[Note: I came up with this idea a couple of weeks ago, but it got left on the backburner over the holidays. Oh, and welcome to anyone coming here from the Carnival of Space.]
One of the biggest mixed blessings of lunar transportation is the lack of an appreciable atmosphere on the moon. While this is a big benefit as far as propulsion efficiency and deep throttling goes, it is also a big drawback for crew safety. Basically, a VTVL vehicle lives or dies on its propulsion system. However, in an atmosphere, even if you have a complete propulsion failure (say catastrophic loss of power, propellant tank rupture, etc.), there is still the option of using an emergency ballistic chute (if your vehicle has one), or “bailing out” and using your own parachute. While this is by no means foolproof, it sure beats the alternative.
The problem is, on the moon there is none of that nice, draggy, “air” stuff that are somewhat non-optional for parachutes.
The traditional solution to this problem has been to use a two-stage lunar lander, and treat the upper stage (the ascent stage) as an escape capsule. But such TSTO designs make reusable lunar transportation a lot more difficult. And you’re still stuck with the tricky situation of what happens if your ascent engine fails?
While a good reusable lunar lander is probably going to borrow heavily from operations, design, and maintenance experience from terrestrial VTVL suborbital vehicles, there’s still the reality that cislunar space is a more dangerous place. Not only are there environmental factors such as micrometeors, radiation, etc. that make failures more likely. But the effects of those failure modes are more severe. There’s a reason why most of the predicted risk for a lunar mission center on the transportation phases to and from the lunar surface. The Moon really is a “Harsh Mistress”.
Ejection Seats: aka “Attempting Suicide to Avoid Certain Death”
So here’s a crazy idea: what about using some sort of ejector seat that used pure rocket power instead of parachutes? Basically it would be a propulsion system with a main engine, and possibly some RCS engines, and some sort of minimal GN&C system. Assuming that either some sort of hypergolic combination (or something like a scaled up version of what Digital Solid State Propulsion is working on) were used, and that the package was sized for say 1200m/s, you’re only talking about a couple of hundred pounds per spacesuited crew member.
Assuming 400lb for the crew member in a space suit, 100lb worth of dry mass (chair structure, engines, tanks, any pressurization systems, valves, etc–probably quite doable), you’re talking about ~250lb of propellant. For a total weight of about 350lb for the ejector seat (at least some of which may have been needed for a non-ejectable landing seat anyway). If you go much higher than 1200m/s, the propellant fraction starts growing fast enough that the system would probably weight too much to make sense, but at an extra cost of only about 250lb per crew member, it might not be too crazy. Especially if it allows you to save weight elsewhere by going to a higher performance but non-hypergolic main propulsion system, for instance.
Since there’s about 2km/s of Delta-V between a low lunar orbit and the lunar surface, the worst case failure would occur partway through the retro burn (or the orbital ascent burn), where you have about half of the delta-V left. 1200m/s gives you a little bit of margin, plus some propellant for RCS ops, off-nominal operations, etc. If you’re most of the way down, or only part of the way up, you abort to the surface (using your legs as landing gear like parachutists do on earth). If you’re more than half way up, or less than half way down, you abort to orbit.
Also, 1200m/s can probably give you a pretty decent suborbital hop. Unfortunately the lack of atmosphere means that once again you have to decelerate as well as accelerate. But we’re still probably talking about a several hundred km range in case your lander malfunctions during a sortie mission.
Lastly, having each crew member have a maneuverable emergency seat like that also makes rendezvous failures between the lander and the lunar orbital station (or CEV or whatever you’re using to get back to earth in) much less likely.
Are there some dangers in such a system? Probably. Ejection seats kill ground crew on a regular basis here on earth. But they still save enough lives on net (in spite of how much more reliable even combat aircraft are compared to rockets) that they still get used. It’s like a launch escape tower. Even if it only has a 75% chance of working, and a nonzero chance of going off at the wrong time, it will probably cut back on overall fatalities by a substantial amount.
Now, obviously, in order to do any good, you obviously need the crew in spacesuits during orbital descent or ascent maneuvers. Also, you need a way of getting the seat out of the ship without undue risk to the crew members. Shrapnel from something like a shaped charge that on earth might just risk causing an ugly injury could cause a loss of pressure integrity in the spacesuit with predictably bad results. Fortunately, due to the lower gravity, lack of aerodynamic forces, etc. it may be easier to make such a system safe and reliable than it would be for say an ejection system for a supersonic jet fighter.
One of the main potential benefits of having something like a lunar ejection seat, is that it frees up the design of your lunar lander a lot more. For instance, you can now use a single-stage (and thus more easily reusable) lunar lander without having to take as much risk of losing the crew. Also, you can pick propellants for the lunar lander more on performance and economics criteria instead of having to use hypergolics on your ascent stage because you’re trying to shoehorn the thing into being an escape capsule.
Now, I don’t know if the idea makes total sense on balance, but I think it’s an interesting one at least worth looking at. What do you all think?
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