[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.
Potential Benefits
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?
Jonathan Goff
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What nav and comm gear would you need? A ring laser gyro and a star tracker? They would have to be calibrated.
You need some kind of orientation mechanism after the chaotic bailout.
The CEV can use radar and radio to guide the free-flying astronauts to final rendezvous.
I always remember the story by Henry Spencer where during Apollo training, there was a computer failure scenario during ascent. The guys flew to the orbit manually looking at charts just fine, but they ended up in the wrong plane. :/
And would the CEV have an airlock in the normal case? Or perhaps one can vent the atmosphere in an emergency and get in through the docking hatch.
Hello Jon,
I think, now we better put the margins/redudancy on the main vehicule than on individual rescue devices. If you have to use such a scheme, assuming you reach without injuries the lunar surface, chances are you will be stranded with little air waiting for inexistent lunar rescue.
In the (far?) future, if a full blown lunar base with quick rescue transport is available, your idea could be interesting. Using lunar ressources to make fuel on site will help, assuming a reusable/refilable lunar VTVL transporter (cosmos 1999 like 😉
Gravityloss,
I wasn’t really thinking of this idea so much in terms of the existing ESAS lunar architecture. Sure the CEV probably wouldn’t have an airlock. But in a more reasonable lunar architecture, you’d probably have a small station in lunar orbit (and/or L1/L2) where translunar transfer vehicles meet up with the lunar vehicles from the surface. In that case, an airlock (and stuff like nav aids and long-range radio communications) are most likely. Eventually you might see tugs capable of rescue operations and such.
A lot of how well rescue systems work depend on how much infrastructure is in place. The reality is that even with good escape systems, a transportation failure on or above the moon is going to be fairly dicey for some time yet.
~Jon
Gabriel,
If you bail out fast enough after a failure, you should actually still be on about the same trajectory as you were before the failure, which means that with the extra propellant margin, you should actually be able to land pretty close to the landing site.
That said, the reason I was even thinking about this was precisely because “beefing up your margins and redundancy” can only take you so far. Most importantly it pressures you to move away from architectures that would be more reusable (single-stage systems), and technologies that provide better performance (cryogenic propulsion systems) in order to get only a little bit of extra safety. It’s the same reason why launch escape systems are used on rockets these days–until you have a truly reusable vehicle that you can fly a lot *in the environment you intend to use it in*, your vehicle reliability is never going to be that good. Adding an escape system can greatly increase that, even if the escape system isn’t ultra-reliable itself.
Sure, some subset of bailouts are going to result in losing a crew member or two anyway. But is that really better than a 100% loss rate for failures without an escape system? I don’t think the trades would end up going that way.
One other thing. If you use nitrous as the oxidizer for your ejector seat (say using an engine like the one XCOR recently announced), it would be possible to use a decomposition device to generate a breathable atmosphere for many days using the residual nitrous. There are also proposed methods for doing the same with an NTO/Hydrazine system, but the danger of contamination or incomplete decomposition are a lot higher (but then again if the alternative is a 100% chance of asphyxiation…I’d take my chances).
~Jon
Jon:
I think that this is a great idea.
I think that the idea would be better if you only designed this ejection seat with the delta-V for abort to orbit, and not for the ejection scenario of abort to landing.
You should also not worry about having individual ejection seats for individual Astronauts, and instead have one “ejection couch” or pod (like ejection system on F-111 fighter jet) for all of the Astronauts who are ejecting. Any scenario where one Astronaut needs to eject will be a scenario where all Astronauts need to eject. You will be able to save a lot of weight on redundant structures, etc. if you do this.
Lastly you should use a fabric or inflatable or “Bigelow-type” pressurized structure for all of the Astronauts to get into that is attached to your “ejection” mechanism.
If you make my above recommended changes, then you have basically created an “ejection seat” that is really a cheap lunar ascent module. The only difference is that a “real” lunar ascent module will have a lot more structure, avionics, ECLS capability, etc, than your ejection pod, but it would also weigh a whole lot more.
This ejection pod would make economic sense in lunar architectures where you separate cargo and human landing vehicles, and where you decide to pre-position equipment and facilities. The economic logic is that you don’t waste a lot of weight, delta-V and cost on an over-designed lunar ascent vehicle, because the ejection pod can get you back to lunar orbit (for rendezvous with your capsule) at extreme low weight and cost. You don’t want to use your ejection pod as a true lunar ascent stage for “normal” travel back to lunar orbit, but it sure could come in handy if your lunar descent has a mishap or if your lunar base malfunctions or if your “normal” lunar ascent vehicle is broken or delayed.
I bet that if you did your calculations you would find that you could design design something that could return 4 Astronauts (without their Space suits) to lunar orbit and that weighs 500 kg to 1,000 kg including fuel. You could probably use a few of the smaller RCS thrusters designed for Apollo/Dragon/CEV as the propulsion system, and you could ignore using a complex 6-DOF guidance system by assuming that you will get close enough to your rendezvous target that the CEV, etc. could use their delta-V to do the docking/rescue maneuver.
What do you think?
Anonymous
Hey Jon,
That’s a great idea, although I would have to go with Anonymous and have an ejection couch rather than a seat.
However, would adding that to the lunar lander drive up the overall price of the lander, thereby making it too expensive?
Jon, I really like your idea to make the escape system’s delta-V only a bit more than half of orbital velocity- that is the trick that makes the concept possible. It’s only half a stage, not a complete 1800 m/s system- indeed, I think you may be a bit too conservative with the 1200 m/s requirement, about 1000 should be enough to abort to orbit or surface. My reasoning:
LLO velocity is 1650 m/s, so if you’ve slowed to less than 750 m/s, you can still get back into orbit with 900 m/s of dV (gravity losses are very low, drag nonexistent)- or land with 250 m/s of margin. If the main system drops dead early in the descent, you abort to orbit, if it dies at anything between 700 and 850 m/s you have your choice, below 700 m/s you have to land, so 1 km/s is plenty. The abort guidance system would be awake and ready to take over at any time, and would make the surface/orbit choice at a clearly defined point.
The escape system would be an integral part of the crew module that gets swapped out for a cargo module, so only manned flights would suffer the payload hit. If the crew cabin is minimal enough to start with, it *is* the escape capsule and the only separation event is the entire crew bugging out. No traffic management problem with four escape systems having to keep out of each others’ way…
Acceleration is mild, the main engine is sized for the emergency landing case, with throttling. This is a very promising concept for those missions where there is enough infrastructure for a prompt rescue mission within the life support limits of the bugout vehicle.
Doug,
Good point. 1000m/s makes the numbers look even better. One possible problem with making the whole crew cabin an escape capsule is that now you’d need landing gear for ground aborts. Unless I’m misunderstanding you.
The other thing is that one of the goals of the lunar ejector seat was having additional electrical and control redundancy so if you have a catastrophic electrical or control failure that you can bail out (not just mechanical failures like engine problems). If the whole crew cabin is the escape capsule, getting real redundancy might be harder.
But avoiding having to get 4 people out of a tight area each with their own rocket engines is probably a good idea.
~Jon
Jon:
Abort to the ground does not make much sense to me, because if your lunar lander breaks, you probably want to go to lunar orbit and leave the moon rather than take your chances on the surface waiting for a rescue lunar lander to come pick you up. A future lunar base probably will not be able to afford spare/fueled-up lunar landers, but every lunar mission will probably have a spacecraft waiting for them in lunar orbit.
Abort to the ground also wastes a lot of weight in terms of additional structure and landing legs. Eventhough abort to orbit might have twice the delta-V as some abort to ground scenarios, if you look at the structural weights associated with this, you will find that abort to orbit will weigh about the same as abort to ground.
Abort to orbit also gives you multiple more abort scenarios, than abort to ground.
Using storable propellants, I estimate 200 kg for the structure (including the engines, avionics, etc.), 50 kg for the inflatable/pressurized bubble, and 50 kg for a weak ECLS (keeps 4 people alive for a few hours in a disaster scenario) based on the small portable ECLS carried on an Astronauts back within the heavier Apollo-style lunar space suits. I assume another 500 kg for 4 Astronauts wearing the lighter 10 kg “flight suits”. The heavier lunar space suits can be carried outside of the “ejection pod”, and can be carried within a second pressurized section of the lunar descent vehicle, where the Astronauts would do their real living during the exploration phase. This gives me 800 kg for the vehicle and the 4 Astronauts, which means that it will be approximately 900 kg of storable propellants for fuel to be able to abort to lunar orbit in all scenarios.
This is a 1,200 kg “ejection pod” that can take 4 Astronauts (or 500 kg) back to the waiting spaceship in lunar orbit under most scenarios, and it can also serve as a cheap or emergency lunar ascent stage if the “real” lunar ascent stage does not work. If you are planning a lunar mission architecture with single-stage/reusable/refuelable landers and with pre-positioning of cargo or other lunar base materials, then this cheap/emergency lunar ascent stage will give you a lot of extra flexibility.
Anonymous
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