Ask anybody taking airplane piloting lessons and they'll tell you that taking off is easy, the incredibly hard part is landing. At least landing safely, any fool can land by augering in. Or what Rob Davidoff calls "lithobraking into a low-altitude synchronous orbit."
Naturally this is an order of magnitude harder when doing a tailsitting rocket (Vertical takeoff, vertical landing or VTVL) landing, which is basically a controlled crash. Try playing a few games of Lunar Lander to get a feel for it.
The poor man's way of landing on a planet with an atmosphere is by utilizing aerobraking and aerocapture. Pretty much all of NASA's manned rockets use this method. What you do is equip your spacecraft with a streamlined heat shield (technical term is "aeroshell"), and use air friction to eliminate your deltaV. Hopefully you can reduce the deltaV to zero before you run out of either heat shield (i.e., "burning up in re-entry") or altitude (i.e., "auger in").
The advantage is that it allows landing without requiring a powerful engine (which is a problem with tiny landing boats or inhabitants with strict laws about nuclear radiation) and the elimination of quite a bit of required reaction mass.
The disadvantage is there is a limit to the deltaV that can be shed, your trajectory has to be incredibly on course, and only very few planets and moons in our solar system have atmospheres. Not to mention the fact that most heat shields have to be replaced after each use, which was one of the major drawbacks of the Space Shuttle.
The general rule is that aerobraking can kill a velocity approximately equal to the escape velocity of the planet where the aerobraking is performed (10 km/s for Venus, 11 km/s for Terra, 5 km/s for Mars, 60 km/s for Jupiter).
Robert Zubrin says mass of the heat shield and thermal structure will be about 15% of the total mass being braked. Which sounds like a lot, but is often much less that the reaction mass required to brake with rocket thrust.
The final bit of landing is done by parachute or belly-landing with glider wings like the Space Shuttle. The trouble with a parachute is it drastically limits the size of the spacecraft. The trouble with glider wings is they really eat up your ship's mass budget, and are just dead weight for most of the mission.
The deltaV limit is due to a couple of factors. The faster you shed deltaV, the more heat the heat shield will have to cope with, and there is a limit to the heat shield's ability to cope. There is also a limit to the amount of atmosphere you can pass through with a given trajectory, but it is possible to plot clever paths that loop back and pass through the atmosphere repeatably.
Your trajectory has to be dead on course. If you are too steep, the generated heat will cause heat shield failure. If you are too shallow, you will ricochet off the atmosphere on a one way trip into the big dark.
Terrestrial planets with atmospheres include Venus, Earth, Mars, Titan, and maybe Pluto. All the gas giants have atmospheres, so much in fact that the pressure will eventually implode your ship. As a side note, aerobraking can be used with gas giants in order to change one's trajectory instead of landing. This was done in the movie 2010, The Year We Make Contact, where they used a ballute as a heat shield.
Aerobraking is the reason that the planet Mercury is the most expensive terrestrial planet to soft-land on, in terms of delta V. All the other planets either have lower gravity or have an atmosphere suitable for aerobraking.
Since there is lots of velocity to get rid of, it turns into lots of heat. A whole lotta heat. 2760 degrees Celsius of heat. We are talking more than enough heat to melt metal.
But the heat shield is build out of materials that can take it. Mostly because the shield ablades away, which puts a limit on how much velocity can be removed before the heat shield is all gone.
However, there is another consideration. Consider a Roman Legionnaire, armed with a sword and a mightly scutum (shield). A smelly barbarian can flail away at your scutum without doing you any harm. Consider though, how well the scutum will protect you if it had a big hole in the middle. The barbarian can stick their sword through the hole and into your heart, sending your ghost into the Elysian Fields. A scutum with a hole in it is worthless.
The same goes for a heat shield with a hole in it. A plume of incandescent atmosphere at 2760°C will shoot through the hole and burn a hole in the hull. Your ship will do an impromptu impression of the Space Shuttle Columbia disaster.
Now NASA's space shuttle did have temporary holes for the shuttle's retractable landing gear, but this had to be precisely engineered. Even then they were weak spots.
For other spacecraft, they would go to extreme lengths to avoid putting holes in the heat shield.
Apollo Command and Service Module
The Apollo Command and Service module assembly presented a problem.
The gumdrop shaped Command module had a heat shield on its base. The Command module was stacked on top of the Service module. So the heat shield separates teh two modules. The problem is that the Service module supplies the Command module with electrical power, air, water, and coolant. How do you get these vital fluids from one module to the other without poking dangerous holes in the heat shield?
Easy, you make a detour by using umbilical cables:
At the end of its mission, the Command module separates from the Service module. The Command module aerobrakes with its heat shield, delivering the astronauts safely to the landing zone. The Service module burns up in reentry and any remaining fragments fall into the Pacific ocean.
Before the two modules can separate, the umbilical must be cut.
Now, the Command module and Service module are rather firmly bolted together by three rods of solid stainless steel whimsically called "tension tie straps." Because it would be catastrophic if the two modules separated before the scheduled time. They are cut by explosive shaped charges The tie straps are about 2-½ inches wide and 4 inches long. The upper end of each tie strap is bolted to the Command module and the lower end bolted to the Service module.
Oh noes! The tie straps actually penetrate the heat shield! Why isn't this a problem? Let me explain
The blazing heat of reentry would ordinarily melt the steel tie strap. However, since the straps are still attached to the rear bulkhead of the entire freaking Command module, the module acts as a heat sink. This wicks away the heat so the tie straps do not get hot enough to melt. The bulkhead does rise in temperature. But since the heat in the grams of straps is spread out in metric tons of bulkhead, the temperature rise is endurable.
So, why didn't the designers make the entire heat shield out of stainless steel? Well, for starters that would increase the heat shield mass by several orders of magnitude and reduce the payload budget to negative numbers. But secondly since every square centimeter would be expose to enough thermal energy to melt, it would be the functional equivalent of having no heat shield at all. Meaning the Command module (and crew) would burn up in reentry and any remaining fragments would fall into the Pacific ocean.
There is some current research into magneto-hydrodynamic force fields as heat shields.
The advantage is you do not have to replace a physical burnt-out heat shield (like the Space Shuttle required), since it is composed of renewable force fields instead of matter.
Disadvantages include the fact that they require electrical power, and that they only work at large velocities. However the latter drawback is not as bad as it seeems. A MHD heat shield can reduce the spacecraft's velocity to the point where a ceramic heat shield can manage the rest of the landing. And ceramic shields do not have to be replaced after each landing.
You can read more about it here.
As mentioned before landing a tailsitting rocket (Vertical takeoff, vertical landing or VTVL) requires lots of training, since the pilot is basically performing a controlled crash. As you can discover for yourself by playing a few games of Lunar Lander.
So the problem of landing is to ensure that your downward velocity and altitude above the landing pad both reach zero simultaneously. The Apollo Lunar Module burned enough to come to a stop at some distance above the ground, and hover. Then it gradually throttled down the engine, which gently lowered the spacecraft to the Lunar surface. For all the Apollo landings this was done manually by a human pilot.
The SpaceX Falcon 9 first stage cannot do that. The problem is at that point in the flight the first stage cannot hover. Not under one Terran gravity at any rate.
When the first stage is trying to land, almost all the fuel is gone, so its mass is minuscule. Even if it turns off eight of the nine engines, and throttles the remaining engine down to minimum (50% of full thrust), the acceleration is still greater than one gee (thrust-to-weight ratio is greater than one). With one engine at minimum the stage will go skyward, with all engines off it will plummet to the ground, but it cannot just hover at a given altitude.
Faced with this problem, the ingenious engineers at SpaceX came up with an innovative solution: the Hover-Slam (unofficial name, SpaceX has not revealed what the official term is). Which player of the game Kerbal Space Program call the Suicide Burn. Robert Heinlein compares it to catching a falling egg on a plate without breaking it.
A sophisticated computer program precisely calculates thrust and altitude to smoothly bring the spacecraft to zero downward velocity on the ground, not hovering at a low altitude. In a single burn. And taking into account the high minimum thrust. Remember that for landing the stage starts with a velocity of four times the speed of sound (about 1,400 m/s).
If you start the burn too late, you crash (you'd reach zero altitude before zero velocity). If you start the burn too soon, you eventually go back upwards and then crash (you'd reach zero velocity before zero altitude, and start to ascend). You have to start it exactly on time, the Falcon 9 landing jacks can only handle a landing jolt up to six meters per second.
The horizontal vectors also need to be very small, or you will skid on the landing pad and tip over. The ship need to be vertical or it will tip over. Keeping in mind that tipping over means the stage explodes (what SpaceX calls Rapid Unscheduled Disassembly or RUD) since the fuel tank walls are necessarily flimsy and the tanks still contains dangerous amounts of fuel.
And you have to hit within a couple of meters the center of the landing pad.
Landing like this probably almost impossible for a human pilot to do (hence "suicide burn"), but SpaceX doesn't care: it uses computer to control everything.
The cherry on the top of the hover-slam sundae is the fact that it is far more fuel efficient than a hover-and-lower. Which always brings a smile to the face of the bean-counters in the accounting department. And more hand-fulls of hair yanked out by your competitors due to their despair at competing with you.
Landing a tail-sitter is a problem. It is almost impossible for the pilot to see the landing site because the exhaust is in the way. At least when the ship is close enough to the ground that the exhaust plume starts blowing dirt around. The Apollo Lunar Module had special outward angled windows to let the pilot see the landing point, while the ship was high enough. When the LEM got too low, the pilot relied upon the landing sensors.
In the Three-man Space Scout note the Ground Reflecting Periscope Mirrors. The three transparent blisters on the flight deck help the pilot to land by providing full ground visibility via a system of reflecting mirrors.
As previously mentioned, when the ship is very close to the ground, the rocket exhaust blows dirt around such that the pilot cannot see anything. So the lunar module had contact probes on the landing pads. These were about 1.5 meters long. When the lunar module was low enough that the probes actually touched the surface, the contact lights on the pilot's control panel would light up. The probes were like a visually impaired person's white cane
Originally there were four contact probes, but later NASA removed the probe on the leg with the ladder. It seemed like a bad idea to send an astronaut in an inflated puncture-prone space suit down a ladder where awaits a pointy contact probe been bent upwards like a spear covered in gold foil.
A contact probe (called a "touch down shoe") was also used in the lunar ships featured in Collier's Man Will Conquer Space Soon! series. Which isn't surprising since Wernher von Braun had a hand in creating both.
The spacecraft in First Men to the Moon by Wernher von Braun had a sort of landing sensor, but I'm not sure I understand how the thing functioned.
The way I am reading it, the spike has to be capable of retracting all the way back into the body of the spacecraft. Otherwise, if the lunar surface is too hard, the spike will be unable to penetrate and the ship will topple over.
My guess is that the spike helps the ship cope if the lunar surface has the firmness of one hundred feet of corn-flakes cereal. The spike will imbed itself into fluffy soil and help prevent the ship from falling over. Otherwise the ship will either slowly sink into the ground, never to be seen again; or uneven settling of the individual landing legs will allow the ship to topple.
Presumably after Luna had been explored a bit, the spike could be omitted.
As a side note, there is a good reason for having either three landing legs/fins or adjustable landing legs. Have you ever had to sit on a stool or chair with four legs, and one of the legs was shorter than the other? You sort of rock back and forth. This is annoying for a seated person, but can be disastrous for a sixty meter tall rocketship. In Andre Norton's space novels, the height of a pilot's skill was to make a "perfect three-fin landing".
The reason for the rocking is that three points automatically determine a plane, but four or more points are not guaranteed to. If you have four or more, the landing jacks had better be adjustable, or you will only be able to land on a perfectly flat surface. Don't even think about a landing on a random spot on the rocky planes of Luna.
Of course there is an argument for four or more landing legs. Imagine you are looking at the rocket from overhead. Draw dotted lines from the foot pad of each leg to the adjacent pads. With three legs you'll have a triangle. The key point is that if from your overhead view the rocket's center of gravity moves outside of the triangle, the rocket will topple over and crash.
This can happen if you have the misfortune to be landing on a slope, and a single pad touches down first on the up slope section. As the other two pads lower, the already landed pad will force the rocket off vertical until it topples. The rocket can also tip a bit if it is moving a bit sideways as it comes down. The two pads bracketing the sideways direction can dig in and stop while the nose of the rocket is still moving, causing a tip.
The advantage of four pads is that now you have the dotted lines forming a square. This increases the distance the center of gravity has to move in order to topple, which increases your safely margin.
If you go with more than three landing pads, just make the landing legs adjustable in length to deal with the "rocking stool" problem.
Mike Williams points out the extra mass problem of four landing pads:
But if safety is primary, Bernard Peek notes that current European safety legislation requires office chairs to have five castors so that losing one is not a catastrophic failure.
Sean Willard brought to my attention an important mathematical proof. It has been proven that if:
- you have a four legged structure
- all the legs are exactly the same length
- the ground is a continuous surface with a local slope of no more than fifteen degrees
- you rotate the structure around the plane of the feet
Please note that the non-rocking position is not guaranteed to be level. If you want the intricate details, you will find them in this paper.
Sean goes on to say:
While landed, for extra safety, keep the control moment gyroscope clutched and powered up. This will help keep the ship from toppling.
However, what if you do have a catastrophic failure and your rocket topples over? If you are at a civilized spaceport, as you float in the burn-recovery medical tank, you can console yourself with the thought that the port will probably have the equipment required to restore your ship to an upright position. Provided, of course, that the ship didn't break its spine and that you or your insurance can cover the cost.
If your ship is a deep space explorer on an uncharted planet, and you have no communication system with which to yell for help, then you have a problem.
WILDERNESS SHIP RAISING
So say you are on an uninhabited planet far from the benefits of civilization with no way to call for help. If the ship isn't too terribly huge, Isaac Kuo suggests that it might be possible to construct a sort of gigantic A-frame and raise the ship with cables. Isaac suggests attaching cables to the ship's nose and hoisting it vertical. The ship's landing jacks are repaired, if necessary, then the ship is lowered to stand on its own three feet.
Eric Tolle points out that care must be taken not to drag the ship's tail. He makes the brilliant suggestion that studying the engineering associated with Egyptian Obelisks would provide answers! In many ways it is the same problem.
Back in 1586 engineers lowered, moved, and erected the Vatican Obelisk (because it wasn't in the aesthetically perfect spot). The Obelisk is about 25 meters tall and weighs 330 tons. This isn't much smaller than the Polaris I worked out in the example. That was about 43 meters tall and massed 378 metric tons.
The Obelisk required about 140 horses and a year of work, but I'm sure things will go quicker with modern machinery and engineering (however, extra time will be required if the natives are shooting at you). The girders for the A-frame could be a standard feature with wilderness spacecraft, perhaps stored in the ship's core and extracted from the nose. Storing the girders as removable parts of the hull may make it easier to access, but they might be damaged in the initial topple.
Isaac says that avoiding tail dragging can be done by anchoring the tail with cables. But a better solution might be raising the ship by cables attached to the midpoint, instead of the nose. The ship is raised entirely free of the ground, then a cable attached to the tail is pulled to pivot the ship into proper nose-upward orientation. You might be able to get away with an A-frame only half the height of the ship.
Garon Whited points out some of the trade-offs:
If that fails, Eric suggest rolling the ship into a nearby lake or ocean and hoping that the ship floats with the nose uppermost. He notes that this will probably terminate the ship's warranty with extreme prejudice. One can hope that the heavy nuclear propulsion system will make the ship tail-heavy. However, Isaac points out that the huge propellant tank will tend to make the ship float sideways, with the propulsion system providing little or no tipping, much like the outboard motor on a speedboat.
The "uneven landing site" problem is so daunting that it is tempting for a science fiction author to invent incredibly high tech solutions that are unobtainium at best and technobabble at worse. Much like David Drake did in his classic series of novels about Hammer's Slammers. Mr. Drake noted the many problems of using caterpillar tracks for armored fighting vehicles. His solution was to make the tanks into hovercraft, using ducted fans so that they could float over irregular ground. All you need to make it work is to equip each tank with a fusion power source capable of supplying the electricity needs of California. Mr. Drake did analyze and accommodate the logical consequences of the tanks possessing so much electrical generation capacity, so this is a case of an author doing the job right.
I will note in passing the jaw-dropping stupidity of the landing legs on the starship Voyager. Not only would they not work, Star Trek starships as a general rule never have a need to land anyway. Shuttlecraft and transporters are a much more efficient solution. The Voyager is not designed to land, it probably cannot support its own structure without technobabble force fields reinforcing the internal girders, the same goes for the ludicrously tiny landing gear, seven hundred thousand metric tons concentrated on those tiny foot pads will poke holes in solid bedrock, once landed the Voyager is suddenly vulnerable to all ground-based hazards, the list goes on and on. About the only reason for landing is to make flashy eye-candy images for the audience in a desperate attempt to prop up the TV ratings.
The standard science fiction gag is to support the spacecraft on the ground using technobabble "force fields" or "pressor/repulsor beams". These are fields or beams of as-yet undiscovered energy that are perfectly adjustable, capable of allowing for any ground level irregularities. They are also also somehow much stronger than steel girders or anything else composed of matter. Their main drawback is that they consume energy, and steel girders do not vanish if the energy becomes exhausted.
However, from the standpoint of basic physics it would imply that any hapless person who walked through one of the beams would be flattened into a thin layer of bloody goo under a crushing force of N where N is equal to the weight of the spacecraft divided by the number of landing leg beams.
In Isaac Asimov's The Currents Of Space, the ships are equipped with an unobtainium "diamagnetic field", which allows the ships to float out to the launch pad. Diamagnetism is when a substance is repelled by both poles of a magnet, but it is unclear if such levitation is possible by the laws of physics.
At a minimum, a landing beacon emits a radio signal that allows the pilot to home in on the landing site, and makes the implicit promise that the landing site is more-or-less flat and not full of bolder/quicksand/pools of bubbling lava/ or other landing hazards. Beacons on boondocks pioneer planets do not provide any more than this, the pilot has to be responsible for avoiding midair collisions with other spacecraft. Beacons at actual starports provide the pilot with all sorts of telemetry (approach rates, wind velocity and direction, local weather conditions, etc). If the starport is handling more than one landing per day, it probably also has a full blown space traffic controller that will assign you approach lanes and scream at you if you stray outside. Massive spaceports at highly developed colonies might not let you land at all, instead you have to dock your ship at the orbital highport and rent a registered and bonded local interface shuttle to transport your crew and cargo to the surface.
If you are trying to land somewhere real primitive (like to rescue some castaways on an otherwise uninhabited planet), you might have to be satisfied with landing beacons in the form of wood bonfires, maybe a few signal flares if you are real lucky.
First-in scouts landing on unexplored planets have no beacons. They must do extensive scans from orbit, try to pick a safe landing site, and take her down by the seat of their pants. This is why NASA and other space agencies spend months on choosing safe landing sites.
And even then Neil Armstrong got a rude surprise when the "perfectly smooth" landing site the Lunar Module was heading for turned out to be full of bolders the size of trucks. The bolders were below the resolution of the Lunar Orbiters that did the mapping. Neil had to go to manual control to avoid crashing.
This is why first-in scouts may have to recover their spacecraft from a topple.
If the piece of wilderness planetary surface that you are attempting to land on is composed of sand, fine soil, regolith, or similar material and your landing rocket thrust is really huge you may have a big problem. The rocket exhaust might create an instant crater. If it is large enough, it could undermine one or more spots reserved for the spacecraft's landing legs resulting in the ship toppling over. If you are really out of luck, it will be big enough to swallow the spacecraft entirely.
This is one of the reasons why first-in scouts have such generous salaries. And why it is impossible for them to buy life-insurance.
The Hercules Single-Stage Reusable rocket lands using thrusters at the upper end of the spacecraft (a "tractor", "Space Truck" or "waterskiing" arrangment), canted 30° outboard from vertical (cosine thrust loss reduces thrust to 87%). This avoids digging a crater.
In addition, lunar regolith is noted for being hideously abrasive (think: microscopic razor blades) and will contain larger rocks. Any of this propelled upwards by the exhaust plume can damage the landing spacecraft. It also can sand-blast and perforate equipment already on the ground that has been stored too near the landing site.
If the plan is to set up a planetary base or something, they would be well-advised to have the first-in scouts look for a nice nearby piece of flat planetary bedrock, resistant enough to widthstand multiple landings and blast-offs. Or supply them with quick-setting concrete or something else suitable to make a landing pad. This is called "avoiding Surface/Plume interaction".
Later if a permanent spaceport is desired on the planet or moon, a major construction effort will be made to create a permanent landing field, with blast deflectors and everything.
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?
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
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.
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)
- Pavers with grouting
- High temperature polymer infusion in soil
- Rock welded pavers
- Bring sheet material from Earth
Outer methods (low temp, resist scouring erosion)
- 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
- Mass brought from Earth;
- Energy required (high energy systems cannot be landed until after the landing pads are built);
- Construction time;
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.
Most of the potent engines have exhausts measured in thousands of degrees. What about landing? We don't want our ships to touch down on a new planet only to immediately sink out of sight in a self-created sea of lava.
(We will ignore that unpleasant little man in the front row who is asking loudly: why don't we keep the rocket in orbit and land with the small shuttle we carried along?)
As a side note, the SpaceX Falcon 9 has retractable landing legs. This is because while landing the white-hot exhaust is blown upward around the rocket base. If the legs were extended into that blowtorch they would be incinerated. Instead they deploy at the very last minute. Well, actually the other reason to deploy at the last minute is the legs interfere with using the grid fins to steer the landing.
As a wild guess, the exhaust temperature is approximately 100 K * Ve2. So an exhaust velocity of 20,000 m/s is on the order of 40,000 K.
The particle energy(approximately the temperature inside the engine) is
Ae = (0.5 * Am * Av2) / B
- Ae = particle energy (Kelvin)
- Am = mass of particle (g) (1.6733e-24 grams for monatomic hydrogen)
- Av = exhaust velocity (cm/s)
- B = Boltzmann's constant: 1.38e-16 (erg K-1)
But more to the point is total plume energy.
Fp = (F * Ve) / 2
- Fp = thrust power (watts)
- F = thrust (newtons)
- Ve = exhaust velocity (m/s)
(Note that exhaust velocity is in two different units in the two equations)
Timothy's masterful analysis does have some alarming consequences. Considering that we might be using a gas core atomic rocket, this means when it lands it becomes a lean, mean, fallout machine.
Belly landers for use on planets with an atmosphere typically land much like aircraft, on long runways. A standard aircraft runway is from 1,829 to 2,438 meters long, longer will handle wide-body aircraft. The Shuttle Landing Facility at the Kennedy Space Center in Florida was a whopping 4,600 meters. Of course the Shuttle is a bit bigger than your average aircraft, and comes in with a totally dead-stick landing.
Belly landers for use on airless planets will probably land vertically on rockets like a tailsitter. Except the rockets will be in the ship's belly instead of the tail. Just like a Space 1999 Eagle Transporter doing a touch-down.
Since the major reason to use a belly lander is to simplify loading and unloading cargo, a spaceport built to accommodate belly landers will probably have lots of cargo warehouses.
For a belly lander the pilot will need similar cockpit windows to a conventional aircraft IF THEY ARE LANDING AT A SPACEPORT WITH A RUNWAY AND AN AIR TRAFFIC CONTROL SYSTEM. If they are landing in a wilderness enviroment in an unknown patch of ground that sort of looked flat from orbital surveillance, more windows/cameras will be needed. Suggestions include a droop-nose like the Concorde Supersonic Transport and helicopter chin bubbles. Both allow the pilot a better view of the irregular makeshift runway.
If the bellylander is a vertical take-off and landing vehicle, it might only need the chin bubbles. If you were building a NASA type vehicle where you couldn't afford the mass penalty of upper windows.
United Launch Alliance lunar lander
Belly landing also made sense for the LUNOX proposal. As did windows facing downwards ("chin bubbles"). Since this design is a vertical take-off and landing (VTOL), the chin bubbles are the only windows it has or needs.
Multi-Mission Space Exploration Vehicle
Eagle Transporter from Space 1999
In Heinlein's Time For The Stars, the torchship Lewis & Clark avoids both the "uneven landing site" problem and the "vaporizing the landing site" problem by the simple expedient of only landing in oceans. This is also true of the landing shuttles carried by starships in Jerry Pournelle's CoDominion universe. Alas, this won't work very well for a non-interstellar spacecraft, as the only planet in the solar system with an ocean is Terra (with the possible exception of a methane ocean on Titan. Some of Jupiter's moons may have oceans of water, but these are covered by many miles of ice.)
Be aware that water landing creates additional problems when it comes to embarking.
Landed in Something Nasty
Safe Landing but Something Happens After
Monsters Stepping on Spaceships