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 rule of thumb 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.
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.
You can make the pilot's job fractionally easier with ground-based assistance, like perfectly flat landing pads and landing beacons. Pioneer beacons are placed on reasonably flat stretches of ground and provide the pilot with basic help. Beacons at starports provide the pilot with all sorts of telemetry.
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.
The Apollo Lunar Module had special outward angled windows to let the pilot see, and even then the landing pads had contact probes like 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 a puncture-prone space suit down a ladder where a pointy contact probe has been bent upwards.
A contact probe was also used in the lunar ships featured in Collier's Man Will Conqure Space Soon! series. Which isn't surprising since Wernher von Braun had a hand in creating both.
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.
United Launch Alliance designed a lunar lander. The two main problems they were attempting to deal with were:
- Loading/unloading cargo from a tail-landing spacecraft means using a crane over tens of meters
- Pilots trying to land a tail-landing rocket are flying blind
The solution to both problems was making the thing a belly lander. For landing purposes, this allowed the addition of windows aimed downward that let the pilot see exactly what they were setting down on.
Belly landing also made sense for the LUNOX proposal. As did windows facing downwards.
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:
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 unobtanium 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 unobtanium "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.
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.
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.
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.
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.
Landed in Something Nasty
Safe Landing but Something Happens After
Monsters Stepping on Spaceships