Tuesday, February 28, 2017

A cheap Lunar Lander for SpaceX lunar tourisim.

SpaceX’s proposed Lunar flyby mission got me thinking (always a bad thing); if Lunar flybys turn out to be a good money-maker, what about Lunar landings? Revenue is revenue, with the added enormous fringe benefit that the architecture to land tourists economically on the Moon could land anything else; cargo, researchers, base components, etc. It would also be helpful for Mars, via both revenue and experience.

What’s lacking currently? A lander, and a service module (the latter to allow Dragon 2 to enter and leave Lunar orbit). 

Operating premise; keep costs down, including R&D. Use existing tech (Dragon2, in this case) as much as possible. That means, in part, keep it as simple as possible. It also means avoiding unneeded mass. It would require lunar orbit rendezvous like Apollo.

First, Dragon 2 needs a propulsion service module to enter and leave lunar orbit. Essentially, a cylinder, slightly smaller in diameter than the F9 (to fit within the Dragon trunk), containing Superdracos (2, for redundancy) and fuel. Theoretically, you should be able to make this with about the same mass ratio as the upper stage. It'll be a short cylinder, mounted behind the trunk, size dictated by needed delta/v and thus tankage capacity. Needed Delta/v is entering low lunar orbit and departing. So, what length and mass? To the rocket equation! A Dragon 2, plus trunk, plus crew, etc, reportedly wet masses about 7385 kg. (In this scenario, the Dragon2 plays the role of the Apollo CSM stack).

The math. It takes about 680 m/s to enter low lunar orbit. About the same for the TEI burn. So, 1360MS. Round up for margins, 1500 m/s. ISP of a SuperDraco is about 240 at sea level. But, the fuel is MMH/NTO, which has a theoretical max of 336, so a superdraco with a vacuum expander bell (with electric actuators for steering) should do a lot better than 240. I'll ballpark it at 275, which I think is conservative. The dry mass of this upper stage should be, using a mass fraction of the S2 (again, I'm being conservative - this service module is just tankage and superdracos, and does not need to support Dragon and Trunk launch loads the way S2 does). I can't find a mass figure, even a ballpark, for a superdraco. So, I'm going to totally ballpark it and take a SWAG, so basically my proposed service module for inside the trunk is a scaled down Stage 2 in mass fraction. S2 Dry mass is 3900 kg. I'll scale that down, as the service module is a lot smaller, with fewer structural demands.

Dragon2 plus internal SD fuel, etc, has reported mass of 7385kg. I'll add 1000kg for crew plus non-life-support consumables, putting it as 8385kg. That, plus dry service module (I've rounded that up to 1000kg - a very poor mass ratio compared to either of the F9 stages), 9385kg. So, per the rocket equation, we have a fuel mass for the service module (to get 1500 m/s delta/v) of 7000kg. That's about 1555 gallons. Even assuming the same density as water (It’s not, it’s denser), it should therefor fit in a cylindrical unit within the trunk.

So, the service module is a cylinder, 1000kg dry mass (It's far smaller than Stage 2, and I'm taking a wild guess as to SuperDraco mass), using 2 Dragon Superdraco engines. That service module (just a propulsion unit; unlike the Apollo SM it does not provide power, O2, etc, to the CM) has another purpose; a derivative of it is the lander.

It'll need a few mods; add legs plus draco thrusters and Dragon avionics. Without cargo, by itself, it has a delta/v of 5.5 kps, more than enough to land on the moon and take off again (you need 3.76 kps for that - Let’s round that up to 4kps for margin.). That delta/v surplus translates directly into cargo mass capacity. It can carry 600kg of payload and still do the job (low lunar orbit, land, and return to orbit). And that’s your crew lander.

You’re probably wondering how, exactly, the crew accommodations on the lander, plus crew, could mass so little. It’s actually easy if you dispense with what you don’t need. It’s a short trip, timewise, so the crew can make it in space suits. That means they don’t need a crew cabin, life support, etc. All they actually need is a lightweight accel couch like in the Apollo CSM, atop the cylinder. A small control panel would be added.  That same lander type, in one-way cargo mode, could land 3400 kg of cargo on the moon - for example a BEAM type module (With a dragon-based life support) for a short term hab, and other supplies. If added capacity is needed on the crew and cargo versions, it could be attained by stretching the tanks; it could land and take off with one superdraco even with an additional couple of tons of fuel, due to the low lunar G. (The stretched version could thus include a stowed expandable hab on the crew lander).

A mission might look like this; a F9 launches with a crew lander and a cargo lander, either stacked or side by side in a shroud, not quite fully fueled (and thus within the F9’s recoverable capacity to LEO. These self-propel to low lunar orbit. A Falcon Heavy launch places a fuel depot (not technically hard, as the fuel is storeable hypergolic.) on a lunar intercept trajectory; onboard regular-Draco thrusters provide thrust to settle it into low Lunar orbit. A second FH launch, Dragon 2 (manned) plus service module, ends with rendezvous in lunar orbit. Crew handles docking plus topping up the two landers. Cargo lander lands the cargo - if successful, a crew descends on the crew lander, stays a few days, then ascends to rejoin Dragon 2 for a return to Earth. Crew lander remains in low lunar orbit, to be refueled from the depot for the next mission.

This architecture is flexible; could be used to set up a base, or have a standby emergency ascent vehicle.

Is this a bit far fetched, and relying on a lot of assumptions and guesses? Yup. I tried to be conservative, but I'm sure I missed some big things. I tried to keep it as cheap as possible, to make it viable for tourism. As part of this, I tried to keep it efficient (such as just couches on the lander, no structure or shell, and no staging, plus possibly reusable.)

A few huge technical challenges/issues; Can a FH push 16385kg (Dragon plus service module) through TLI? If they can, as claimed, throw 10 tons at mars, maybe, but my guess is probably not – there’s just not that much delta/v difference between Mars intercept and Moon intercept points. If they do stretch the second stage, then I'd feel better about it being plausible. Another hard point is Lunar orbital rendezvous; hard to do. Can't use the GPS based system they use for ISS; GPS won't work in lunar orbit. The DragonEye laser docking system would - but navigation to close proximity will probably require either crew or a lot of work on the automated navigation system. Further problem, the fuel depot; storable fuel has been transferred in orbit before (such as to ISS) but I have no idea how hard it would be to accomplish for this. Vastly easier than with cryo fuels, though.

I'm sure there are major flaws I didn't see - and I'd appreciate criticism and correction.

Note: I plagiarized large parts of this from a post on Nasaspaceflight.com. I admit it. I also admit that the poster I plagiarized this from happens to be me.


ken anthony said...

What a pleasant surprise CJ. I read about a lunar lander like the one you describe years ago. They could certainly do it and it would be quite a ride.

I'd like to see somebody put a reusable SSTO lander in lunar orbit as it's own mission. It would be a totally sunk cost. Thereafter, it would be leased to anybody that wanted to use it for round trips to the lunar surface. It would then make sense that the lander be mated with a depot while waiting in lunar orbit for customers (or perhaps a Lagrange point?) That would vastly simplify any mission to just reaching the lander.

C J said...

Thanks Ken!

There are flaws with my concept, some only just occurring to me. For example, the space suit issue; working space suits are required. So, if during a lunar EVA, somebody's suit starts malfunctioning (leak, life support, etc) then they can't ascend to the dragon. They die. So, my concept would thus require spare suits for each member - a quite costly issue, as well as quite a bit of mass and bulk. Might be viable for a fixed landing site that's revisited often, but not for much else.

Basically, I was just trying to find a way to dispense with a lot of the mass and complexity of a lunar landing - especially the ascent.

I recall making the same proposal for a Mars ascent vehicle a while back; basically just a stage with seats on top. That view and ride might be a big plus for tourists. :)

I'm going to keep working on this to see if I can find some solutions. Perhaps one of those old shuttle "rescue balls" with an O2 tank instead of a backup space suit for emergency ascent in case of suit failure? We're only talking about a half hour duration for ascent and rendezvous - the astronaut stays in the surface hab until being rolled out (literally) in the rescue ball for the launch window, hauled up on top of the lander (easy in 1/6 G), and tied down in place of a removed seat.

I like your idea of a leased SSTO lander. I like it a lot. Low lunar orbit would be best - too much delta/v to and from an L point. It'd have to be a slightly higher lunar orbit (100km or so) not low elliptical; those aren't too stable long-term.

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