Introduction

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 augring 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.

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.

From Chaos Manor Debates by Jerry Pournelle (2007)

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.

From DOUBLE STAR by Robert Heinlein, 1956

Landing in Lava

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 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

where

  • 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

where

  • 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)

[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).

Timothy Little

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.

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.

Aerobraking

The poor man's way of landing on a planet with an atmosphere is by utilizing aerobraking. Pretty much all of NASA's manned rockets use this method. What you do is equip your spacecraft with a streamlined heat shield, 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). 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 atmosphere. 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 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.

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.

From Wait It Out by Larry Niven (1968), collected in All The Myriad Ways

...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...

From The Mote in God's Eye by Larry Niven and Jerry Pournelle (1975)

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.

From 2010 by Sir Arthur C. Clarke (1982)

Magnetohydrodynamic 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.

MHD Aerobraking and Thermal Protection Part I: Introduction

I’ve been meaning to write for a while about a rather fascinating, but not very well known, area of research that I think might have significant implications for several areas of space transportation. The research I am referring to is focused on exploiting Magneto-hydrodynamic forces to manipulate weakly-ionized plasmas caused by hypersonic flight in rarefied flows–ie using magnets to shove around the hot flamey stuff caused by slamming into the thin air above us at crazy-high speeds. I’m going to be a tease, and not go into some of the ramifications until later posts in this series, but for now I want to give a bit more of an explanation than I’ve found available in the popular press so far.

Oh, and one small caveat before I jump in–while I think there’s some real potential here, electromagnetics is a topic that I’m truly awful at. I’ve never had another class, including a PhD level turbulent fluid dynamics class that made me feel like such a brow-dragging neanderthal as my Physics 122 class on Electromagnetism. This may be yet another niche technology that while somewhat interesting, ends up not being all that useful. But it looks at least possible that this may become a game changing technology in many space transportation fields. Without further ado, let’s go over some of the basics.

Some Background on MHD Aerobraking and Thermal Protection
The basic concept behind MHD Thermal Protection is that during hypersonic flight, above about Mach 12, the shockwave formed in front of a blunt-bodied vehicle reaches a high enough temperature to form a weakly ionized plasma that is conductive enough to be manipulated by strong magnetic fields. A powerful magnet near the leading part of the vehicle interacts with charged particles in the plasma via the Lorentz force. This force bends the trajectory of charged particles, creates large hall currents, which if I’m understanding correctly repel the magnetic field. These charged particles also impact with the uncharged gas particles nearby (the plasma is only “weakly ionized”) transmitting these forces to them as well. Here’s an interesting diagram I’ll reference from one of the papers I’ll talk about more later (“Trajectory Analysis of Electromagnetic Aerobraking Flight Based on Rarefied Flow Analysis” by Otsu, Katsurayama, and Abe–well worth the $28):

If the magnet is strong enough, this leads to two interesting effects–first, the distance from the vehicle to the bow shock increases (I think the plasma density between the bow shock and the vehicle also decreases, but I’m less sure about that). This can significantly reduce the heat transferred into the vehicle for a given velocity and altitude. The other big effect is that the Lorentz forces create forces that can produce drag or lift. At high altitudes these Lorentz forces can greatly augment the aerodynamic drag forces, effectively making it as though the vehicle had a much lower ballistic coefficient. It should be noted that this force is electrically controllable. In fact, depending on the sophistication of the magnetic apparatus and its location within and orientation with respect to the vehicle, it can possibly also produce lift as well as control torques without the need for aero control surfaces.

Both of these help from a reentry thermal standpoint, because by the time you hit the denser air, where the heating is the highest, you’re going a lot slower than you would’ve been otherwise, and a lot of that earlier braking is done at much lower heating loads than would have been the case without the electromagnetic effects.

Several of the papers I’ve read introduce an interaction parameter term, Q, that relates the relative strength of the Lorentz forces to drag forces. The relationship takes the form:

Sigma is the conductivity of the weakly ionized plasma, B is the magnetic field strength, L is a reference length (I think related to the magnet configuration), rho is atmospheric density, and V is velocity. As you can see, for a given magnet, the drag forces start dominating as the conductivity drops and as the atmospheric density increases. Atmospheric density increases dramatically as you descend from orbit, so for a reentry application, you get most of your benefit from the first little bit of descent.

We’ll go more into some of these ramifications starting in my next installment.

MHD Aerobraking and Thermal Protection Part II: Atmospheric Reentry for RLVs

In this installment, I want to dig a lot deeper into the mechanics of how one might maximize the utility of MHD effects for orbital reentry. But first, I wanted to spend a few seconds discussing what is important for an RLV TPS system.

RLV Thermal Protection Systems
Protection from the harsh heating environment caused by atmospheric reentry is one of the biggest challenges for reusable vehicles–far more difficult than the often harped-on rocket equation or the “inefficiency of chemical propulsion”. The problem isn’t even the weight of the thermal protection system as much as it is the maintenance requirements. Ideally you’d like a TPS solution that requires very little maintenance, and can be “tested” easily and quickly on the ground before flight, even if it cost you a little extra weight. You’d also prefer something that was relatively simple operationally to use, with a minimum number of failure modes. MHD thermal protection seems like an interesting match for these requirements. I should note however that there are other promising ideas out there such as transpiration cooling that might also work on their own or in combination with MHD thermal protection, but they are beyond the scope of this blog post.

Some Take-Aways from the Literature on MHD Reentry TPS
There have been several interesting papers on this topic, including the JS&R article “Experiment on Drag Enhancement for a Blunt Body with Electrodynamic Heat Shield” that got me thinking about this more seriously, a second JS&R article that goes into experimental proof of the heat flux reduction “Experimental Verification of Heat-Flux Mitigation by Electromagnetic Fields in Partially-Ionized-Argon Flows”, and another JS&R article from a year and a half ago “Numerical Analysis of Reentry Trajectory Coupled with Magnetohydrodynamics Flow Control” that I’ll be leaning on pretty heavily for this discussion. You can purchase the articles from AIAA, or if you already have a subscription to the Journal of Spacecraft and Rockets, you can read them for free.

I’ll briefly summarize some of my takeaways before going into my thoughts on how to move things forward from there:

  1. Both analytically and experimentally, magnetic reentry TPS appears to provide large reductions in both peak heating and in total heat load. The third paper above suggested a 30% reduction in peak heat load and a 40% reduction in total heat load for ballistic reentries. Under the conditions tested in the second paper, heat reductions up to 85% were shown.
  2. The magnetic braking effects dominate aerodynamic braking effects at high altitudes. This is mostly due to lower density meaning that atmospheric drag is fairly low, while also lower density means that Joule heating caused by the currents (the loop marked “J” in the previous post) induced by the magnetic fields increases the electrical conductivity more effectively than at lower altitudes.
  3. The more deceleration that can be done high up in the atmosphere, the lower the peak heating and the lower the total heat load. The heat flux is roughly proportional to the cube of the velocity.
  4. The heat flux reduction from this scheme is dominated by the increased shock layer thickness at high altitudes, and at lower altitudes is dominated by the much lower velocity by the time you get there by getting extra braking high up.
  5. Conductivity of the plasma is one of the keys to making this work. The conductivity in these cases was entirely due to the temperature in the plasma–higher velocities lead to higher temperatures, and Joule heating also leads to higher temperatures. As velocities slow down, conductivity drops, as does the effectiveness of the braking system. Below about Mach 12, the only way to keep the flow ionized enough to control magnetically is to add energy via some mechanism.
  6. Because of the large induced currents, this idea only works if the heat shield is an electrical insulator. If it is a conductor, you’ll just generate hall currents in the heat shield which will null out a lot of the benefit of the approach.

Thoughts on Maximizing the Effectiveness of MHD Reentry TPS
Based on these takeaways, and the discussion in the last post, I’ve come up with a few ideas for how to maximize the effectiveness of an MHD heat shield.

  1. Use a lifting reentry. Just as it is possible to offset the CG of a reentry body to generate some aerodynamic lift, it may also be possible to locate and orient the magnet in a way to create both lift and drag. If you do a force balance on a body in a circular orbit, the downward gravitational force is exactly balanced out by a fictitious centrifugal force due to your forward velocity. As you decelerate though, that centrifugal force component decreases, but by using lift, you can counteract some of that gravitational force. This allows you to stay up at a higher altitude longer, which allows you to do more of your deceleration in the lower density air. This is already used by all manned space capsules as well as the shuttle in order to keep reentry decelerations to a reasonably low level, and also to reduce the peak heating. This is even more beneficial for magnetic braking concepts, because you can do more of your deceleration at a point where the magnetic effects dominate, electrical conductivities are high, and heat fluxes are low.
  2. Use as strong of a magnet as you can reasonably work with. While there are diminishing returns according to all of these papers, a stronger magnet does help provide more deceleration and shoves the boundary layer away further.
  3. Use an alkali seed. As velocities decrease, it gets harder and harder to maintain the electrical conductivity in the plasma at a high enough value to maintain useful levels of Lorentz interaction. This is similar to the challenge with MHD electric generators. In order to keep the conductivity high, injecting an alkali metal into the stream can help. Alkali metals, particularly Potassium and Cesium have very low ionization energies compared to air. In a weakly ionized plasma, most of the atoms are actually not atomized–almost all of the conductivity is provided by the small number of atoms that are. So, a little bit of seeding can go a long way. This helps you keep your magnetic deceleration forces high even as altitude and velocity drop. The other nice thing about seeding, is that depending on what the fluid is, it might also cut down on the radiative heat transfer from the hot shock layer back to the heat shield.
  4. Heat the plasma. This may sound counterintuitive, but you might actually get better thermal protection if you start heating the plasma once you get to a certain point. Below Mach 12, even with seeding, there just isn’t enough heat rise caused by the shock layer to keep the plasma sufficiently ionized. But, it is actually possible via several different means to dump a bit of energy back into the shock layer to push the gas back into an ionized state. It’s unclear at this point if this is worth doing, but if the system is light and simple enough it might be worth considering. As it is, you’ll have a lot of stored energy in the superconducting magnet, and you probably want to dump that somehow before landing–using it to keep the incoming air ionized a bit longer to get a little more deceleration before you hit the thick air might be worth it.

All told, you’re still going to need some sort of thermal protection for the last bit of deceleration, but the heat loads and max temperatures are so much lower if you can dump say half the reentry velocity while you’re still high up, that the problem becomes a lot easier to deal with. If you could only get down to Mach 12 with this system, that would cut the peak and total heat loads by at least a factor of 8x. The heat fluxes at this point would be low enough that you wouldn’t need ablative materials, and could probably use a ceramic tough enough that it was low maintenance.

Anyhow, the key questions I have at this point are: a) what sort of effective “L/D” ratio can you get by varying the location and orientation of the magnet, b) how much does seeding help, c) how long can you stay up in the high altitudes, d) what is the maximum amount of velocity decrease you can provide via this method, e) how strong of a magnet could you reasonably hold on an RLV, f) how does the strong magnetic field interact with the operation of the RLV itself–what does it do to solenoid valves, electric actuators, etc. and is there a way to shield against these issues?

In the next segments, I’m going to talk about another, possibly even more interesting application of this concept, as well as some thoughts on how we can reduce this technology to practice.

MHD Aerobraking and Thermal Protection Part III: Aerobraking and Aerocapture

While using electromagnetic effects for atmospheric reentry and thermal protection is interesting, it’s only one of several promising options that have been proposed over the years. There is another application though, where exploiting magnet-hydrodynamic effects could be a much bigger “game changer” — aerobraking and aerocapture for reusable in-space vehicles.

Traditional Aerobraking and Aerocapture
One of the challenges of orbital mechanics is that it takes just as much energy to descend into a gravity well as it does to ascend out of it. One technique that has been used for lowering the propellant cost of descent into the gravity well of a planet with an atmosphere is aerobraking. Aerobraking is the process of taking a spacecraft in an ellpitical orbit around a planet with an atmosphere, and using atmospheric drag at the lowest altitude portion of its trajectory to slowly decrease the altitude of the high end of the elliptical orbit. This process has been used now on about a half-dozen planetary missions, in some cases reducing the propulsion requirements by 1km/s or more, over the course of a couple hundred passes. Aerobraking has been traditionally been done by satellites that aren’t explicitly shaped like a reentry vehicle–in fact most of the drag for typical aerobraking vehicles is produced by using the spacecraft’s solar panels as massive drag brakes!

A more aggressive maneuver called aerocapture takes a spacecraft in a hyperbolic (interplanetary) orbit and in a single pass decelerates that vehicle into an elliptical orbit around a planetary body. Typically the term refers to maneuvers where the ending orbit has an apoasis near the altitude of a circular orbit, though it could also be used to describe a maneuver that uses a single pass through the atmosphere to replace the “capture braking burn” that would normally be used. Aerocapture is a lot more challenging, since the deceleration has to take place a lot lower in the atmosphere in order to provide the required deceleration in such a short distance. This implies much higher forces and heat-fluxes, which require some sort of aeroshield/TPS system.

Here are a few of the main challenges of aerobraking and aerocapture:

  1. Dynamic Pressure Loads: Dynamic pressure is the pressure felt on the vehicle by the impingement of the atmospheric molecules. The equation for dynamic pressure is q = 1/2 * rho * V^2, where lower case q is the dynamic pressure, rho is the instantaneous atmospheric density, and V is the instantaneous relative velocity. For MRO, the dynamic pressure limits were set at 0.35 Pascals, which correlates to moving at about .76m/s at sea level (ie a slow walking pace). To give you an idea of how this compares with orbital reentrythe peak dynamic pressure of say a Soyuz in its emergency ballistic reentry mode, is over 40,000 Pa of dynamic pressure, and even a low-G lifting reentry is still in the 10kPa+ range. Direct entry into the Venusian atmosphere from a hyperbolic interplanetary orbit gets you into the 1MPa range! Another fun comparison is that the max-Q Xombie or Xoie have seen in flight was around 250Pa. Most of the very low allowable dynamic pressure load for past aerobraking efforts has been driven by the fact that most aerobraking craft to-date have used large flimsy solar panels as their main drag structure.
  2. Peak Heat Flux: The shockwave caused by slamming into gas particles at hypersonic velocities compresses and heats the gas particles to substantial temperatures. Heat from this shock wave is convected and radiated into the aerobraking spacecraft. The equation for heat flux is Q = 1/2 * rho * Ch * V^3. Capital Q is the heat flux (in W/m^2), rho and V are the same as before, and Ch is the heat transfer coefficient. The heat transfer coefficient, I think, represents what portion of that heating goes into the vehicle itself instead of being carried off by the now quite ruffled atmospheric gas molecules who didn’t see you coming. Yes it is confusing that dynamic pressure is lower-case q, and heat flux is capital Q.Once again, to give you some scale, the worst case pass for Odyssey had an estimated heat flux of about 500 W/m^2, which is about 40% of the heat you get in LEO from the solar radiation. For that Soyuz reentry case mentioned earlier, the total heat generated at max-q is in the 240 MW/m^2 range–several times higher than the heat flux at the throat of the SSME or RD-180. The Venusian direct entry example according to one source would actually be in the 4000MW/m^2 range! Fortunately, I think that for atmospheric reentry the Ch term is relatively low–most of that heat gets carried away by the atmosphere.As with dynamic pressure loads, the reason why peak heating rates are kept so low for most aerobraking missions is that you’re using the large solar panels as most of the drag surface, and they can only take so much heating before their temperatures rise to levels that could permanently degrade their performance.
  3. Atmospheric Density Variations: If atmospheric density was nice, constant, and well-known, aerobraking could proceed a lot faster and in a lot fewer passes. The problem is that at the altitudes where aerobraking takes place (100+km), the density can vary significantly over length scales as small as 20km. This can be driven by many processes including variations in the solar wind and solar radiation due to sun cycles, weather effects like dust storms for Mars aerobraking, and other effects. Going off of some data from the Odyssey mission, variations as big as 2-3x were seen in density from pass to pass. A second-order effect of density variations is that both the drag coefficient and the heat transfer coefficient will vary with atmospheric conditions by noticeable amounts. Unfortunately, in many cases you don’t know the density along a given trajectory in advance, so you have to plan for not the average density, but the worst case pass density. Which means that most of the time you’re getting less deceleration and heating than you could actually withstand, but some of the times you might actually find yourself pushing your limits more than you would like. This drives you to taking more passes than you’d really like to take in an ideal situation. These variations get more and more pronounced at higher aerobraking altitudes, where atmospheric density is measured in kilograms per cubic kilometer.Once again, this is an area where using large, sensitive solar panels as your drag devices really hurts. Because you can’t stand high dynamic pressures or heat fluxes, you have to do your passes higher up in the atmosphere. But due to variability in density at those higher altitudes, you end up getting driven even further up to deal with worst case variations. That said, even aerocapture trajectories are high enough altitude that atmospheric variations can be important challenges to deal with.
  4. Aerobraking Duration: For most previous Mars and Venus aerobraking missions, velocity changes in the 1-1.2km/s range have taken between 70-150 days, over several hundred passes. While this is fine for unmanned missions, it’s harder to do for manned missions, where radiation concerns make you want to minimize your time spent in-transit. The large number of cycles is also a difficulty for missions aerobraking at earth, where each pass will take you through the Van Allen belts. Lastly, for reusable in-space transports, the total turn-time is an important economic parameter–the more missions you can fly in the same period of time, the fewer vehicles you need to support a given mass throughput.

A couple more quick observations before we jump into using MHD forces to enhance aerobraking:

  • For typical aerobraking, the parameter you can control easiest is the periapsis altitude, and thus indirectly the average density. In other words, if you want to double the drag on a pass, you lower your periapsis to an altitude that has about double the average density. This also means that to a first order approximation (ie ignoring the relation between density and the heat transfer coefficient) heat flux for traditional aerobraking is going to scale fairly linearly with drag.
  • Ballistic coefficient ends up being really important for aerobraking as well–this is the whole reason why the solar panels are used unstowed for aerobraking. Higher ballistic coefficients mean that you have to dip lower into the atmosphere (and thus get a higher heat flux) to get the same amount of deceleration per pass.
  • In spite of the disadvantages of using solar panels as your drag brakes, there are some real advantages to being able to use a aerobraking scheme that doesn’t require your vehicle to be explicitly crammed into a typically reentry-vehicle shape behind a massive heat shield. It would be nice for instance to be able to get tanker vehicles or orbital tugs back from lunar trajectories or martian trajectories without them having to carry a big aerobraking shield like you see in all the old literature.

Anyhow, that was a quick introduction to aerobraking by a complete non-expert.

Some Backstory on Why I’m Interested in Aerobraking
I started looking into this a few months ago as an alternative to propulsive retrobraking for Centaur-derived cislunar tanker vehicles. While a Centaur stage actually can do a lunar round trip fully propulsively, with at least some payload delivered to the Moon, the “gearing ratio” (initial mass in LEO compared to payload delivered to LUNO or the Lunar Surface) was pretty pathetic. Just to use some ballpark numbers, without digging up my more precise calculations, I’m getting around 8000lbs payload to LUNO if you drop it off in orbit and the Centaur only returns to earth, dropping to only 2500lb if the Centaur has to haul the payload all the way there and all the way back propulsively. However, if you could do 3km/s worth of aerobraking (assuming about 1200m/s worth of burns between the Trans-Earth Injection burn and any periapsis raising maneuvers, including the final circularization), all of the sudden you’re talking about almost 20,000lb of payload on the dropoff mission, and about 13000lb on the round-trip maneuver. Depending on how massive and expensive the aerobraking system weighs, it makes a massive difference in the performance of a reusable cis-lunar architecture. For a long time though, I had sort of dismissed aerobraking, because any aeroshield big enough to allow single-pass aerobraking (or few enough passes to be interesting) also ended up looking like it would either be very heavy, or very bulky, or require lots of orbital assembly or some sort of new deployable technology. Not that any of those other than being too heavy was a total show-stopper, but it definitely made it less attractive for a near-term commercial operation.

Another line of thought I had been wondering about recently was manned cislunar transportation, especially in light of the Augustine Committee report. One of the big suggestions they made that rubbed a lot of HLV-advocates wrong was the idea of launching the crew on commercial LEO taxi vehicles, and flying Orion up to LEO unmanned. A lot of people said this was just silly–if you’re launching Orion may as well launch it manned, even though this would require adding launch escape and emergency detection capabilities to the HLV. I started thinking down the lines of what Orion could look like if it was designed from the start not to carry astronauts until they got to space. The LAS would go away, as would all the structural requirements for taking those sorts of loads, being able to rapidly drop the service module, etc. The whole thing could fit inside a fairing, thus simplifying aerodynamics and loads on the front end of Orion. Heck, it could even be attached to the rest of the stack in whatever orientation made the most sense for mission ops–it wouldn’t be constrained by needing to be on the top in an orientation where the capsule could “get out of Dodge” in a hurry if something “went south” with the HLV. The more I thought about it, the more I realized that Orion could end up looking like a drastically different vehicle if it was optimized for in-space use and reentry instead of needing to also handle manned ascent to orbit as well. Then I made an interesting leap of logic. What if Orion was only meant to be used in space? I originally sort of dismissed this, since most single-pass aerobraking schemes I knew of would require the thing to be designed like a reentry capsule anyway.

Jumping back to the Centaur-based tug idea, I toyed around with the idea of doing a blog series, seeing if I could make an aerobraking simulator to figure out if a Centaur could without any sort of fancy aerobraking shield actually do a multi-pass aerobraking mission that would get it back to LEO within a reasonable amount of time (say three weeks or less). However, I stumbled on the papers about magnetic aerobraking right about this point in my thought process, which may possibly provide a solution to both of these problems.

While I don’t have anywhere near the analytical chops to know for sure how far you can push this technology, if it could enable single-pass or at least small number of pass aerobraking without requiring a huge traditional aerobraking shield, interesting things might become possible. Magnetic aerobraking could potentially revolutionize cislunar transportation, enabling low-cost reusable manned and unmanned deliveries based on modified versions of existing LOX/LH2 upper stages, and could allow fully-reusable in-space only manned vehicles that weren’t just an overglorified 1960s-style reentry capsules. But more on that later.

For now let’s get back to how we can use magneto-hydrodynamic interactions to enhance traditional aerobraking, and see if we can figure out if this idea has merit at all.

Magnetic Aerobraking
Going back to our previous two discussions, one of the key takeaways was that the enhanced braking and thermal protection provided by strong magnetic fields was strongest at high altitudes where atmospheric density was lowest. At high altitudes, the ambient atmospheric density is low, but Joule heating caused by the interactions between ions in the shock layer and the superconducting magnet keeps the electrical conductivity of the plasma in the shock layer high. Also, for aerobraking or aerocapture short of reentry, by definition you are both always at a speed and altitude high enough that you don’t have to worry about the shock layer losing sufficient conductivity for MHD effects to dominate aerodynamic drag effects. The magnetic interaction parameter (Qmhd) introduced in my first post in this series can easily be in the 250-1000+ range at high altitudes compared to down in the 5-50 range you might see during atmospheric reentry. For example, the paper I cited in my first article (Otsu et al) showed that for a vehicle coming back from a GTO-like orbit, you could cut the return time by 70% with a 0.1T magnet, which is about 5x weaker than the magnet assumed for most of the reentry magnetic TPS studies. While magnetic effects may be helpful for reentry, they truly come into their own for aerobraking and aerocapture.

A few other thoughts:

  • While the total drag for a magnetic aerobraking concept can actually be several times the drag of a similar non-magnetic vehicle, the gas-dynamic portion of the total drag actually decreases substantially in the case of magnetic aerobraking. This is due to a much lower velocity behind the shock layer in the magnetic case. Figure 9 from the Fujino et al paper I used in the last post (“Numerical Analysis of Reentry Trajectory Coupled with Magnetohydrodynamics Flow Control”, JS&R Vol 45 No 5, pg 911-920) illustrates this beautifully:
  • For a vehicle using magnetic braking, most of the total drag force is actually reacted electromagnetically through the magnet itself, not through the surface of the vehicle. The dynamic pressure that the vehicle surface itself sees is greatly reduced compared to what you would expect at that altitude and entry velocity.
  • While in the above case, the dynamic pressure reduction was about 4x at ~75km, this effect is likely to be even more pronounced at the altitudes used for aerobraking (90-120km) where the electromagnetic interaction parameter is substantially higher (40-160x higher) than it is in the case shown above for atmospheric reentry.
  • The heat flux seen by the aerobraking vehicle will also be greatly reduced compared to a non-magnetic aerobraking system at a similar altitude and velocity. This is due to the much thicker shock layer standoff distance and the lower velocity of the particles behind the shock layer. The Fujino et al paper estimated that the heat flux would roughly be cut in half at 75km with a 0.5T magnet (due to a boundary layer between the bow shock that is twice as thick at that magnetic interaction parameter).
  • For higher parameters in the 100-1000 range that you would likely see for aerobraking, this effect should be even more pronounced. The trend in shocklayer thickness vs. Qmhd shown in Fig 3 of Fujino et al was linear over the Qmhd range of 0-6. If it continued out linearly up into the Qmhd 100-1000 range, the shock layer standoff distance would be in the range of 100-125x thicker than without MHD effects, implying a drastically reduced heat flux at aerobraking altitudes. Unfortunately without having them run the actual analysis, it would be hard to know precisely how well this would work.
  • All these factors mean that the same vehicle could use a lower periapsis with a magnetic braking system than without. The dynamic pressure and heat flux that the vehicle sees at a given periapsis altitude is going to be at least 2-4x and possibly more than an order of magnitude less than it would be without the magnetic field. Even in the most conservative case (ie assuming that the effect at 100km and aerobraking speeds is no better than at 75km in spite of having a Q 40-160x higher) this would allow you to go to an altitude with at least double the density while keeping the heat flux and dynamic pressure loads within tolerances. With an effective total drag 4x higher at a given altitude combined with being able to go to a lower periapsis, you get bare minimum a 8x reduction in total aerobraking time compared to the non-magnetic case.
  • For the aggressive, “I don’t know if I’m extrapolating way too far” case, you could get even larger reductions in aerobraking time. Going back to my linear extrapolation on shock layer standoff vs. Qmhd (and thus heat flux vs Qmhd), at Qmhd=250 this would put the shock layer standoff at about 25-30x thicker than the non-MHD case. The example in Otsu et al gave a Qmhd of 250 using a 0.1T magnet and a 100km periapsis. Since Qmhd is proportional to B^2 and inversely proportional to rho. If you increased the magnetic field from 0.1 to 0.5T (similar to what was being suggested for the reentry studies done by Fujino et al and some of the others), you could maintain a Qmhd of 250 even if you increased the local density by a factor of 25. At Qmhd of 250, the effective drag coefficient is about 3x higher than the non magnetic version. That would give up to a 75x reduction in aerobraking time compared to the non-magnetic case.
  • One other advantage of magnetic aerobraking is that you can drastically vary your effective drag coefficient electrically. Also, the heating and dynamic pressure are far more driven by the magnetic field strength than by the atmospheric density for the MHD aerobraking case. These mean that you can afford to take deeper passes without having to worry as much about variability. If the density is higher than expected, and you have some head-room on your magnet, you can increase the MHD field strength a bit to keep the shock layer back and the dynamic pressure down. This also could cut trip times in half just by allowing you to base your planning off of the average atmospheric density instead of having to take the mean + 3 standard deviations as your predicted atmospheric density.

I’m rapidly coming up to the point where I’m pretty sure I no longer know what I’m talking about. At least from here, it looks like there’s a good chance that MHD aerobraking could allow for aerocapture (at least into a high eccentricity elliptical orbit), and very rapid aerobraking down to a circular orbit compared to the non-magnetic case. I think you can extrapolate the conclusions of these papers in these ways, but without having the people with the analysis tools actually verify these claims, I’d still take them with the appropriate sized grain of salt. Also, my intuition on how a MHD aerobraking vehicle would compensate for density variations is not very good. That alone could be a paper or a thesis.

So, whether this ends up being a mild curiosity that ends up only being useful in niche applications, or a game-change remains to be seen, but the potential for this being a game-change is real.

Pilot View

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.

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:

  1. Loading/unloading cargo from a tail-landing spacecraft means using a crane over tens of meters
  2. 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.

Landing Legs

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:

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: Ship with 3 feet can be stable up to foot distance from center of gravity cos(60°)*D or 0.5*D.

Since 0.707 / 0.5 = 1.414, for a three-footed rocket to have the same stability as a four-footed rocket it would have to have its feet at a distance D3 from the center, where D3 = 1.414 * D4)

It may well be that three such extensions require less mass than adding a fourth leg.

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).

Mike Williams

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:

  1. you have a four legged structure
  2. all the legs are exactly the same length
  3. the ground is a continuous surface with a local slope of no more than fifteen degrees
  4. you rotate the structure around the plane of the feet
it will always be possible to find a position where the structure does not rock back and forth. And you won't have to rotate the feet more than ninety degrees, either.

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:

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.

Sean Willard

Recovering from a Topple

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.

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.

From THE BEAST MASTER by Andre Norton, 1956

(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.

From Grendel by Larry Niven (1968)

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:

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...

Garon Whited

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.

Technobabble Landing Legs

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 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.


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.


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.


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.

From Wait It Out by Larry Niven (1968), collected in All The Myriad Ways

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.

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.

From Starman Jones by Robert A. Heinlein (1953)

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.

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.

From The Currents Of Space by Isaac Asimov

Water Landings

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.)

Unhappy Landings

Monsters Stepping On Spaceships

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