floyd. He was glad that he had Zenia to worry about; it took his mind away from his own fears. Just for a moment, before he managed to expel the thought, he had a vision of the walls suddenly glowing cherry red, and caving in upon him. Like the nightmare fantasy of Edgar Allan Poe's 'The Pit and the Pendulum', which he'd forgotten for thirty years.
     But that would never happen. If the heat shield failed, the ship would crumble instantly, hammered flat by a solid wall of gas. There would be no pain; his nervous system would not have time to react before it ceased to exist. He had experienced more consoling thoughts, but this one was not to be despised.

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     When Floyd reached the observation deck — a discreet few minutes after Zenia — Jupiter already seemed farther away. But that must be an illusion based on his knowledge, not the evidence of his eyes. They had barely emerged from the Jovian atmosphere, and the planet still filled half the sky.
     And now they were — as intended — its prisoners. During the last incandescent hour, they had deliberately jettisoned the excess speed that could have carried them right out of the Solar System, and on to the stars. Now they were travelling in an ellipse — a classical Hohmann orbit — which would shuttle them back between Jupiter and the orbit of Io, 350,000 kilometres higher. If they did not — or could not — fire their motors again, Leonov would swing back and forth between these limits, completing one revolution every nineteen hours. It would become the closest of Jupiter's moons — though not for long. Each time it grazed the atmosphere it would lose altitude, until it spiralled into destruction.
     Floyd had never really enjoyed vodka, but he joined the others without any reservations in drinking a triumphant toast to the ship's designers, coupled with a vote of thanks to Sir Isaac Newton. Then Tanya put the bottle firmly back in its cupboard; there was still much to be done.
     Though they were all expecting it, everyone jumped at the sudden muffled thud of explosive charges, and the jolt of separation. A few seconds later, a large, still-glowing disk floated into view, slowly turning end-over-end as it drifted away from the ship.
     'Look!' cried Max. 'A flying saucer! Who's got a camera?' There was a distinct note of hysterical relief in the laughter that followed. It was interrupted by the captain, in a more serious vein.
     'Goodbye, faithful heat shield! You did a wonderful job.'
     'But what a waste!' said Sasha. 'There's at least a couple of tons left, Think of all the extra payload we could have carried!'
     'If that's good, conservative Russian engineering,' retorted Floyd, 'then I'm all for it. Far better a few tons too much — than one milligram too little.'
     Everyone applauded those noble sentiments as the jettisoned shield cooled to yellow, then red, and finally became as black as the space around it. It vanished from sight while only a few kilometres away, though occasionally the sudden reappearance of an eclipsed star would betray its presence.
     'Preliminary orbit check completed,' said Vasili. 'We're within ten metres a second of our right vector. Not bad for a first try.'
     There was a subdued sigh of relief at the news, and a few minutes later Vasili made another announcement.
     'Changing attitude for course correction; delta vee six metres a second. Twenty-second burn coming up in one minute.'

• 

• 

Dear Caroline: I miss you terribly. The time has come to put ourselves in an orbit around Io, which is where Discovery is. We don't have enough fuel to slow ourselves down, so we are about to use a technique called aerobraking.

The theory is, we will enter the outer layer of Jupiter's atmosphere using what is called a "ballute" for a shield. The atmosphere will slow us down, and Jupiter's gravity will grab hold of us and slingshot us around behind the dark side. If all goes well, we'll wind up in a gentle orbit around Io.

It's dynamite on paper. Of course the people who came up with the numbers on the paper aren't here.

Since no one has ever done this before, everyone up here is as scared as I am. The difference is, they're busy. I have nothing to do but wait for it to happen. I hope this is all worth it.

• 

• 

      “Altitude seventy-five hundred feet, manual attitude control is good.”
     We copy, Conestoga. You’re now in final approach phase.
     “Landing gear lowered, landing shoe deployed. Passing high gate at angels seven-three and all systems are nominal.”
     You’re go for landing.

     His palms were sticky with sweat; Parnell wiped them off on his trousers, then took a second to glance around A-deck. On the opposite side of the flight deck Ryer had her hands locked on the control yoke, her eyes flickering across the myriad dials and digital displays at her station, while Lewitt carefully watched the engine status board at his console.
     Conestoga had passed “high gate,” the point of no return. At this juncture, it was a captive of lunar gravity and was committed to touchdown except in the most dire emergency, in which case firing engines to achieve escape velocity meant using the fuel reserves. If that happened, they would have no recourse except to limp home.
     That wasn’t going to happen, though. So far, the descent had been smooth. One of the CRTs on Parnell’s console displayed a map of the landing site, with a series of concentric circles expanding outward from the ground-zero mark. He tapped a command into his keyboard and the computer responded by pinging twice; the tiny crosshatch designating Conestoga’s position was slightly to the left of the circles.

     “Altitude two thousand feet and closing,” he said. “Landing beacon is acquired, thirty-five degrees from mark. Over.”
     “Correcting attitude,” Ryer said. “Closing in on landing beacon.”
     Parnell felt the ship tremble as Ryer fired thrusters to compensate for Conestoga’s drift. The crosshatch moved closer to the bull’s-eye as the moonship homed in on the automatic radio beacon at Tranquillity Base.

     Roger that, Conestoga. You’re looking good. Over. Main-Ops was monitoring radar telemetry sent from the base, playing backseat driver.
     “Fuel reserves nominal,” Lewitt said. “The shoe is down, gear is locked.” The shoe was a vertical probe which extended straight down from the center of the engine array, designed to absorb most of the landing shock and stabilize Conestoga once it was on the ground.
     “Altitude one thousand two hundred feet, seven degrees off the zero and closing.” Parnell glanced away from his board, looking across the compartment at Ryer. “Need to goose it a little there, Cris. There’s a boulder field you need to …”
     “I know, I know. I’ve been here before.” Ryer was fighting the yoke with one hand as she worked the thrusters with the other. Landing four hundred tons of flying skyscraper on a dime was not a job Parnell envied. Ryer had the skill and guts needed to pull it off, but he had done this once himself, and he didn’t recall being so goddamned nervous back in ’69….
     Forget it, he told himself. Let her do her job.

     Conestoga was now only a thousand feet above the base’s cleared landing area. Although Ryer had managed to compensate for engine drift, the ship’s terminal velocity was sixty-five mph above touchdown speed. “Angels one and we’re on the beam,” he said. “Coming in a little fast …”
     As he spoke, he felt the entire fuselage shudder as Ryer throttled up the engines. She was braking Conestoga just in time; the ship listed one degree starboard, but she gimbaled the port thrusters and quickly brought the mammoth vessel back in line.
     We copy, Conestoga, you’re on the mark. We check you at altitude seven hundred fifty feet and closing. Over.

     “Roger that, Main-Ops. Altitude seven-two-five, all systems a-okay for touchdown.” For the first time since they had commenced final approach, Gene looked up at the TV monitors. The lunar horizon was no longer curved, but instead lay as flat as a Kansas prairie, gray volcanic maria with short rounded hills in the far distance. Dust was already being kicked up by the engines, but through the dirty haze he could make out boulders and small impact craters, and glimpses of unnatural man-made shapes reflecting sunlight….
     No time for sightseeing. He pulled his eyes away from the screens and back to the console where they belonged. “Altitude four hundred, altitude three-five-oh …”
     We copy, Conestoga …

     “Reserves down by point two percent,” Lewitt said.
     “Throttling back five percent,” Ryer responded.
     “Staying on the beam, zero drift. Altitude one-fifty, one twenty-five. Throttle up a notch there …”
     “Engines up one percent …”
     “Altitude seven-fiver, attitude nominal …”
     We copy, Conestoga. Looking good.
     “Altitude fifty, engines down two percent.” The entire vessel shook as if it were caughtin a minor earthquake. Nothing could be seen on the screens now except dust and dense shadows. Parnell licked his dry lips. “Altitude twenty-five, altitude twenty …”
     “Engines back two percent …”
     “Shoe contact light!” Lewitt shouted.
     “Altitude fifteen … twelve … ten …”
     Now he could hear an almost impossible roar as Conestoga’s engines baked the hard volcanic floor. “Eight … six … five … cut main engines.”
     “Roger. Cutting main engines. Engine arm off.”
     The trembling stopped, and for an eternal half-second there was the sensation of falling … then the landing gear slammed into the regolith.
     “Touchdown!” Ryer yelled.

     Conestoga teetered on its legs like a drunk fighting for balance. For an instant it seemed as if the towering vessel would keel over and crash on its side, but then the gyros told the hydraulics which end was supposed to be up, and the moonship remained erect. The fuselage creaked as gravity settled old bulkheads and deck plates into unfamiliar positions, but after a moment that, too, passed.

     And then there was nothing but silence.

     Parnell took a deep, shuddering breath. “Wheel Command, this is Conestoga. We have landed. Over.”
     We copy, Conestoga. Good job.
     “Engines safed,” Lewitt said, his fingers quickly moving across his console to click switches. “Internal pressure okay, landing gear intact, main computer reset on standby mode. All systems green.”

Our Moon being an airless planet, a torchship can land on it. But the Tom Paine, being a torchship, was really intended to stay in space and be serviced only at space stations in orbit; she had to be landed in a cradle. I wish I had been awake to see it, for they say that catching an egg on a plate is easy by comparison. Dak was one of the half dozen pilots who could do it.

• 

     With Thurlow at the controls and Matt in the co-pilot's seat the jeep started down. It started with an orbital speed of better than four miles per second, the speed of the Aes Triplex in her tight circular orbit around the equator of Venus. The lieutenant's purpose was to kill this speed exactly over his destination, then balance the jeep down on its tail. A jet landing was necessary, as the jeep had no wings.
     He needed to do this precisely, with the least use of fuel. He was helped somewhat by riding "with the current" from west to east; the 940-mile-per-hour rotational speed of Venus at her equator was profit rather than loss. However, exact placement was another matter. A departure time was selected so that the entire descending curve would be on the day side of the planet in order to use the Sun as a reckoning point for placement in longitude; placement in latitude would have to depend on dead reckoning by careful choice of course.
     The Sun is the only possible celestial body to use in air navigation at Venus, and even Sol is lost to the naked eye as soon as one is inside the planet-wide blanket of cloud. Matt "shot the Sun" by keeping one eye glued on the eyepiece of an infra-red adapter which had been fitted to the ship's octant, and was enabled thereby to coach his skipper from a prepared flight plan. It had not been considrred practical to cut a cam for the automatic robot; too little was known about the atmospheric conditions to be expected.
     When Matt informed his pilot that they were about thirty miles up, by radar, and approaching the proper longitude, is given by the infra-red image of the Sun, Thurlow brought the jeep down toward their target, ever lower and slower, and finally braked her with the jet to let her drop in a parabola distorted by air resistance.
     They were enveloped in the ever-present Venerian clouds. The pilot's port was utterly useless to them. Matt now started watching the surface under them, using an infra-red-sensitive "cloud piercer."
     Thurlow watched his radar altimeter, checking it against height-time plan for grounding.
     "If we are going to dodge around any, it's got to be now," he said quietly to Matt. "What do you see?"
     "Looks fairly smooth. Can't tell much."
     Thurlow sneaked a look. "It's not water, anyway — and it's not forest. I guess we'll chance it."
     Down they dropped, with Matt watching the ghostly infra-red-produced picture narrowly at the end, ready to tell Thurlow to give her full power if it were a meadow.
     Thurlow eased off his jet — and cut it. There was a bump as if they had fallen a couple of feet. They were down, landed on Venus.
     "Whew!" said the pilot and wiped sweat from his forehead. "I don't want to have to try that every day."
     "Nice landing, Skipper!" called out Oscar.
     "Yea bo!" agreed Tex.
     "Thanks, fellows. Well, let's get the stilts down." He punched a stud on the control board. Like most rockets built for jet landings, the jeep was fitted with three stabilizing jacks which came telescoping out of the craft's sides and slanting downward. Hydraulic pressure forced them down until they touched something solid enough to hold them, whereupon the thrusting force was automatically cut off and they locked in place, propping the rocket on three sides, tripod fashion, and holding it erect.
     Thurlow waited until three little green lights appeared under the stud controlling the stilts, then unclutched the jeep's stabilizing gyros. The jeep held steady, he unstrapped. "All right, men. Let's take a look. Matt and Tex, stay inside. Oscar, if you don't mind my mentioning it, since it's your home town, you should do the honors."

• 

(ed note: in the year 2050, our heros are members of the Southwestern Rocket Society (SRS) fan club. The fans want to travel in space in the worst way, but civilians are not allowed to fly in their own ships.

On a field trip to Luna Louis' rocket junkyard they are stunned to find the space ship Absyrtis sitting in the lot. As it turns out that ship was Mr. Louis' last command when he was in the UN Space Force, and when the ship was decommissioned he managed to obtain it at scrap metal prices.

Club president Chubb Delany has an insane idea. He tells Mr. Louis that the club would love to refurbish the old ship, and fly it on a short hop to Luna. With Mr. Louis as captain.

Mr. Louis says if the club will promise that, he will give the ship to them free, along with any used rocket parts in the lot needed for the refurbishing.

They refurbish the ship pretty good, considering they are amateurs. They have to do lots of damage control once in space, due to all the things that were not refurbished as well as they thought. But the real fun comes when they are making the landing on Luna at Dianaport.)

      “All divisions stand by to commence pre-landing checks at will! Executive officer report to the control room. It is now approximately ten hours to zero on landing at Dianaport!”
     Chubb left the details of the ship up to Bert and concentrated on the mechanics of their landing at Dianaport. He knew it was a critical test of the ship and crew. Lift was mere button-pushing. It takes considerably more skill to bring a space craft to rest with another object—considerably more skill indeed. He planned for a straight-in approach. An involute would have been better, but he did not want to stretch his luck in that complex a maneuver. With the Moon’s slow rotation, he did not have to allow for much of a drift angle. But because of Dianaport's position in Mare Nubium, he would have to correct quite a bit to get a truly radial approach.
     He had it worked down to one application of thrust for correction of trajectory and another for landing and was about to set up a prob to see if it could be done with one prolonged acceleration period when Bert interrupted him.
     “My checks don’t look so good, skipper,” the first mate told him. “Servo systems on the guidance nozzles are wild. We’ve apparently lost the dither on the transfer valves, and the system’s become critically damped.”
     “Oh, great balls of fire! Can it be fixed?” Chubb exploded.
     “I don’t know. I’ll have to check.”
     “Well, check—and fix it! How am I going to get this crate down without guidance nozzles?”
“I’ll try, skipper …”     “Try, hell! Do it! If you need help, I’ll lend a hand. I’m familiar with that lash-up.”
     “Okay. But brace yourself for something else …”
     “Break it gently,” Chubb moaned.
     “Danforth's got running rabbits on the approach radar, and the range tracking element's gone out of calibration.”
     Chubb looked dismayed. “One more failure, and we can forget about trying to land; we’ll be lucky to get close enough to crash.”
     “How about going into circum-lunar orbit, skipper?”
     “That’s not a bad idea, but I don’t know if it can be done.” He scratched his head. “Let me feed it to Isaac Newton here. Get busy on that guidance and radar—and don’t Waste my time telling me it can’t be fixed. Get the lead out and use that time to figure out how it can be fixed —then let me know how you did it. And move! We haven’t got six months to play around this time!”
     Thirty minutes later, Chubb had determined that the Absyrtis could not make circum-lunar orbit. The mechanics of the maneuver were too expensive and too complex to be attempted. He told Bert, “It’s Dianaport or nothing, chum. Can I give you a hand with that servo problem?”
     LeRoy was almost at his wit’s end. The million little troubles he was havingin the power room were almost beyond him. He called Bert to report it. Chubb overheard. He muscled Bert away from the intercom and told LeRoy, “Look, you spent six months with that tangle of plumbing! Don’t tell me you can’t make it work now! Get out those spares you made! Get on the stick, man!”
     “But, skipper, this rusty old …”
     “If you want to gripe, join the Space Marines!” Chubb paused and went on in a quieter tone of voice, “What’s the trouble, LeRoy? Can I help?”
     Chubb could. He was a trained hydaulic engineer. LeRoy wasn’t the only one he helped in those long hours before turn-over. He wasn't doing the job Louis could have done; but it didn’t worry him because he was doing the best he could. He cajoled, cursed, encouraged, reassured, and bawled out his division chiefs, but he was always ready to help them. The crew responded. At turn-over time, there were still troubles, but the crew was of the state of mind where they would have gotten out and pushed if that would have done any good.
     The men of the Absyrtis were no longer the South-western Rocket Society; they were the crew of the space ship Absyrtis, a rusty, obsolete old bucket but still their pride and joy. Amateurs they were and would admit the fact—but unqualified, no. Heaven help the man who had the temerity to say they were not competent. Hadn’t they taken a piece of rotting junk and remade it into a space ship which was able to break free of the Earth’s stubborn gravity?
     They were not thoroughly satisfied with that, however. There was a matter of landing, the final portion of the examination. When they landed—not if—they would not have to take the ridicule of the seasoned spacemen. The landing would be their accolade.
     There are no adequate adjectives to describe their utter determination to get that ship down on Luna and eventually back to White Sands.
     It was reflected in the manner in which they performed turn-over, swinging the ship around so that the rocket nozzles pointed toward Luna. The radar was still jumping, but Danforth was reading it; the servos had no dither, but Bert was working the guidance nozzles by direct coupling, hoping that the hydraulic lines would hold; and the power plant—well, it looked good, but only the actual operation would tell the story.
     They came in boldly but carefully on their approach.
     “Dianaport Traffic reports that we are in the groove,” came the report from Bob Danforth.
     “Do they sound worried?" Bert asked.
     “No. Doubtful, perhaps. They’re going to hold traffic when we get in the final approach leg—and the meat wagons are standing by.”
     “Tell them we won’t need them,” Chubb remarked from under the astrostat hood.
     “How about some radar and doppler data?” Bert asked the electronics officer.
     “In half a shake. I’m having a little trouble reading the scopes and getting the aft radar locked in. We’re still a little far out; I’m getting a double trace on my scopes, but you’ll have data shortly.”
     “Well, start pushing doppler difference frequencies up to the computer as soon as you can.” The ballistic computer was standing by, its memory banks loaded with all the information Chubb and Bert could give it. The program data was also inside it. It was merely waiting patiently for final position data in order to swing into action.
     “Line up looks good,” Chubb said, pushing his astrostat hood up. “Give or take a half r-c-h, I think we can live with it. What’s the minus time for correction maneuver?”
     “I won’t know exactly until we get some radar. Wup!”
     “Electronics to control! Do you read that radar data okay?” Danforth’s voice came over the phones.
     Chubb and Bert checked the winking tell-tale lights and swinging dials on their boards.
     “It looks rough, but the computer’s taking it,” Bert told him. He looked over at Chubb. “Computer and autopilot tracking, skipper. Minus five minutes to correction time.”
     “Okay, Bert. All hands to acceleration stations and strapped. Get the power plant hot and ready to go. Give me thirty-second counts.”
     “Aye, sir. Do you want all hand in pressure suits?”
     “Negative. We don’t have enough to go around. Have Greg seal all compartments in case we spring a leak under thrust.”
     It was, as usual, a long wait.
     After making one final adjustment on the autopilot panel, Chubb floated over to his couch and started strapping in. He studied the data presentations on his panel as he did so, trying to get the feel of this landing. He had studied the techniques of landing in texts, but realized it was more of an arcane art than a science. It took What they called “touch”, and he didn’t have any. He would have to feel his way down by inches, but he didn’t dare waste any time about it. The Absyrtis had only so much propellant left in her tanks, and he didn’t want to waste it by needless maneuvering or fighting gravity too long.
     “All hands ready, sir,” Bert reported, jostling him from his concentration. “Ninety seconds to zero!”
     “Let’s have pumps!”
     “Roger! Power room, stand by to fire!”
     “Power room here! Pumps coming up! Tank pressures coming up! Reactor at pre-fire heat!” LeRoy’s voice replied. “I think she’ll hold! I think she’ll hold!”
     “Read-back check on parameters!” Chubb ordered.
     Bert consulted his panel. “Set for all units one-third thrust—Three degrees pitch positive and seven degrees yaw left on the guidance nozzles for five seconds thrust duration. Do you concur?”
     “Concur.”

• 

     “Sixty seconds to zero!”
     In theory, the Absyrtis could have made the Earth-Luna jump with two applications of thrust: one at lift and one at landing. However, it would have required accuracy far beyond practical engineering ability. Instruments could not be read that closely, neither by man nor machine. And the instruments could not possibly be expected to have that sort of accuracy. It was much cheaper, both in terms of all-up ship mass and sheer complexity of machinery, to allow for deviations and carry along a meneuvering margin in the propellant tanks.
     “Thirty seconds to zero!”
     “Power plant hot!” The pumps were shaking the ship again.
     Chubb stared at the pin-point stars in his astrostat, making a last check. Pushing a switch, he changed the mirror angles to show three separate stars which would coincide in the eyepiece once the correction had been made.
     “—fifteen—fourteen—thirteen—” Bert was chanting in a monotone.
     Chubb braced himself, relaxing and at the same time preparing himself for the push that was coming. “Fire on autopilot!”
     “—four—three—two—one—”
     It was a hard start, even at reduced thrust. The ship bucked violently, then settled down to shaking. But it didn’t last long. The motors shut down with their usual clamoring and bellowing, a wet, sloppy blubber that was heard even in the control room.
     Chubb squinted through the astrostat
.     “Looks good here,”“Bert remarked.
     “Power plant in cut-off!” LeRoy called. “She works smooth!”
     “Smooth, my foot!” Greg’s voice snapped. “What are you burning for go-juice?”
     “Shut up!” Chubb roared. The three stars were perfectly aligned and holding their positions. “Steady as she goes, Bert. Give me some ranges and bearings?”
     “Dianaport or Divana Space Station?”
     “Both.”
     He lay silent as the data flowed into the computer. Watching, he saw it was assimilating the data continuously and was holding to program. It was a reliable piece of equipment, but with the fouled-up radar, Chubb felt he would probably have to make part of the landing either on manual control or manual over-ride.
     The Absyrtis was in quite close and falling toward the Moon’s surface at a little better than a kilometer per second. Bert was reading Moon-relative vectors now, using his own astrostat as a drift indicator.
     So far, so good, thought Chubb. He was just beginning to correlate all the data into a fair mental picture when LeRoy’s voice screamed over the interphone, “Control room, we’ve lost a pump! Number Four has seized her bearings! Chambers three and five are out!”
     That was almost too much for Chubb. He hesitated for a split-second, then saw the approach rate indicator. “Bert! All available chambers stand by for full thrust! Program the computer for four-g by chambers one, two, four, and six! Guidance corrections accordingly! Move! I want to know if we can do it with what we’ve got!”
     So near, he thought disparingly, and yet so far. Three- hundred and eighty-three thousand kilometers behind them and less than a thousand to go. A half-year of work, hours of sweat and worry, a man’s life—and a failure with the goal in sight.
     Bert had the keyboard console swung over his couch and was setting up the new prob. LeRoy called up, “Skipper, can we make it on four chambers?”
     “Pipe down and keep your britches on!”
     “Whew!” Bert breathed. “We can make it! Four and a half minutes to zero!”
     “LeRoy,” Chubb called the power man, speaking in a firm but quiet tone, “we’re going to have to mash it in under high-g. Can the plant take it? How does it look?”
     "I've got my fingers crossed. We’ll have to do something about the vibration at high thrust; it shakes everything loose.”
     Chubb thought back, trying to remember his theory of combustion. “Look, are your pumps running fast enough? Try increasing the injector pressures.” He was shooting in the dark, but it might work.
     “It may bust the chambers.”
     “Not those iron maidens it won’t.” The Absyrtis had been built long before the thin-walled, light-weight modern rocket engines had become standard equipment. They were such an integral part of the ship’s structure that they had never been torn out during the many modifications; the ship-yards had merely modified the injectors.
     "We'll have to cut the over-speed trips on the turbines!”
     “Do it! Let ‘em run hot! Those turb buckets will stand it for a couple minutes! Just hang on, LeRoy, and twist her tail hard!—Or there’ll be a new lunar crater called Absyrtis!”
     “Two minutes to zero, skipper!” Bert armounced. “Doppler indicates our approach rate is within one-percent of calculated. Dianaport and Divana have confirmed. Autopilot is tracking. Dianaport Traffic says we’re eight degrees north of the groove and to correct. They’re holding traffic and have cleared the vicinity of Landing Pad Twenty-three for us.”
     “I’ll bet they’re standing by. Meat wagons for us and cops for LeRoy.” Chubb growled. “Okay,” he went on, then paused before giving the long-awaited order. “Bert, stand by to take her down;”
     “All hands Stand by for landing! Ninety seconds to zero!”
     “All boards Test-Fly to FLY!”
     “Boards to FLY!”
     “Power room here! Pumps coming up!—Up!—Up!—Up!—Good Lord!—Up!”
     The sound was a scream in the deck plates.
     “Hold it, LeRoy! Hold her, boy!”
     “Seventy-five seconds to zero!”
     “Autopilot?”
     “Tracking, skipper! Corrections noted and ready for compensation!”
     “Radar?”
     “Locked on Dianaport beacons!”
     The rocky craters lining Mare Nubium were plainly visible through the conning blister now. There was no doubt that the Absyrtis was falling toward the Moon.
     “Bert,” Chubb said privately to his exec, “I don’t know what’s going to happen after we hit dirt down there—but whatever it is, so help me if we get out of the sky safely I’ll figure it was worth it.”
     “Sure, it was worth it! Sixty seconds to zero!”
     “We’!l make it—We’ve got to make it!”
     “Control, these pumps may not hold up much longer at this speed!” LeRoy yelled over the interphone.
     “Electronics reporting! Divana just went into our blind spot toward!”
     “We go down on one radar, then!”
     “Skipper, that’s suicide!”
     “Shut up and do as you’re told!”
     “Forty-five seconds!”
     “Control room, these pumps are shaking everything! We’ve got a leak!”
     “Where? How bad?”
     “Squirting out around a fitting!”
     “Tighten that fitting or seal it! Hurry!”
     “Thirty seconds to zero”
     “The Lord is my shepherd; I shall not want …”
     “Shut up, Greg! I can’t hear LeRoy!” Chubb yelled.
     “Twenty-five!”
     “We’ll let it leak, Chubb. We’ll just get a little wet, that’s all”
     “Twenty!”
     “All hands brace for landing!”
     “Fifteen!” Bert screamed. “All green! Autopilot in command!”
     “Like hell! I’m on manual over-ride.”
     “Ten!—Nine—eight—.”
     It looked—well, they might make it—if the radar didn’t fizzle out—if the computer didn’t go wild—if the guidance system didn’t jitter—if the propulsion system worked. There were too many “if’s” for Chubb. He braced his fingers over the manual controls, not sure of whether he could land the ship or not, but willing to make a try.
     “Three—two—one—.”
     This time one chamber lit off before the rest. He felt the ship start to swing under the unbalanced thrust. Then all chambers were lit and correction was possible. The Absyrtis gyrated wildly, then settled down to a bone-crushing four-g's of acceleration.
     She hammered and rattled. Chubb clamped his jaws tight to keep from biting his tongue. He alternated his glances between the panel with the meters blurred by vibration and the conning blister.
     They were falling too fast! No, that was his imagination. He couldn’t read the approach rate indicator clearly enough. View aft on the tv monitor was useless; the vibration in the boat-tail had thrown the camera unit out of synch.
     But that couldn’t be Mare Nubium below them! It was much too rugged! Mountain peaks were spearing toward the ship.
     There was a weak voice in his headphones. “They’re holding! They’re holding!” LeRoy was screaming.
     The acceleration increased slightly, then slacked off as the autopilot felt its way down, the ground clutter on the radar confusing it to some extent.
     Then the ship cocked over at a ten-degree angle. Chubb almost over-rode it then, but it righted itself before he could move against the acceleration. Almost immediately, he was glad; through the brilliant orange haze of the jet aft, he could see the tiny checkerboard of Dianaport.
     What had Luna Louis once said? Why, he had been right! The old bucket did know her way into Dianaport by heart!
     The autopiot slacked off and let them fall at a half-g. The lunar soil rushed up at the ship. Chubb panicked invountarily, the old, ingrained fear of falling taking over. He tried to grope for switches. Something must be wrong! He got his hand over the panel, and a sudden burst of acceleration slammed his hand down on another switch.
     “What did I hit? What did I hit?” he screamed to himself.
     But it happened too quickly. Another burst of acceleration, a fall of about, a second’s duration, and then a back-wrenching jolt. The rocket nozzles blubbered and the dust swirled up around the conning blister.
     They were down on the Moon.
     And Chubb discovered what switch he had accidentally hit. He found himself staring at a luminous light panel on the overhead which said plainly:
     “You have just hit the panic switch, provided by the electronics division for your convenience in times of stress.”
     He started to laugh." “Jack down,” he managed to get out. “Secure all in-flight operations. Prepare to …” He couldn’t go on. He was laughing so hard that tears were running down his cheeks, pulled by the feeble lunar gravity. It was not hilarious laughter; there were sobs mingled in.
     Bert was a bit quieter, but he was doing the same thing. They lay there together on their couches, having trouble believing that they had made it at last.

---

     Chubb and Bert finally got the ship secured as the big loading towers were pulling up to the ship. The skipper of the Absyrtis would have liked to have seen the lunar landscape outside, but by the time things had quieted down to the point where he was able to take a look, the pressurized towers had cut off the view.
     “Electronics,” Chubb called, “give Dianaport Traffic a yelp and tell them we’re down and secured—and find out when the quarantine crew will be out …”
     “Wait one, skipper! I’m talking with them right now!” Danforth’s voice came back, strangely excited.
     “How about it, Bert? Have we got ground power yet?” Chubb asked his exec, checking.
     “Ready to switch now.”
     “Okay, all hands unstrap at will and stand by for …”
     “Chubb! Uh—Control, this is electronics! Skipper! We’ve got a reception waiting for us!…”
     “I’ll bet”
     “No, not what you’re thinking! We’re heroes, skipper! The mayor of Dianaport and all the colonial officials are waiting to greet us as soon as we get our locks open! This whole place is going nuts!”

     "Nice landing, Skipper!" called out Oscar.
     "Yea bo!" agreed Tex.
     "Thanks, fellows. Well, let's get the stilts down." He punched a stud on the control board. Like most rockets built for jet landings, the jeep was fitted with three stabilizing jacks which came telescoping out of the craft's sides and slanting downward. Hydraulic pressure forced them down until they touched something solid enough to hold them, whereupon the thrusting force was automatically cut off and they locked in place, propping the rocket on three sides, tripod fashion, and holding it erect.
     Thurlow waited until three little green lights appeared under the stud controlling the stilts, then unclutched the jeep's stabilizing gyros. The jeep held steady, he unstrapped. "All right, men. Let's take a look. Matt and Tex, stay inside. Oscar, if you don't mind my mentioning it, since it's your home town, you should do the honors."

A possibly significant advantage of three landing jacks is that they all get approximately evenly loaded when you land on an uneven surface. With four jacks you either have to build them strong enough that the heavily loaded jacks can take the strain, or add some sort of compensating mechanism (which would require extra mass).

If the feet are distance D from the centre, then with 4 feet the rocket can be stable with the center of gravity up to cos(45°)*D from the centre (i.e., 0.707*D). For a ship with 3 feet, to achieve the same stability, you'd need to extend the feet to be 1.414*D from the centre.

---

(ed note: For a ship with 3 feet the rocket can be stable with the center of gravity up to cos(60°)*D from the centre (i.e., 0.5*D)

A four-footed rocket with feet 10 meters from center would have a safe area within 7.07 meters of center. But for a three-footed rocket with feet 10 meters from center, the safe area would be only 5 meters in diameter.

How do you make a three-footed rocket's safe area the same as the four-footed rocket? You have to make the distance of the feet from center larger.

How much larger? cos(45°) / cos(60°) = 1.414. So you take the foot distance of the four-footed rocket, muliply by 1.414, and you have the required foot distance of the three-footed rocket.

In this case, 10 meters times 1.414 equals 14.14 meters. A three-footed rocket with a foot distance of 14.14 meters would have a safe area of 7.07 meters, the same as a four-footed rocket with a foot distance of 10 meters.)

---

It may well be that three such extensions require less mass than adding a fourth leg. (that is, three legs that are 14.14 meters long might have less mass than four legs that are 10 meters long)

There are probably swings and roundabouts, with one system being slightly superior than the other depending on other factors of the design, such as the mass of the feet (which still mass the same when you increase the length of three legs, but if you have four legs you need to budget for a whole extra foot).

• 

• 

• 

One could therefore devise a system comprising high-resolution landing radar or lidar (perhaps four antennae, one on each leg), computer, and attitude jets, to automatically rotate the ship as it lands to the optimum orientation. It would be quite difficult for a human helmsman, but maybe not fiendishly so, given the right instrumentation.

• 

Bear in mind that Starship is Spacex’s new name for what was once the BFS second stage spacecraft article of its BFR/BFS system. To be useful at Mars or the moon, this spacecraft must be able to make rough-field landings. Its mass is heaviest when fueled for launch. Initially, it must be refueled for use at Mars, and eventually, also the moon. These static loads are larger than the landing weights, even factored for dynamical impact.

There are two parts to this: (1) tip-over on rough or sloping surfaces, and (2) not exceeding the bearing load capability of the natural surfaces. There is also a new idea presented here for creating very large landing pad surfaces that fold so as not obstruct airflow, in a very practical way.

The tip-over problem was well-explored in another article on this site, as part of updates to the basic performance evaluation article. That article was “Reverse-Engineering the 2017 Version of the Spacex BFR” dated April 17, 2018. The same article identified soil bearing strengths as likely inadequate to support the spacecraft when refueled for launch.

The related article “Relevant Data for the 2018 BFS Second Stage” dated September 24, 2018, included among other things a way to reconfigure the round tip-mounted landing pads into oblong pads of increased area. Those results were still inadequate for the loose fine sand-like surfaces of much of Mars.

• 

What is analyzed here is a different landing pad idea, depicted in Figure 1. Essentially, panels resembling landing gear bay doors are built into each side of each fin tip, with hinge lines at the aft trailing edge (which is the touchdown surface otherwise). Unfolded hydraulically, these panels become very large landing pad surfaces. Folded, they do not protrude into the ascent or descent airstreams at all.

The same figure shows an 1100 metric ton fueled mass, which really could be as large as 1300-something tons. However, the BFS weight statement is still not known publicly with any certainty. This figure is in the same ballpark, given all the other uncertainties.

Assuming 1100 metric tons of mass, the weights on Earth, moon, and Mars are given in the figure. The bearing pressures associated with those weights are also shown, assuming 2 m by 2 m parallelogram-shaped folding panels are used. This assumes 2 panels per fin, and 3 fins. Just that initial assumption provides some 10 times the bearing area as the roughly 1 m diameter round tip pads shown in the Spacex illustrations of this vehicle.

• 

Figure 2 presents safe load-bearing strengths for civil engineering purposes of various types of Earthly surfaces. These came from an older-vintage Marks’ Mechanical Engineer’s Handbook. No such reference yet exists for lunar or Martian soils. However, experiences from the Apollo missions verify that the lunar regolith is similar to fine, loose Earthly sand.

Experiences with the various Mars landers and rovers suggest that much of Mars is similar to lunar regolith and to Earthly fine, loose sand. Some of Mars seems to have a mix of sand, gravel and larger rocks, perhaps similar to Earthly surfaces such as loose beds of medium and coarse sand, or perhaps even as substantial as beds of coarse sand with gravel. These require picking, not a spade, to remove. A hard clay requiring picking would also be of similar bearing strength.

All of this is indicated in Figure 2.

• 

Figure 3 compares applied bearing loads to soil strengths for the moon, Mars, and Earth. The moon is the least demanding problem because of its lowest gravity. A design adequate for the moon is adequate only for some of Mars: the indicated folding-panel parallelogram dimensions would be 2.2 m by 2.2 m, for about 29 sq. m bearing area. On Earth, sites must be strong, well-packed sand/gravel or hard clay. No beaches, no sand dunes, no soft desert.

• 

If we increase that pad area to be capable all over Mars, the folding panel parallelogram dimensions become 2.67 m b 2.67 m, for about 43 sq.m bearing area. This is part of the comparative pad area summary and comparison given in Figure 4. There is a trade-off here: the bigger these folding panels can be, the more of Mars (that is otherwise fairly smooth and level) is a feasible landing and takeoff site. The moon is not a problem, nor is much of dry land Earth (most anything requiring picking or blasting would be adequate).

This would simply not be the case with those 1 m diameter round tip-mounted landing pads that Spacex illustrates. That total pad area is about 2.4 sq.m, give or take a small amount. What is needed for rough-field capability on Mars (or the Earth) apparently falls in the 30-45 sq.m range. You simply cannot do that, in any practical way, with smallish fixed-geometry tip-mounted landing pads. Those will require thick reinforced-concrete landing fields, or else thick solid rock.

I know they have their hands full at Spacex trying to make this vehicle a reality. But some of the thinking I have explored here, also needs to go into Spacex’s designs!

Update 2-5-19:

As a follow-up, I put some more-traceable masses for the BFS/”Starship” weight statements into a spreadsheet, with gravity data, and empirical data for the safe bearing capability of various Earthly surfaces. This included the selections (and rationales) as for which surfaces resemble the moon and Mars, and which might be the widespread worst cases for emergency landings on Earth. Sources are indicated.

• 

Masses and gravity are given in Figure A. The residual propellant remaining is nothing but a wild guess, knowing that a dry-tanks landing is truly risky. As it turns out, the presence or absence of residual propellant mass at touchdown does not drive the sizing of landing pad area. Refilled launch weight drives this.

• 

The safe surface bearing-pressure capability data for a variety of Earthly surfaces is given in Figure B. These are civil engineering data from an old-vintage Marks’ Mechanical Engineer’s Handbook. These surfaces are rather variable in properties, as is typical of geology. They represent safe bearing pressure loads so that your structure or object does not try to sink slowly into the surface, even over long periods of time. It is conservative, ethical practice to use the min pressure values for design. In the handbook, the table presented both metric and US customary values, for which it was obvious the metric were converted from the US customary source values.

To this table I have added the notations about which surfaces resemble the bulk of the moon and Mars, and the rationales for those selections. I have also indicated the most common soft-surface emergency landing surfaces for Earth, excluding soft sand beaches and deserts (and swamps). The rationale for that is simple experience. These are my best estimates, if I had to do this.

I worked out local-weight weight statements for Earth, Mars, and the moon, for two configurations. One is arrival, with the larger payload sent from Earth, and only residual propellant left at touchdown. The other is at departure, with full propellant load, but a reduced payload, for the return to Earth. This reflects exactly what Spacex says the scenarios are for Mars.

Spacex says there needs to be no lunar refilling for return to the Earth from the moon, but this analysis anticipates that eventually lunar refilling might be attempted, for trips from the moon to destinations other than Earth. It does not matter; as it turns out, the lunar launch case does not determine the design requirement for landing pad area. But I had to check, as a due diligence item.

What I found was that local-weight launch weights exceeded local-weight touchdown weights by roughly a factor of 5. Therefore, it is only local launch weights that govern max bearing pressure exerted upon the local surfaces. One of these is the worst case that drives the design requirements.

Surfaces on the moon, and the great majority of Mars, resemble soft fine sand. There are places on Mars with somewhat-stronger surfaces, but these are definitely not the majority of possible landing sites. You do not want a rough-field landing design restricted to rare site opportunities. That would be rather pointless. As for the range of properties, you have to select the min capability.

• 

Figure C combines the local-weight weight statements with the appropriate selected surface bearing capabilities, to produce min total landing pad areas for Earth, Mars, and the moon. Mars governs, and by a significant margin. The total pad area result obtained here is not at all far from the seat-of-the-pants 43 sq.m in the original article just above. But this updated result is more traceable, and therefore the more reliable value. It is just about 46.2 sq.m.

The same figure also gives a selection of parallelogram dimensions for each fold-out landing pad panel. This is a function of the number of landing-leg fins, the number of panels per fin, and the aspect ratio of those panels (the height to base ratio for the parallelogram shape). Beyond scope here is the structural design of such panels; it seems likely that the lightest version would be nearer aspect ratio 1. That would be a panel 2.77 x 2.77 m size, vs 2.67 x 2.67 in the original article. That’s pretty close!

Again, I must point out that this is the sort of design, and design analysis, that is needed by Spacex to really provide a rough-field capability for its BFS/”Starship” spacecraft on Mars (or anywhere else). The round tip pads Spacex currently shows are roughly a meter diameter, for a total of about 2.4 sq.m total area, roughly some factor 20 smaller than what I determined here.

Implications

Without the large total landing pad area I found, the craft is restricted to very thick reinforced concrete landing pads, or smooth, level stretches of thick, solid rock. Otherwise, while you might possibly land (and no guarantees about that!), you’ll sink-in unevenly, and tip-over “for sure”, upon refilling propellant for launch.

At 2.4 sq.m and Mars launch weight, the applied bearing pressure is ~0.39 MPa, similar to the 0.38 MPa min capability of Earthly coarse sand & gravel, and similar to only a minority of Mars! And even that still lacks the factor 2 margin you need for the dynamical impact effects of the touchdown: the fin tips will inevitably stab deeply into the surface, risking getting stuck like tent stakes. Very likely, they will stab-in unevenly, risking a tip-over, even if the site is level. So the chances of a successful landing with the depicted design are very poor, and there is no chance at all of a successful refilled launch.

If the site actually resembles the soft sand that is the bulk of Mars, there is no chance at all of a successful landing, and by far. The applied bearing load is 0.39 MPa, even without the factor 2 for the dynamics. The soft sand capability is only 0.1 MPa. That’s about factor 4 outside-of-the-ballpark, and about factor 8 wrong with dynamics allowed-for! That’s a crash, period. That’s exactly what will happen with three 1-meter-diameter fixed-geometry fin-tip landing pads!

Conclusion

Something about the design Spacex currently shows, must change substantially, before even an unmanned cargo ship flies to Mars. They need 47 sq.m of pad area, not 2.4 sq.m. That’s what the best-available data actually says.

He was right, the digging was recent and it was not yet finished, for only half of the soil had been cleared away from around the fins of the ship. The cruiser had been buried after it had been landed, partly to help conceal it, partly to keep it steady in a proper position for a take-off where there was no cradle to hold it. If a storm here had battered it off fin level, with no port cranes to right it, the ship would be useless scrap until it rusted away.

(ed note: Beowulf Shaeffer is in a flying car, while Bellamy is in a spacecraft called "Drunkard's Walk" with an unreasonably powerful engine. Bellamy is trying to kill Beowulf. Beowulf bashes his flying car into the side of Drunkard's Walk then crashes into the ground. Drunkard's Walk lands using a "gravity drag" {don't ask})

     The car was on its nose in high fern grass. All the plastic windows had become flying shards, including the windshield; they littered the car. The windshield frame was crushed and bent. I hung from the crash web, unable to unfasten it with my crippled hands, unable to move even if I were free. And I watched the Drunkard's Walk, its fusion drive off, floating down ahead of me on its gravity drag. I didn't notice the anomaly then. I was dazed, and I saw what I expected to see: a spaceship landing. Bellamy? He didn't see it, either, but he would have if he'd looked to the side when he came down the landing ladder. He came down the ladder with his eyes fixed on mine and Emil's sonic in his hand. He stepped out into the fern grass, walked over to the car, and peered in through the bent windshield frame.

---

     I could walk, barely. I could keep walking because he kept prodding the small of my back with the gun.

---

     We were halfway to the ship when I saw it. The anomaly. I said, "Bellamy, what's holding your ship up?"
     He prodded me. "Walk."
     "Your gyros. That's what's holding the ship up."
     He prodded me without answering. I walked. Any moment now he'd see ...
     "What the —" He'd seen it. He stared in pure amazement, and then he ran. I stuck out a foot to trip him, lost my balance, and fell on my face. Bellamy passed me without a glance.
     One of the landing legs wasn't down. I'd smashed it into the hull. He hadn't seen it on the indicators, so I must have smashed the sensors, too. The odd thing was that we'd both missed it, though it was the leg facing us.
     The Drunkard's Walk stood on two legs, wildly unbalanced, like a ballet dancer halfway through a leap. Only her gyros held her monstrous mass against gravity. Somewhere in her belly they must be spinning faster and faster ... I could hear the whine now, high-pitched, rising ...
     Bellamy reached the ladder and started up. He'd have to use the steering jets now, and quickly. With steering jets that size, the gyros — which served more or less the same purpose — must be small, little more than an afterthought.

---

     Bellamy had almost reached the air lock when the ship screamed like a wounded god.
     The gyros had taken too much punishment. That metal scream must have been the death agony of the mountings. Bellamy stopped. He looked down, and the ground was too far. He looked up, and there was no time. Then he turned and looked at me.
     I read his mind then, though I'm no telepath.
     Bey! What'll I DO?
     I had no answer for him. The ship screamed, and I hit the dirt. Well, I didn't hit it; I allowed myself to collapse. I was on the way down when Bellamy looked at me, and in the next instant the Drunkard's Walk spun end for end, shrieking.
     The nose gouged a narrow furrow in the soil, but the landing legs came down hard, dug deep, and held. Bellamy sailed high over my head, and I lost him in the sky. The ship poised, braced against her landing legs, taking spin from her dying flywheels. Then she jumped.
     The landing legs acted like springs, hurling her somersaulting into the air. She landed and jumped again, screaming, tumbling, like a wounded jackrabbit trying to flee the hunter. I wanted to cry. I'd done it; I was guilty; no ship should be killed like this.
     Somewhere in her belly the gyroscope flywheels were coming to rest in a tangle of torn metal.
     The ship landed and rolled. Bouncing. Rolling. I watched as she receded, and finally the Drunkard's Walk came to rest, dead, far across the blue-green veldt.
     I stood up and started walking.
     I passed Bellamy on the way. If you'd like to imagine what he looked like, go right ahead.
     It was nearly dark when I reached the ship.
     What I saw was a ship on its side, with one landing leg up. It's hard to damage hullmetal, especially at the low subsonic speeds the Drunkard's Walk was making when she did all that jumping. I found the air lock and climbed in.
     The lifesystem was a scrambled mess. Parts of it, the most rugged parts, were almost intact, but thin partitions between sections showed ragged, gaping holes. The flywheel must have passed here.

---

     The bouncing flywheel hadn't reached the control cone.
     Things lighted up when I turned on the communications board. I had to manipulate switches with the heel of my hand. I turned on everything that looked like it had something to do with communications, rolled all the volume knobs to maximum between my palms, and let it go at that, making no attempt to aim a com laser, talk into anything, or tap out code. If anything was working on that board — and something was delivering power, even if the machinery to use it was damaged — then the base would get just the impression I wanted them to have. Someone was trying to communicate with broken equipment.

There are substantial trade-offs, however, depending on which method you use. Midpoint lifting requires an A-frame to support the entire weight of the ship in whatever gravity you happen to be. Lifting the ship by the nose requires longer, but less sturdy members for the A-frame. Shorter, more sturdy members are more likely to survive the initial crash, but may very well mass more than the longer ones. Longer members may be "recycled" into shorter members if they are damaged. Shorter members may be replaced by native materials -- trees, stone blocks, etc -- provided such are of sufficient strength.

It all boils down to your ship design. How much emergency gear can you carry? Is it better to stock up on ship repair materials to maximize the chance of getting off that space-rock, or on survival equipment to stay alive until rescue comes? Which then brings us back to the question of how far out your ship is meant to go...

• 

      The Far Traveler came to Botany Bay, to Paddington, dropping down to the Bradman Oval—which sports arena, since the landing of the Survey Service's Discovery, had become a spaceport of sorts. Discovery was gone, to an unknown destination, taking with her the mutineers and the friends that they had made on the newly discovered Lost Colony. The destroyer Vega, dispatched from Lindisfarne Base to apprehend the mutineers, was still in the Oval, still lying on her side, inoperative until such time as the salvage tugs should arrive to raise her to the perpendicular. Discovery, under the command of her rebellious first lieutenant, had toppled the other ship before making her escape.

---

     "This is too much!" yelled The Far Traveler's captain.
     Grimes blinked, thinking at first that the other was referring to the matutinal snack. But this was unlikely, he realized. "Calm down, calm down," he soothed. "Take a pew. Have a cuppa. And a scone …"
     "Calm down, you say? How would you feel in my shoes? I was engaged as a yachtmaster, not a tugmaster. I should have been consulted. But she, as per bloody usual, has gone over my head!"
     "What is all this about?" demanded Grimes.
     "You mean that you don't know either, Captain?"
     "No. Sit down, have some tea and tell me all about it. Shirley—a mug for Captain Billinger, please."

     "She," said Billinger after a tranquilizing sip, "is rolling in money—but that doesn't inhibit her from grabbing every chance to make more of the filthy stuff. She has signed a contract with your pal Delamere, engaging to raise Vega to liftoff position. She just happened to mention it to me, casual like."
     "You're not a tugmaster," agreed Grimes, "and a space-yacht is certainly not a tug. Looks to me as though she's bitten off more than she—or you—can chew."
     "Maybe not," said Billinger slowly, "maybe not She's a powerful little brute—The Far Traveler, I mean. She's engines in her that wouldn't be out of place in a battleship. But I should have been consulted."
     "So should I," said Grimes. "So should I. After all, this is my spaceport, such as it is." And then, more to himself than to the other, "But Frankie won't be too popular, signing away a large hunk of the taxpayers' money when the Survey Service's own tugs are well on the way to here."
     "They're not," said Billinger. "It seems that there's been some indefinite delay. Delamere got a Carlottigram about it Or so she says."
     "And so Frankie keeps his jets clear," murmured Grimes in a disappointed voice. "He would."

     And just how would this affect him? he wondered. Vega lying helplessly on her side was one thing, Vega restored to the perpendicular, to the lift-off position, would be an altogether different and definitely dangerous kettle of fish. Even should her drives, inertial and reaction, require adjustments or repairs she would be able to deploy her quite considerable weaponry—her automatic cannon, missile launchers and lasers. The city of Paddington would lay at her mercy. And then?

---

     "Good. No doubt you gentlemen feel that you are entitled to be apprised as to what has been arranged between Commander Delamere and myself. The commander will supply the towing wires from his stores. It will be necessary to pierce The Far Traveler's shell plating about the stern to secure the towing lugs. I am informed that the welding of steel onto gold is impracticable—and, of course, the modified gold that was used to build the ship on Electra is unobtainable here. Commander Delamere assures me, however, that his artificers will be able to make good the hull after the job has been completed. All dust and shavings will be carefully collected and melted down to plug the holes."

---

     Grimes stood with Wheeldon on the close-cropped grass of the Oval—the groundsmen were still carrying out their duties although no one knew when, if ever, play would be resumed—a scant five meters from the recumbent hulk of Vega. She was no more than a huge, useless, metal tube, pointed at one end and with vanes at the other. It did not seem possible that she would ever fly, had ever flown. Like a giant submarine, improbably beached on grassland, she looked—a submarine devoid of conning tower and control surfaces. Grimes remembered a visit he had paid to one of the ship-building yards on Atlantia where he, with other Survey Service officers, had witnessed the launching of a big, underseas oil tanker. And this operation, of which he was in charge, was a launching of sorts …

     Forward of the crippled destroyer stood The Far Traveler, a fragile seeming golden tower, a gleaming spire supported by the flying buttresses that were her stern vanes. Between each of these there was a steel towing lug, the dull gray of the base metal contrasting harshly with the rich, burnished yellow of the yacht's shell plating. Grimes had inspected these fittings and, reluctantly, had admitted that Delamere's artificers had made a good job. To each of the three lugs was shackled a length of wire rope, silvery metal cordage that, in spite of its apparent flimsiness, was certified to possess a safe working load measured in thousands of tons. It, like the Baroness's yacht, was a product of Electra, yet another example of arcane metallurgical arts and sciences. It was hellishly expensive—but when it came to the supply of stores and equipment to its ships the Survey Service had occasional spasms of profligacy. That wire must have been in Vega's storerooms for years. Nobody had dreamed that it would ever be used.

     Lugs had been welded to the destroyer's skin just abaft the circular transparencies of the control-room viewports. To each of these a length of the superwire was shackled. All three towlines were still slack, of course, and would be so until The Far Traveler took the strain. Grimes didn't much care for the setup. The problem would be to maintain an equal stress on all parts. He would have liked to have installed self-tensioning winches in either the yacht or the warship but, although such devices were in common use by Botany Bay's shipping, none were available capable of coping with the enormous strains that would be inevitable in an operation of this kind. As it was, he must do his damnedest to ensure that at least two of the wires were taking the weight at all times, and that there were no kinks. He could visualize all too clearly what would happen if there were—a broken end whipping through the air with all the viciousness of a striking snake, decapitating or bloodily bisecting anybody unlucky enough to be in the way. And he, Grimes, was liable to be one such. He had to direct things from a position where he could see at once if anything was going wrong. Delamere and the Baroness and all Vega's crew, with the exception of one engineer officer, were watching from the safety of the stands. And Mavis, with her entourage, was also getting a grandstand view …

• 

     He actuated his transceiver. "Port Captain to Far Traveler. Stand by."
       "Standing by," came Billinger's voice in the headset.
     "Port Captain to Vega. Stand by."
     "Standing by," replied the engineer in the destroyer's inertial drive room.
     Ships, thought Grimes, should be fitted with inertial drive units developing sufficient lateral thrust to cope with this sort of situation. But I'll use whatever thrust Frankie's engineer can give me …
     "Port Captain to Far Traveler. Lift off!"
     The yacht's inertial drive started up, cacophonous in the still air—. She lifted slowly. The wire cables started to come clear of the grass.
     "Hold her at that, Billinger. Hold her … Now … Cant her, cant her … Just five degrees short of the critical angle …"

     The Far Traveler was not only a floating tower, hanging twenty meters clear of the ground, but was becoming a leaning tower, toppling slowly and deliberately until her long axis was at an angle of forty degrees from the vertical. Billinger should have no trouble holding her in that position. In a normal vessel anxious officers and petty officers would be sweating over their controls; in the fully-automated yacht servo-mechanisms would be doing all the work.
     "Port Captain to Vega … Maximum lateral thrust, directed down!"
     The destroyer came to life, snarling, protesting. The combined racket from the two ships was deafening.
     "Lift her, Billinger. Lift her! Maintain your angle …"
     The Far Traveler lifted. The cables—two of them— tautened. They … thrummed, an ominous note audible even above the hammering of the inertial drive units. But the sharp stem of Vega was coming clear of the grass, a patch of dead, crushed, dirty yellow showing in sharp contrast to the living green.

     "Thirty-five degrees, Billinger …"
     The change in the yacht's attitude was almost imperceptible but the threatening song of the bar-taut wires was louder.
     "Increase your thrust if you can, Vega!"
     "It'll bugger my innie if I do …"
     "It's not my innie," growled Grimes. "Increase your thrust!"
     More dead yellow was showing under the warship.

     "Billinger—thirty degrees … Twenty-five … And roll her … Roll her to port … Just a touch … Hold it!"
     For a moment it seemed that all the weight would be on one cable only but now two had the strain once more.
     "Billinger! Twenty degrees …"
     Vega was lifting nicely, coming up from the long depression that she had made with her inert tonnage. Grimes noticed worm-like things squirming among the dead grass stems—but this was no time for the study of natural history. He was trying to estimate the angle made by the destroyer's long axis with the ground. Soon he would be able to tell the engineer to apply a component of fore-and-aft thrust. 
     "Billinger, ten degrees …"

     Then it happened. One of the taut wires snapped, about halfway along its length. The broken ends whipped viciously—the upper one harmlessly but the lower one slashing down to the grass close to where Grimes was standing. It missed him. He hardly noticed it.
     "Billinger, roll to starboard! Roll!" He had to get the weight back on to two wires instead of only one. "Hold her! And lift! Lift!"
     Would the cables hold? "Vega! Fore and aft thrust! Now!"
     The destroyer, her sharp bows pointing upward and rising all the time, surged ahead. Two of her stern vanes gouged long, ugly furrows in the grass. There should have been a spaceman officer in her control room to take charge of her during these final stages of the operation—but Delamere, when Grimes had raised this point, had insisted that it would not be necessary. (The obvious man for the job, of course, would have been Vega's captain—and Frankie, as Grimes well knew, was always inclined to regard the safety of his own skin as of paramount importance.)
     Vega lifted, lifted, coming closer and closer to the vertical. Two of her vanes were in contact with the ground, the third was almost so. Grimes looked up to the taut cables. He could see bright strands of broken wire protruding from one of them. It would be a matter of seconds only before it parted, as had the first one. Obviously those safe working load certificates had been dangerously misleading … "Vega! Full lateral thrust! Now!"

     "The innie's flat out!"
     Damn all engineers! thought Grimes. At crucial moments their precious machinery was always of greater importance to them than the ship.
     "Double maximum thrust—or you've had it!" The officer must have realized at last that this was an emergency. The destroyer's inertial drive not only hammered but … howled. The ship shuddered and teetered and then, suddenly, lifted her forward end, so rapidly that for an instant the cables hung slack. But Billinger quickly took the weight again and gave one last, mighty jerk. The stranded cable parted but the remaining towline held. The broken end slashed down to the grass on the other side of the destroyer from Grimes.
     Vega came to the perpendicular and stood there, rocking slightly on her vanes.
     "Billinger—'vast towing! Vega—cut inertial drivel"
     "It's cut itself …" said Vega's engineer smugly.
 
     And then, only then, was Grimes able to look down to see what the end of the first snapped cable had done. He stared, and swallowed, and vomited. He stood there, retching uncontrollably, befouling his clothing. But it didn't much matter. His footwear and lower legs were already spattered with blood and tatters of human flesh. The flying wire had cut the unfortunate Wheeldon—not very neatly—in two.

(ed note: Rick Sternbach dropped me an email with some commentary. Which reminds me that more people read this website than I realize so I would do well by being more polite.)

Yeah, a lot of folks assumed Voyager was sitting all 700,000 MT on a planetary surface, and yeah, they looked a bit small and silly, but there were amazingly few places in which to stow the legs within the engineering hull and have them look like they’d work.

I did want to give you a little info on the true nature of the legs, if it’ll help. Yes, the ship is using it’s structural integrity field (SIF) generators to reinforce the framing and plating. The ship is also using its impulse propulsion system (IPS) to support most of the mass, and the legs and feet are really what we called the ground hover footpad system (GHFS), mostly to keep the vessel from sliding about while on a planet.

Normally, the impulse engines can move a Starfleet ship at multiples of 1g, so countering the gravity of an Earthlike world is a walk in the park. Of course, in Star Trek, we don’t feel the acceleration because of the inertial dampers. Or magic.

I won’t argue the wisdom of actually landing a starship, since that was really a producer thing, but based on all the other Trekish engineering I’ve played with for the franchise, I can accept using the legs more as simple stabilizers. If the impulse space-time driver coils flake out, well, then we’ve got a problem.

• 

But he had not eyes for it. To the west where avenue and buildings ended was the field and on it space ships, stretching away for miles — fast little military darts, stubby Moon shuttles, winged ships that served the satellite stations, robot freighters, graceless and powerful. But directly in front of the gate hardly half a mile away was a great ship that he knew at once, the starship Asgard. He knew her history, Uncle Chet had served in her. A hundred years earlier she had been built out in space as a space-to-space rocket ship; she was then the Prince of Wales. Years passed, her tubes were ripped out and a mass-conversion torch was kindled in her; she became the Einstein. More years passed, for nearly twenty she swung empty around Luna, a lifeless, outmoded hulk. Now in place of the torch she had Horst-Conrad impellers that clutched at the fabric of space itself; thanks to them she was now able to touch Mother Terra. To commemorate her rebirth she had been dubbed Asgard, heavenly home of the gods.

Her massive, pear-shaped body was poised on its smaller end, steadied by an invisible scaffolding of thrust beams. Max knew where they must be, for there was a ring of barricades spotted around her to keep the careless from wandering into the deadly loci.

The ship rolled out of the hangar like an air-borne whale, moving slowly, its diamagnetized hull clearing the smooth-packed clay of the field by three inches.

Terens watched Genro handling the controls with finger-tip precision. The ship was a live thing under his touch. The small replica of the field that was upon the visiplate shifted and changed with each tiny motion of every contact.

The ship came to a halt, pinpointed at the lip of a take-off pit. The diamagnetic field strengthened progressively towards the ship's prow and it began tipping upward. Terens was mercifully unaware of this as the pilot room turned on its universal gimbals to meet the shifting gravity. Majestically, the ship's rear flanges fitted into the appropriate grooves of the pit. It stood upright, pointing to the sky.

The duralite cover of the take-off pit slipped into its recess, revealing the neutralized lining, a hundred yards deep, that received the first energy thrusts of the hyperatomic motors.

• 

(ed note: Mercenaries from evil global corporations are trying to capture the nation of the United Mitanni Commonwealth. Protagonist Sandy Balwin aka Sendi Bold One is leading the liberation effort. His main job is the liberation of the country's Vamori-Free Space port)

      As CIC and Spaclmpy commander, my missions were the recapture of Vamori-Free followed by the Topawa assault. I planned to move fast and alter plans in the face of new situations. That's the classic formula for winning a battle or a war. Like General Nathan Bedford Forrest, I wanted to git thar fustest with the mostest men.
     Holding and defending a space port had never been done before, and the Freedom Army consisted of land warriors whose only experience with vertical envelopment had been with armored aerodynes. They planned to defend Vamori-Free on the ground and moved in a few shoulder-launched SAMs to defend against low-level tacair. They didn't know how anyone could attack and invade a space port from space.
     I did.
     A space port is mostly space.
     Vamori-Free Space Port covered more than 7,500 square kilometers and stretched more than 150 kilometers along seacoast. It was larger than some nations. During normal operations, 25,000 people lived and worked there.
     We mounted a two-pronged effort against the Vamori-Topa objectives. Pahtu's river offensive would pin down Free Army forces at Topawa. Moti's Northern Impy would to hit the western edge of Vamori-Free by following the power transmission lines from Oidak.
     Then my space contingent would strike Vamori-Free and land.
     It sounds easier than it was.
     The tacair strikes were to be followed by a second pass of Omer's skalavans to cover the landing of Ursila's packets and free trader ships manned by as many swat teams as we could put together from Citlmpy people in space. Some had to be flown by a single pilot because we were short of pilots. There was no ground power for landing aids at Vamori-Free (because the invaders have cut the power) and shipborne radars don't have the precision necessary for landing, so Omer had the crucial task of dropping a landing beacon on his second pass.

---

     We were over Vamori-Free's western horizon now, but I couldn't pick up Omer's implanted landing beacon. "Slugger Leader, this is Bold One. Do you receive signals from Prong Alpha?"
     Ursila's voice came back, "Bold One, this is Slugger Leader. Negative on Prong Alpha. Something's gone wrong, Sendi!"
     "Boomer Leader, this is Bold One coming out of blackout with Slugger. We are negative on Prong Alpha. Repeat, negative on Prong Alpha. Any problem?"
     "Uh, Bold One, this is Stomper Leader. Purple Stomper was too high, and Boomer Leader made his second pass low to deploy Prong Alpha. They had a mid-air."
     Ohmygawd! Omer! Omer! The Mad Russian Space Jockey had let it all hang out … and some damned fool was where he shouldn't have been and cut it off. At a closure rate of Mach fifteen, neither had seen each other. The aerodyne may have been on Omer's screen, but he probably didn't have time to look at it.
     It was my fault for mixing Mach zero aerodynes with Mach fifteen space ships! There was no time to grieve then. We were less than five minutes from landing with no landing beacon to steer us in. If I didn't do something instantly, there would be a lot of pranged space ships.
     "Slugger Leader, this is Bold One. Did you monitor that?"
     "Affirmative! Sendi, I'll get down using calibrated eyeballs, but some of my pilots can't do it without help," Ursila reported. "Get a beacon down there. Any beacon!"
     "Stomper Leader, this is Bold One. Can you put your aerodyne in Area Twenty-four with its beacon squawking four-zeroes?"
     "I'll draw fire if I do!"
     "If you don't, you'll have real fire all around you from burning space ships! Or we'll overfly and leave you up the crick. Take your choice. We need your beacon in four minutes."
     "I can't risk my …"
     "Stomper Leader, this is Citlmpy Prime One." It was Kivalina who cut in. "Comply with Bold One's request now! Everyone's risking everything here. Topawa or the Dilkons!"
     That was The General's last call to his warriors before the First Battle of Oidak.
     The reaction and reply were immediate: "Topawa or the Dilkons! Stomper Leader is cushioning in Area Twenty-four now, beacon squawking four zeroes. But please don't land on me!"
     "Don't worry. And thank you," I told him. "Sluggers all from Bold One, critical instructions. Tape for replay. No time to repeat. Landing beacon is squawking four zeroes at Area Two-four. Repeat: Area Two-four. Have your computer offset for your specific landing coordinates. No need to acknowledge. Comply and execute … or you'll have a damned rough landing unless you put it in the ocean which isn't much softer."

      There was actually quite a long process used to do landing site selection. The original batch of Apollo landing sites (I'm referring to the ones that appear on lunar maps before Apollo 11) had to meet, as closely as possible four criteria:

     First, the site had to be 'clear' of major obstacles to the degree that could be discerned from overhead photography. In particular, each site (there were 7 of them) had to have an ellipse that was something like 4 or 5 miles long East-West, and something like 1 or 2 miles North-South (those numbers are to the best of my recollection) that was free of major terrain problems. However, we should keep in mind that at the time of the first landing mission, the limit of photographic resolution that we had of those sites was of stuff that was about 30 feet (9 meters) across. So, there could well have been obstacles for the crew to contend with that were too small to show up in the pictures we had at the time.

     Second, each site had to be within approximately 10 degrees north or south of the lunar equator. The idea here was that for the early missions it was desired to use something called a 'free return trajectory' enroute to the moon. This trajectory was desired because it is such that, in the event of a complete failure of the Apollo Service Propulsion System engine (the SPS engine) the spacecraft could go around the moon and return to a satisfactory re-entry condition at the earth (now, in fact, the accuracy required for this was much too great to achieve after translunar injection, so mid-course corrections would have been required on the way back, but they would be quite small — well within the capability of the RCS to perform). The free return trajectory requires that the inclination of the approach relative to the moon not exceed about 10 degrees, so this sets the maximum inclination of the lunar orbit at about 10 degrees if the spacecraft does no plane change during the Lunar Orbit Insertion maneuver.

     Third, the sites needed to be spaced across the face of the moon such that a maximum number of launch opportunities from the earth could be had in case of launch scrubs. This was a part of the constraint on lighting from the sun at each landing site when a landing was being performed. To give the crew best visibility, it was desired that the sun be something like 5 degrees to 20 degrees above the lunar horizon at the time of landing, and (of course) at the crew's back. In other words, just past sunrise at the landing site. The approach trajectory during landing had a somewhat steeper flight path angle than this, which meant that the LM came down 'over top of' the sun for best visibility. Actually, the angle limits changed during the mission planning phases, but there was always a 'window' of from 13 to about 15 degrees that was considered 'allowable' for the sun's position. Now, the moon rotates on its axis every 28 days or so. This means that the sun moves over the moon's surface at something like 13 degrees per day. If we have only a 13 degree (or so) allowable range of position of the sun at landing, it means that a particular landing site is available only 1 day per month. For example, if we want to land at Apollo landing site #2, and we set a launch date, and we scrub the launch for some reason, we are out of luck at Site #2 if we want to stay within our sun position constraint. So, it has to be 'on to another landing site'. If we can wait for the next month, we can try landing site #2 again. If we don't want to wait, we have to have another site that is at least 13 degrees west of landing site #2 (if we have to scrub for 2 days, we have to have a site that is at least 26 degrees west of landing site #2).

     One last constraint — early on, it was decided that we knew (to the required precision) the absolute position on the moon of features that were within 45 degrees of the North-South centerline — in other words a band of 45 degrees east to 45 degrees west of the North-South centerline. Outside of this boundary, the people responsible for this decided that the 'error bars' on positions of landmarks were too large, so they decided to pick sites within the plus or minus 45 degree longitude constraint.

     The above resulted in 7 original sites spread across the face of the moon, all within 10 degrees of the equator, and all within about 45 degrees east to 45 degrees west.

• 

(ed note: The arrogant Styor aliens have destroyed the the Terran trade post on Klor, by hovering over it with a spacecraft and torching the place with the rocket exhaust. Our Hero Kade is rooting around in the ruins of the base, looking for the interstellar communicator. What he finds is a landing beacon. Placing it on level ground will allow a spacecraft to safely land. The oppressed primitive Ikkinni natives call it a "star drum".)

     But what Kade wanted was still there; the com. He splashed to that shelf. However as he reached for the starting button he saw another object, poised directly before the communicator. And he had been briefed in the proper use of that sectional rod mounted on a firm base.
     Now he knew that the men who had waited in that room, or some member of the ship's crew, had suspected—or hoped—for his escape. There would be no answer to any message sent from the com. Perhaps the installation itself had been booby-trapped to prevent examination by native or Styor—but he did not need it.
     Kade caught up that tube. Sealed into it were delicate works, the technology of which was beyond him. But it would work when and if he desired. Cradling it against him, the Terran made his way back along the waterlogged passage. He had only to locate a proper site, set up what he carried, and there would be a new landing field on Klor, one not supervised by the Styor.

---

     A full week of the longer Klorian days passed before their small party caught up with an Ikkinni war party. Kakgil called a conference of scouts who knew the land while Kade set up his signal tube in demonstration, explained the terrain needed and why. Hunters compared notes, grew heated in dispute, finally agreed and voiced their suggestions through Iskug, who had joined the band.
     "Two suns, two sleeps away, there is a place where long ago the Spearman struck deep into the earth." He rounded his hands into a cup. "It has seen the ships from the stars. If it who drives such a ship is skillful, the ship could be set into this place as so." He inserted a finger tip into the curled fingers of his other hand.
     "This is the only place?" Iskug's description was too graphic to be reassuring. The Ikkinni agreed that the described crater was the best and safest landing the range had to offer.
     Later Kade, standing at the end of a grueling climb and looking down into that hole, was not sure. There was floor space enough, yes, to set a scout down. And the surface appeared as level as any ground. But the fitting of the ship into the hollow required skill such as only a veteran pilot would possess. However Trade pilots were top men.
     They made their way to the floor of the crater. The eruption which had caused the blowout must have been a cataclysmic one. Kade held the signal at shoulder level, triggered a thumb button, and slowly turned, giving the hidden lens the complete picture of this rock-walled well for broadcasting. Then he walked to what he judged was the center of the open space and secured the tube on the ground with latching earth spikes. Last of all he brought his hand down sharply on the pointed tip of the cone. There was no way for him to know whether the broadcaster was really working; his answer could only come, in time, from off-world.

---

     Kade followed the line of Dokital's pointing spear tip. The newcomer was no Ikkinni, collared or free, nor the Overman of a squad. Away from a carrying-chair, the other marks of his Klorian godship, a Styor was climbing stiffly up the rugged slope. He held one arm bent at chest level and divided his attention between his footing and a band about his wrist. In his other hand he carried the ultimate in the aliens' armament—the needler!
     Flight was cut off. The Terran judged that the wristband was some kind of tracking device, perhaps centered on his own thought waves. He could walk backward, step out into the space of the crater, and crash down to end near the signal. Only then the Styor might use that signal for bait.
     On the other hand, suppose he was needled down. Would the alien pass the signal unnoticed? The Styor was astute enough to investigate why the off-worlder had camped here. Either way the bejeweled, slim humanoid had all the cards on his side. Kade had overestimated the sloth of the pampered lords, underestimated their desire to make sure of the last Terran.
     About the Styor's middle was an anti-person belt. No overlord would risk his precious skin with the slightest chance of a counterattack. The spear in Kade's hold, any Ikkinni net, a rock thrown by a desperate man, would rebound from the aura now about the alien as from a dura-steel wall.
     Unless—Kade searched the ground about him for some suggestion of offence or defense. The Styor could probably track them if they tried to run for it. He did not know the range of the instrument the alien wore. On the other hand he was not going to be needled down without some counterattack, no matter how feeble.
     More to gain time than by any plan the Terran signalled Dokital away from the hut, along the edge of the crater. The rough terrain hid them from actual sighting by the Styor, though his locator would bring him on their track.
     Single file the two walked a narrow line along the drop. An idea grew in Kade's mind. A chance he was now desperate enough to try.
     The Styor reached the hut, did not even glance into its empty interior, but came on, treading the same way the fugitives had taken. Again Kade signed to Dokital, sending the Ikkinni away from him. Then the Terran halted, balancing his spear in his hand. A few feet beyond, the ancient bowl of the crater was split with a crack wide enough to offer protection to a slender Terran body. He marked that down.
     He was waiting as the Styor's head arose, the alien's eyes raised from the device on his wrist to the man before him. Then Kade hurled his spear.
     The aim was true, though the point struck that invisible guard a good six inches away from the Styor's chest. And the involuntary reaction of the other carried through even as Kade had hoped. A flinch backward set the alien's booted heel on a patch of smooth stone. There was a wide flail of arms as the Styor went backward into thin air.
     His safety belt would save his life, but now he would have the inner wall of the crater to climb. The Terran's attack had bought them a measure of time. Kade sped to the crevice, Dokital joining him. The Styor was floating down, settling to the floor of the crater. But they had only gained a few moments of time, no real escape. Only—
     Kade's arm went about Dokital, he carried the native with him in a rush as from overhead came a clap of sound louder than any thunder. Stone scraped skin raw as they tumbled into the rock crack. Above there was a flare of blinding light, and Kade hid his eyes with one bruised arm. The roar of a ship's tail flames as it braked into the heart of the crater was deafening.
     Perhaps the Styor had had one instant of horror, a second's realization of descending death—then nothing at all. The same end he or his fellows had visited on the Trade post had already been his.
     As the Terran and the Ikkinni crawled from their refuge the fumes of molten sand arose from that cup. Set neatly in the center was the star-ship. Kade climbed to the rim of the rock wall, waved at that expanse of pitted metal although no hatch had yet opened. But the response came soon enough, a ramp swung out to ground against the mountain some feet below him. He slid down, hearing his boots clang against its surface, hardly yet able to believe in that opportune arrival.

     Right that moment I was feeling unusually expendable, almost expended, because I was hearing the sweetest sound in the universe, the beacon the retrieval boat would land on, sounding our recall. The beacon is a robot rocket, fired ahead of the retrieval boat, just a spike that buries itself in the ground and starts broadcasting that welcome, welcome music. The retrieval boat homes in on it automatically three minutes later and you had better be on hand, because the bus can't wait and there won't be another one along.

---

     We heard the beacon cut off as the retrieval boat landed on it—I saw it land … and it was too far away. We heard the acting platoon sergeant call out: "In succession, prepare to embark!"
     And Jelly called out, "Belay that order!"
     We broke at last into the open and saw the boat standing on its tail, heard the ululation of its take-off warning—saw the platoon still on the ground around it, in interdiction circle, crouching behind the shield they had formed.
     Heard Jelly shout, "In succession, man the boat—move!"
     And we were still too far away! I could see them peel off from the first squad, swarm into the boat as the interdiction circle tightened.
     And a single figure broke out of the circle, came toward us at a speed possible only to a command suit.
     Jelly caught us while we were in the air, grabbed Flores by his Y-rack and helped Three jumps got us to the boat. Everybody else was inside but the door was still open. We got him in and closed it while the boat pilot screamed that we had made her miss rendezvous and now we had all bought it! Jelly paid no attention to her; we laid Flores down and lay down beside him. As the blast hit us Jelly was saying to himself, "All present, Lieutenant. Three men hurt—but all present!"
     I'll say this for Captain Deladrier: they don't make any better pilots. A rendezvous, boat to ship in orbit, is precisely calculated. I don't know how, but it is, and you don't change it. You can't.
     Only she did. She saw in her scope that the boat had failed to blast on time; she braked back, picked up speed again—and matched and took us in, just by eye and touch, no time to compute it.

      Lunar lander engine exhaust blows soil, rocks, and dust at high velocity and will damage surrounding hardware such as lunar outposts, mining operations, or historic sites unless the ejecta are properly mitigated.

     Twenty years of research have developed a consistent picture of the physics of rocket exhaust blowing lunar soil, but significant gaps exist. No currently-available modeling method can fully predict the effects.

     Understanding and Modeling the Physics: Our prior work characterized the different regimes of transport that can occur under various plume and planetary environment conditions. While rocket exhaust can deeply crater Martian regolith, the lunar effects are largely restricted to surface scouring a few centimeters of looser material. Lunar regolith is highly compacted deeper than a few centimeters and the lack of an atmosphere to collimate the plume prevents abrupt pressure gradients from the surface that would otherwise cause the soil to deform into a crater. However, a possible exception may occur in the permanently shadowed regions where soil may be looser (as suggested by several lines of evidence) or with a larger lander on a soft crater rim.

     We quantified several types of damage to neighboring hardware via analysis of the Surveyor III spacecraft that was sandblasted by the Apollo 12 landing and by performing hypervelocity impacts of appropriate particle sizes and velocities onto additional materials.

     Mitigating via ISRU: Our results show plume ejecta are impossible to simply block with a berm or fence because particles colliding in flight scatter over the barrier. Also, larger particles like rocks loft over the barrier and arc down into the other side, and the berms themselves scatter the particles in lunar vacuum. Berms can reduce ejecta damage, but full mitigation requires construction of a landing pad. Center for Lunar and Asteroid Surface Science (CLASS) team members have prototyped and studied technologies including sintering lunar regolith with microwaves, sunlight, and/or infrared radiation, application of polymers to regolith, the use of gravel and pavers, lunar concrete, and more. They have tested robotics for grading and compacting lunar landing zones. Our team members also tested 3D printing of regolith that can construct walls, and non-ISRU solutions such as deployment of inflatable blast barriers. Each method has drawbacks, so downselection to a set of complementary technologies is required.

     Next Steps: We are pursuing three approaches to fully address the plume challenges. First, we are continuing research into the physics to close the gaps, leading to more predictive computer models. These will set better requirements for landing operations and landing pad construction. Second, we are testing and assessing each mitigation technology including sintering lunar regolith and other methods to create competent surfaces, robotics for bulldozing and bermbuilding, and the use of gravel or pavers. This will lead to a recommended downselection. Third, we are organizing a series of robotics competitions for landing pad construction technologies in conjunction with Machine Learning companies to further advance the necessary robotic capabilities.

---

---

Crater formation for ~40 MT landers will be qualitatively worse than Apollo

Analysis of Surveyor, Apollo, and Chang’e data plus terrestrial experiments and computer modeling determined the following:

---

• Different regimes of gas/granular behavior exist (like different behaviors of frozen water vs. liquid water vs. vapor).
• (Non)Occurrence of each regime depends on size of the lander + environmental conditions (atmosphere, soil permeability, etc.).
• Martian and the Lunar plume effects are not comparable.
• Apollo LM effects were dominated by Viscous Erosion regime (smooth & streaking stages) with rare occurrences of Bearing Capacity Failure (in terrain modification stage).

---

     A crater is not directly observable but is detectable by presence of

• erosive crestline failure between lunar sedimentary strata (micro-scarps) (A);
• headed (B) & unheaded (C) erosion remnants

---

• New image analysis technique (Lane & Metzger, 2014) determined erosion scales as plume shear stress to the 2.5 power.
• Therefore, it scales as vehicle mass to the 2.5 power.
• All data agree that about 2.5 tons of soil were ejected by each LM landing

---

• Effects for ~1 ton Commercial Lunar Payload Services (CLPS) landers will be tiny: ~20 kg ejecta predicted
• Chang’e 4 (~0.8 t) ejecta measured at 19 kg, confirming the 2.5 power index
• For CLPS landers, there will be blowing dust and some rolling gravel but no other significant effects
• CLPS erosive depression predicted 0.25 cm deep, too tiny for microscarps. Probably impossible to measure anything in imagery.
• CLPS crater impossible to identify in computer simulations:

6 degree sun angle. Left: simulated lunar terrain; center: Chang'e 4 erosion on a smooth surface. Right: both added together

---

Applying the 2.5 power index to a 40 t lander predicts ~470 t ejected soil, forming a crater many meters deep

     However, we cannot extrapolate this far.

     It is likely that additional regimes will “turn on” at these high thrust levels. Bearing capacity failure? Diffusion-driven shearing? Bulk failure?

---

---

Ejecta Characteristics

For the LM, some ejecta exceeded lunar escape velocity (2.43 km/s)

Particle Size	Ejecta Speed for LM
Dust	1000 – 3000 m/s
Sand	100 – 1000 m/s
Gravel	~30 m/s
Cobbles	~10 m/s

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Ejecta were dispersed globally though flux was small a great distances. Can destroy orbiting spacecraft.

---

For CLPS landers, ejecta travel multiple kilometers (up to 10s or 100s).

---

For 40 t landers, ejecta particle velocities are nearly double the LM’s, so will travel much farther and disperse globally with vastly larger impact flux

• 

---

Analysis of impact damage on returned Surveyor 3 hardware shows extensive surface cracking, pitting, and dust impregnation.

SEM images of Surveyor 3 surface. Left: original condition. Right: after sandblasting by Apollo 12 LM.

---

For 40 t landers, the higher ejecta quantity and higher ejecta velocities indicate great damage can occur to an outpost or an ISRU mining operation. Mitigation is necessary.

---

---

Mitigation Techniques

Berms may help but are not a complete solution

• Evidence indicates ejecta “bounce” off terrain (dust bounces off sand, sand bounces off rocks)
• 40 t landers will cause too much cratering under the lander

Landing pad requirements differ for inner and outer zones

May use different construction methods in inner and outer zones

Inner methods (high temp, gas impermeable)

• Sintering
• Pavers with grouting
• High temperature polymer infusion in soil
• Rock welded pavers
• Bring sheet material from Earth

Outer methods (low temp, resist scouring erosion)

• Sintering
• Pavers (grouting not required)
• Low temperature polymer
• Rock filtration system

Many groups have done tests of various technologies. These & future tests provide input for trade studies.

Preliminary trade study is in work

Considerations:

• Mass brought from Earth;
• Energy required (high energy systems cannot be landed until after the landing pads are built);
• Construction time;
• Reliability.

---

Conclusions & Future Work

• Human-class landers (~40 t) will cause severe pluming effects.
• CLPS pluming will be very minimal but this may be deceptive because scaling is a 2.5 power law of vehicle mass.
• Pluming can damage surrounding hardware including ISRU operations, habitats, and scientific equipment.
• Pluming can damage or destroy spacecraft in lunar orbit if the timing is unfortunate.
• Need to continue developing individual mitigation technologies.
• Need to complete the mitigation trade study.
• Need to develop robotics to implement the mitigation techniques.
• This work is feed-forward to Mars.
• The CLASS Planetary Landing Team is set up to advance this effort.

• 

      As the United States strategizes its return of humans to the Moon in 2024 and beyond per Space Policy Directive One, there are many challenges in technology and engineering that must be overcome to ensure a successful mission.

     One mission element that separates human lunar missions of the present from the Apollo era is lander scale, where Artemis landers propose a landed mass of ~20-60mT versus the ~10mT Apollo lunar module. The engine plume or multi-engine plume from these large landers may pose a range of risks, from high-velocity ejecta abrasion damaging the lander to ejecta damaging other lunar landers or orbital assets, or even creating a crater under the lander as deep as the columnated engine plume. Small landers in the ~1-5mT range, such as those expected to fly on upcoming NASA Commercial Lunar Payload Service (CLPS) missions, are also at risk from ejecta plumes, albeit a smaller risk than the large landers. In all cases, scientific payloads could be damaged due to abrasion from high-velocity regolith ejecta.

     There are multiple schools of thought for mitigating lander plume effects. Ideas include choosing locations with more favorable surface conditions and controlling vehicle throttling and descent trajectories, but the solution that retires this risk long-term to establish a sustainable lunar presence is developing lunar infrastructure to land on. There are many approaches to landing pad design, some using in-situ resources and sintering regolith into a hardened surface, others involving bringing pad materials from Earth. These methods are reliant on multiple systems that are at low development phases and require one or more dedicated lunar missions to establish.

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     The Masten in-Flight Alumina Spray Technique (FAST) Landing Pad changes the approach to landing on planetary bodies by mitigating the landing plume effects by creating a landing pad under the lander as it descends onto a surface. This approach uses engineered particles injected into the rocket plume to build up a coating over the regolith at the landing location. The hardened regolith would have greater thermal resistance and ablation resistance to reduce regolith erosion rates and deep cratering. This innovation would enable large and small landers to safely perform transportation to any region on the Moon without major risks posed by engine plume effects. A lander could land in relatively close proximity to other surface assets without pre-existing infrastructure, which greatly expands potential landing locations and minimizes the need for pad construction missions.

     Using a traditional landing pad concept, vehicles would be constrained to land in specific lunar regions, which restricts the ability to gather science across the lunar domain. The FAST concept enhances overall lunar access and access to other planetary surfaces, including Mars, where loose regolith characteristics pose critical mission risks. The cost savings of this innovation is clear, for every dedicated lunar pad building or preparation logistics mission that does not need to be sent saves $120+ million. Providing mission assurance for crewed landings by mitigating deep cratering effects and reducing required shielding saves mass and cost for the Artemis program.

• 

Landers that generate their own pads as they touch down would mean faster, cheaper, and safer planetary exploration

      Planetary landings are a messy business that can be dangerous for anything nearby, but they can also be risky for the landers themselves. Engine plumes can kick dust, dirt, and rocks back up toward the spacecraft, endangering engines, science payloads (this happened to a weather instrument on the Curiosity Mars rover), and potentially even astronauts.

     We’ve managed so far because most unmanned probes, and even the Apollo lunar landers, have been light enough that their engine plumes have been relatively mild. But as we look toward scaling up our presence on the moon, we’re going to need rockets that are much, much bigger. NASA’s proposed Artemis landers will be somewhere between double and quadruple the mass of Apollo, and modeling suggests that one of these could displace something like 470 tons (!) of lunar soil during landing.

     Through NASA’s Innovative Advanced Concepts (NAIC) program, the space agency is funding a creative new approach toward making planetary landings safer for large spacecraft. Masten Space Systems is developing a concept for “Instant Landing Pads,” where a spacecraft builds its own landing pad as it descends toward the surface. By not requiring landing pads to be constructed in advance, this technique would be safer, cheaper, and help us establish a base on the moon as quickly and efficiently as possible.

     In all of the renderings of lunar landings that we’ve seen so far (from folks like Blue Origin, Boeing, SpaceX, and NASA itself), the landers themselves have touched down on the lunar surface directly, without a landing pad. It’s certainly possible to do this with large landers, if you’re choosy about where you land and add enough shielding to your vehicle for it to be able to withstand whatever it might kick up. And depending on the size and power of the engine (or engines) and the surface that they’re interacting with, this can be a lot of material—Masten has been doing some testing on Earth, and you can see how much of a problem this can be:

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     Finding the right landing location to avoid effects like this, and adding enough shielding to protect the lander, would be restrictive to any lunar program. Shielding is a significant amount of mass that takes away from payload, and if there’s no safe landing area near where you want your moon base to be, you’re out of luck.

     The conventional solution is to send smaller landers first, and use local material to construct a landing pad. A project called PISCES is working on doing this with robots, for example. This could certainly work, but you’re adding months or years of lead time to your overall mission, and Masten estimates that it would cost over $100 million for every dedicated lunar pad-building or preparation-logistics mission.

     What Masten wants to do instead is to put a pad, instantly, on a planetary surface underneath any rocket just a few seconds before landing.

     The system that Masten is developing with NIAC funding is called FAST, or in-Flight Alumina Spray Technique. Here’s how it would work: a few hundred meters above the surface of the moon (or Mars, or anywhere else you want to land), your lander comes to a hover. Alumina pellets are then fed into the engine exhaust nozzle, where they get partially melted in the engine plume and blasted down onto the surface. Most planetary surfaces that a spacecraft would be landing on are cold enough that the alumina cools and hardens on contact, and over the course of about 15 seconds, something like 300 kilograms of alumina gets layered into a totally functional landing pad. You then land as normal, ablating the pad a little bit but not digging a crater under yourself or blasting dirt and rocks all over the place.

     Masten has been testing rockets on Earth for years; its fleet of terrestrial test vehicles has accumulated more than 600 rocket-powered landings (on landing pads). This idea came directly out of testing how rocket engine plumes kick up material, Masten chief engineer Matthew Kuhns tells us. “I started brainstorming ideas around ways you could land without needing a precursor mission to create landing pads. Lots of crazy ideas later, this one stood out.”

     NIAC is all about funding ideas that seem crazy, but that have enough technical feasibility that they could ultimately pay off.

     “We will spend the next nine months looking at how this would benefit the Artemis moon landings,” Kuhns says. “NIAC projects as a rule are very ambitious and usually 10+ years out to use, but in this case since we can build on a terrestrial technology I think we can move a bit faster.”

     Masten will be teaming up with Honeybee Robotics to figure out exactly how engines can be modified to use FAST. FAST requires a system to transport the landing-pad material into the engine, which is basically the opposite of a pneumatic sampling system that Honeybee has been working on. Testing is still to come, says Kuhns, but with Masten’s rocket experience, we’re hoping that moving “a bit faster” is a bit of an understatement.

     We also asked Kuhns if he sees any problem with leaving these instant landing pads scattered about on the moon. “That would be a good problem to have,” he told us. “It would mean many many missions to the moon, a sustainable presence, and lots of science. Depending on their location and material, you could actually do science with the FAST landing pads and use them as laser or radio reflective arrays.”

     NIAC projects are typically funded through three different phases as their technology-readiness level increases. A year from now, we hope to see these Instant Landing Pads make it to phase two, which will bring them that much closer to helping us return to the moon.

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Apollo Dust Storm

     When Apollo 12 landed on the moon in November of 1969, Commander Pete Conrad managed to bring the lunar module down within about 182 meters of its target—the unmanned Surveyor III probe, which had landed there a few years earlier.

     Conrad was almost too precise, landing 152 meters closer to Surveyor III than the actual landing area that NASA was aiming for. NASA’s concern (and the reason for landing farther away) was that the lunar module’s descent stage engine might kick up a bunch of dust that would get all over the probe. As it turned out, and as the astronauts discovered when they made their way over to Surveyor III, it was in fact covered in dust.

     Later examination of some of the Surveyor 3 parts that the astronauts brought back revealed that the probe had been indirectly sandblasted as Apollo 12 landed, and that “had Surveyor III been exposed to the direct spray [of the landing lunar module], the damage would have been orders of magnitude higher.”

Two of the most difficult hurdles for human missions to Mars are the challenge of landing human-class (>20t) payloads on the surface, and extracting water from the environment both for human consumption and for fuel.

Beyond the problems of entry and descent, thruster-fired landings have the potential of damaging the landing site, scattering debris, and even excavating a crater that could swallow the lander. This could jeopardize both the landed infrastructure as well as the payload. Preparation of a landing surface resistant to thruster fire may be a critical pre-requisite for ensuring crew safety and mission success.

However, a mitigation for both these problems is advanced preparation of a landing site, as well as stockpiling of water resources, before human-class landers are sent. Preparation of a landing zone will require traversal of a large surface area, as well as a high energy process for hardening the surface. Extracting water from regolith is a high temperature process, with a high volume of regolith required to produce sufficient water. A single mission that performs these two processes while traversing the same terrain would retire two mission-critical risks: landing safely and demonstrating resource extraction.

This study modelled several options for hardening surface regolith while simultaneously extracting water content from the regolith. The energy requirements for extracting water from different grades of regolith were assessed, and preliminary attempts to model the in-place water extraction processes were undertaken. In addition, several methods for hardening regolith to prepare landing zones were analysed, including production of regolith tiles, solar sintering and microwave sintering. Two microwave concepts were studied in detail, and models created to assess their performance in the two mission goals: water extraction and regolith sintering. The result is that a precursor mission to Mars with a 5t rover with a 20-kW power supply could effectively prepare a safe 100m-radius landing zone for human-scale landers while stockpiling 4-6t water, which would be enough water resources to produce methane propellant and crew consumables for a 4-person 2-year mission.

1 Introduction

The entry, descent, and landing stage of a Mars mission is one of the most critical stages of a Mars mission. The largest payload that has successfully landed on Mars is the Curiosity rover, which is an order of magnitude smaller than the minimum required mass for human-class landers. Larger payloads will likely require thruster firings close to the surface, which raise the possibility of disturbing the terrain and creating a crater for the lander to fall into. Landing pad preparation requires shaping the terrain over a broad area, likely using high-energy processes, which is a similar mode of operation to resource extraction.

2 Problem statements

2.1 Disruption of regolith and potential cratering

According to recent research [1] on plume cratering effects caused by propulsive landing systems, there is significant risk in landing human mission assets on Mars (i.e. in the 20-40t range). Studies from Apollo landings on the moon, showed that lunar regolith and dust was blasted in a horizontal direction from the lander, causing damage to nearby instruments (the Surveyor III lander which was 160m from the Apollo 12 lander). So, effects on nearby extra-terrestrial base assets have long been known to be a problem. In addition, the dust raised from landings could cause a nuisance, especially to deployed solar panels. The standard mitigation strategy for this is to set the landing zone a safe distance from the base, and have 2m berms constructed between the base and the landing zone to mitigate horizontal ejecta. However, research in the last 10 years has shown that propulsive requirements near the surface for large landers on Mars could cause significant cratering. Experiments have been performed on rocket plumes as they near sand, to help understand the physics of the plume effects on surfaces. In addition, computational flow models have been developed to study the theoretical differences in the gravity regimes on the moon, Mars and Earth.

Researchers have discovered four mechanisms of soil movement caused by hot plume gases, in addition to the mechanical shock caused by the initial overpressure shockwave hitting the soil. These are viscous erosion (VE), bearing capacity failure (BCF), diffused gas eruption (DGE) and diffusion-driven flow (DDF). These are:

• BCF is the bulk shearing caused by the localized region of high pressure gas underneath the rocket plume. BCF drives soil perpendicular to the crater’s surface, the surface of which may change as the crater enlarges underneath the plume. This is the primary mechanism considered by previous NASA JPL studies to identify and mitigate risks for human Mars missions [2]. The study identified previous terrestrial tests and analysis estimating that Mars soil bearing capacity fails at 20 kPa, and this rough estimate is taken as sufficient criteria to eliminate the risk. However, the additional surface failure mechanisms below are less well-understood.
• VE is the effect of tangential gas above the soil, stripping away the top layer of particles.
• DGE is an effect of the gas driven into the soil, which might cause soil eruptions in another place.
• DDF is an additional distributed force on the soil, caused by the cumulative drag of the gas through the soil. DDF drives soil tangential to the crater’s surface, away from the tip of the plume.

On Mars, the authors conclude that a 40t spacecraft landing on Mars will fluidize and crater the soil. They are particularly concerned about the regolith being ejected in high trajectories to nearby base elements, perhaps even sufficiently vertically to strike the descending spacecraft. In addition, upon engine cut-off, the fluidized soil may collapse into a crater larger than the craft itself. Most mission planners at NASA seem to be ignoring this risk, indeed an abstract highlighting these risks was proposed to the 2012 Concepts and Approaches for Mars Exploration workshop at the Lunar and Planetary Institute, but was not accepted for presentation [3]. In this study, the authors highlight:

“The need for landing pads is the consensus of the plume/soil research community after a decade of concentrated research, including experiments, physics-bases simulations, and mission analysis. This finding has not yet been communicated widely outside the community of researchers…”

Therefore, it is a key requirement that techniques are developed to protect the surface from rocket plume damage, this would include protection from the four key mechanisms above. However, the researchers in this field believe that mission planners are likely to be sceptical of the need for these systems, and so the physics models need to be improved and validated for Mars environment and architectures. Without this work, it is unlikely that an exact determination can be made of the minimum requirements for soil improvement measures.

3.2 Landing zone preparation requirements

To mitigate the landing zone risks, several things are required:

• A hardened, level landing area for the final touchdown of the spacecraft. Levelling is particularly important in keeping the trajectory of ejected soil near the horizontal plane — since most movement is tangential to the surface where the exhaust plume strikes. Humps or divets in the landing area would provide some non-horizontal surfaces that would cause raised trajectories for ejected particles.
• A surrounding area where the dust is fixed, to reduce the dust raised by the horizontal movement of particles.
• Berms situated between the landing zone and the existing surface base elements, to catch the particles ejected tangentially.
• An exclusion zone between the landing zone and the base elements (possibly with roads prepared between sites to enable dust-free transport).

The size of each of these zones will depend on the transportation system and specifically the landing strategy and lander requirements. To summarize the requirements for a landing pad on the moon or Mars, one study assumed the lander requirements for an Altair-type lunar lander would amount to a 30m diameter hardened landing pad, surrounded by a 70m dust fixation zone. The study recommended a descent trajectory that minimized horizontal movement as high above the terrain as possible, and ending with a final descent as vertical as possible, to minimize the possible disturbance of elements on the ground. [5] Studies on Mars EDL systems tend not to hypothesize on the landing footprint accuracy, however, the EMC technology roadmap indicates that a landing accuracy of < 100 m for human missions is the desirable design goal. [6]

The required thickness of the landing surface is also ill-defined, since the effects of rocket plumes on loose aggregate soil in low-gravity and low-pressure environments is not well modelled. At a minimum, the surface needs to withstand the bearing capacity failure induced by the pressure from the rocket plume. Since the proposed Mars Descent Module (MDM) thrusters are 100 kN with a .8 m diameter [7], each produces a pressure of ~200 kPa, assuming all thrust is directed downwards. Therefore, the surface should be able to withstand this. However, as noted in Section 2.1, there are 3 other disruptive forces on the surface that need to be resisted: viscous erosion (VE), diffused gas eruption (DGE) and diffusion-driven flow (DDF). Since viscous erosion is the disruption of particles on the surface, it should be mitigated by sintering, which will melt loose particles into an aggregate. The other two disruptive forces are caused by diffusion of gas between soil particles, which should also be drastically reduced by the reduction/elimination of gaps between particles in the sintered aggregate. To select the minimum thickness for our model, we will rely on experimental results of different sintering methods, to find a tested surface that meets the minimum capacity. As a minimum baseline for our model, therefore, we will use 5 cm, which is the thickest test result that produced the strongest material. For more details, see the technologies and experimental test results are reviewed in the next section.

(ed note: The unmanned rover is constantly moving over the landing pad area. The leading edge heats a strip of regolith to 575K {sublimation point}. As it moves over the hot strip, rover captures the escaped gases in a cold trap. As the trailing edge moves over the hot strip it heats it to 1400K {sintering point}, making it suitable for a landing pad. So rover is constantly sublimating, capturing, and sintering.)

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• 

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4.2 Model description and calculations

The model uses simple geometry to determine a spiral path of a rover to lay a sintered regolith area in 2 concentric circles. The inner circle is the hardened landing zone (HLZ), which is characterized by a thicker layer of sintered regolith to resist the landing thrusters of the Mars Descent Module. The outer circle is the dust mitigation zone (DMZ), consisting of a thin layer of sintered regolith to reduce the dust scattering from the horizontally-deflected gases of the engine plumes. The required sintered volume of each zone is calculated, and this proportion is used to assign the available mission time to each zone, as a rudimentary way of balancing the power requirements – the overall volume to be sintered corresponds to the energy output for each zone. The time available for each zone is subdivided into individual “steps” for the rover, each of which volatilizes and sinters a certain small footprint of area under the rover. The energy requirements for each step is determined based on the step size, depth required, and the regolith properties. Given the time allocated for each step, the necessary power can be calculated.

(ed note: lots of equations and minutiae omitted. See paper for details)

The Excel solver was used to optimize the following input parameters, while maximizing the water captured, given the corresponding constraints:

• The HLZ sinter depth: must be between 5-10 cm.
• The DMZ sinter depth: must be between 2 mm - 5 cm.
• The HLZ diameter must be greater than 30m and less than the DMZ diameter (which was fixed at 200m)

• 

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[1] P. Metzger, X. Li, C. Immer, and J. Lane, “ISRU implications for lunar and martian plume effects,” in 47th AIAA Aerospace Sciences Meeting including The New Horizons Forum and Aerospace Exposition, 2009, p. 1204.

[2] D. W. Beaty et al., “An analysis of the precursor measurements of Mars needed to reduce the risk of the first human missions to Mars.” unpublished white paper, 2005.

[3] P. Metzger, P. Hintze, and R. Mueller, “Precursor Activities To Solve Plume Cratering Problems For Human-Class Mars Landers,” presented at the Concepts and Approaches for Mars Exploration, Houston, Texas, 2012, vol. 4359.

[5] B. Khoshnevis, A. Carlson, and M. Thangavelu, “ISRU-Based Robotic Construction Technologies For Lunar And Martian Infrastructures,” Apr. 2017.

[6] D. A. Craig, P. Troutman, and N. Herrmann, “Pioneering Space Through the Evolvable Mars Campaign,” 2015.

[7] B. G. W. K. D. Drake, “Human Exploration of Mars Design Reference Architecture 5.0, Addendum #2,” Mar. 2014.

[1] Exhaust power. Say we have a gas core rocket with an exhaust velocity of 50,000 m/s and 1,000,000 newtons of thrust. We've got 25,000,000,000 watts on our hands (25 gigawatts). Say that the exhaust is concentrated mainly in a 10 meter diameter circle on the final descent. The area is about 80 square meters, for a heat flux of roughly 300 megawatts per square meter. Which will vaporize the surface layer of about anything. The question becomes: how deep?

[2] Heat radiation. We're talking about very high temperatures here, so maybe the surface might radiate heat away faster than the exhaust deposits it? If nothing else, it will give a rough upper limit to the temperatures involved. The 300 MW/m² flux correspond roughly to black-body radiation at 8500 K. So the temperatures may well be of that order of magnitude.

[3] Heat Conduction. Let's say the surface gets really hot. How fast is the heat conducted below the surface? Many rocks seem to have conductivities on the order of 2 W/m-K or so. At that rate, the temperature gradient would have to be incredibly high to balance the influx; 150 MK/m. I suspect that indicates that the surface would probably ablate to vapour and escape, with only the thinnest layer liquid at any given time. Maybe a micrometre or so on average.

[4] Quantity vaporized. As a rough guideline, many rocks at normal temperatures have heat capacities of about 1 kJ/kg-K. Extrapolating wildly, I could assume that they don't vary by too much at higher temperatures. If their temperature is raised on the order of 7000 K, then they will carry off something like 7 MJ/kg. Now, we can probably assume a density of about 3000 kg/m³, so that's 21 GJ/m³. If the final part of the landing takes about 20 seconds, that's up to about 23 m³ of rock. Dividing by the assumed area, we get a very shallow crater about 30 cm deep (about a foot).

So, based on the above figures I'd guess that a plume of exhaust like that would erode the surface, breaking it down into vapour or even decomposing it, with the resulting gases dissipating rapidly. With heat conduction so low compared to the incoming energy, there simply won't be enough time for a pool of lava to form. Any liquid would be blasted outward by superhot gases, vaporizing and then recondensing along the way.

My guess is that you'd get a broad but shallow crater, surrounded for quite some distance by ash condensed from the escaping gases and of course any debris that might have been carried along for the ride.

• 

     "Nice landing, Skipper!" called out Oscar.
     "Yea bo!" agreed Tex.
     "Thanks, fellows. Well, let's get the stilts down." He punched a stud on the control board. Like most rockets built for jet landings, the jeep was fitted with three stabilizing jacks which came telescoping out of the craft's sides and slanting downward. Hydraulic pressure forced them down until they touched something solid enough to hold them, whereupon the thrusting force was automatically cut off and they locked in place, propping the rocket on three sides, tripod fashion, and holding it erect.
     Thurlow waited until three little green lights appeared under the stud controlling the stilts, then unclutched the jeep's stabilizing gyros. The jeep held steady, he unstrapped. "All right, men. Let's take a look. Matt and Tex, stay inside. Oscar, if you don't mind my mentioning it, since it's your home town, you should do the honors."
     "Right!" Oscar unstrapped and hurried to the lock. There was no need to check the air, since Venus is man-inhabited, and all of them, as members of the Patrol, had been immunized to the virulent Venerian fungi.
     Thurlow crowded close behind him. Matt unstrapped and came down to sit by Tex in the passenger rest Oscar had left. The space around the lock was too limited in the little craft to make it worthwhile to do anything but wait.
     Oscar stared out into the mist. "Well, how does it feel to be home?" asked Thurlow.
     "Swell! What a beautiful, beautiful day!"
     Thurlow smiled at Oscar's back and said, "Let's get the ladder down and see where we are." The access door was more than fifty feet above the jeep's fins, with no convenient loading elevator.
     "Okay." Oscar turned and squeezed past Thurlow. The jeep settled suddenly on the side away from the door, seemed to catch itself, then started to fall over with increasing speed.
     "The gyros!" yelled Thurlow. "Matt, clutch the gyros!" He tried to scramble past Oscar; they fouled each other, then the two fell sprawling backwards as the jeep toppled over.
     At the pilot's yell Matt tried to comply — but he had been sprawled out, relaxing. He grabbed the sides of the rest, trying to force himself up and back to the control station, but the rest tilted backwards; he found himself "skinning the cat" out of it, and then was resting on the side of the craft, which was now horizontal.

(ed note: re 'skinning the cat', "In America, as any country boy knows, this means to hang by the hands from a branch or bar, draw the legs up through the arms and over the branch, and pull oneself up into a sitting position." -- "A Hog on Ice" by Charles Earle Funk)

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     A quick look around was enough to confirm Oscar's prediction; the jeep lay on her side with her fins barely touching solid ground. The nose was lower than the tail and sinking in thin, yellow mud. The mud stretched away into the mist, like a flat field, its surface carpeted with a greenish-yellow fungus except for a small space adjacent to the ship where the ship, in failing, had splashed a gap in the surface.
     Matt had no time to take the scene in; the mud was almost up to the door.

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     "Quit talking and get busy." Oscar trotted the length of the craft, taking the end of the line with him. He made it to the bank by stepping from a tail fin.
     Matt and Tex had no trouble carrying Thurlow as far as the fins, but the last few feet, from fins to bank, were awkward. They had to work close to the jet tube, still sizzling hot, and balance themselves in a trough formed by a fin and the converging side of the ship. They finally made it by letting Oscar take most of the lieutenant's weight by hauling from the bank with his one good arm,
     When they had gotten Thurlow laid out on the turf Matt jumped back aboard the jeep. Oscar shouted at him. "Hey, Matt — where do you think you're going?"
     "Back inside."
     "Don't do it. Come back here." Matt hesitated, Oscar added, "That's an order, Matt."
     Matt answered, "I'll only be a minute. We've got no weapons and no survival kits. I'll duck in and toss them out."
     "Don't try it." Matt stood still a moment, balanced between Oscar's unquestioned seniority and the novelty of taking direct orders from his roommate. "Look at the door, Matt," Oscar added. "You'd be trapped."
     Matt looked. The far end of the door was already in the mud and a steady stream was slopping into the ship, like molasses. As he looked the jeep rolled about a quarter turn, seeking a new stability. Matt made it to the bank in one flying leap.

The Nerva-K behaved perfectly. We hovered for several minutes to melt our way through various layers of frozen gases and get ourselves something solid to land on. Condensing volatiles steamed around us and boiled below, so that we settled in a soft white glow of fog lit by the hydrogen flame.

Black wet ground appeared below the curve of the landing skirt. I let the ship drop carefully, carefully … and we touched.

It took us an hour to check the ship and get ready to go outside.

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I was screwing down my helmet when Jerome started shouting obscenities into the helmet mike. I cut the checklist short and followed him out.

One look told it all.

The black wet dirt beneath our landing skirt had been dirty ice, water ice mixed haphazardly with lighter gases and ordinary rock. The heat draining out of the Nerva jet had melted that ice. The rocks within the ice had sunk, and so had the landing vehicle, so that when the water froze again it was halfway up the hull. Our landing craft was sunk solid in the ice.

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We did have one chance. The landing vehicle was designed to move about on Pluto’s surface; and so she had a skirt instead of landing jacks. Half a gravity of thrust would have given us a ground effect, safer and cheaper than using the ship like a ballistic missile. The landing skirt must have trapped gas underneath when the ship sank, leaving the Nerva-K engine in a bubble cavity.

We could melt our way out.

I know we were as careful as two terrified men could be. The heat rose in the Nerva-K, agonizingly slow. In flight there would have been a coolant effect as cold hydrogen fuel ran through the pile. We couldn’t use that. But the environment of the motor was terribly cold. The two factors might compensate, or—Suddenly dials went wild. Something had cracked from the savage temperature differential. Jerome used the damper rods without effect. Maybe they’d melted. Maybe wiring had cracked, or resistors had become superconductors in the cold. Maybe the pile—but it doesn’t matter now.

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After the fiasco with the Nerva-K, one of us had to go down and see how much damage had been done. That meant tunneling down with the flame of a jet backpack, then crawling under the landing skirt. We didn’t talk about the implications. We were probably dead. The man who went down into the bubble cavity was even more probably dead; but what of it? Dead is dead.

I feel no guilt. I’d have gone myself if I’d lost the toss.

The Nerva-K had spewed fused bits of the fission pile all over the bubble cavity. We were trapped for good. Rather, I was trapped, and Jerome was dead. The bubble cavity was a hell of radiation.