Thursday, October 07, 2010

Thoughts on a SpaceX Lunar Architecture

It's no secret that the SpaceX Dragon capsule has a very impressive heat shield believed to be capable of direct lunar return. Official statements from SpaceX that they intend to add deployable landing gear and leverage the thrusters in order to land on land in the future prompts an obvious suggestion: if it can land on Earth, could it land on the Moon too?



The Dragon capsule has thus far been launched on the Falcon 9 booster, and although that booster is able to put 2473kg into lunar transfer orbit, after using the Draco thusters on the Dragon to enter low lunar orbit the total mass would be under 1876kg.. this seems a bit light for a crewed configuration, especially when you consider that only 1422kg of it could be returned to Earth. And that's just lunar orbit.

We need a bigger rocket, and the official SpaceX plan right now is called the Falcon 9 Heavy. One should not be confused by the name, the F9H is not "heavy lift" in the sense often used by space advocates and policy makers - who should more correctly be using the term super heavy lift. So how heavy is the F9H? Comparison is usually done in terms of lift to LEO, but for our purposes lift to LTO is more interesting at 10622kg. A slight improvement on the Delta IV Heavy at 9984kg.

Now we can imagine a Dragon-lander flying direct from lunar transfer orbit to the surface, it would have a mass of 3766kg when it landed which is quite respectable. For a cargo flight this is fine, delivering 2737kg of payload, but it's unlikely the vehicle would have enough fuel left to attempt an ascent.

Having determined that a single stage direct descent vehicle is unlikely, we're now forced to choose a mission mode. The size of the launch vehicle has already dictated that LEO should be bypassed, so our choice comes down to lunar-orbit rendezvous (the mode used by Apollo) or lunar-surface rendezvous, aka, refueling on the surface. So much has been said about LOR already, so let's run the numbers for LSR.

Having landed a crewed Dragon-lander on the surface, and assuming no fuel is left, we would require 6014kg of propellant to return to Earth. This is not too bad, at 3 fuel landings, but we can do better. If we can carry just 540kg of fuel in reserve we can eliminate the third fuel landing. Another alternative is to throw 338kg of payload out.

Of the 2737kg payload delivered, we have to determine how much is needed for the crew and their supplies, and how much can be fuel. The pressurized volume of the Dragon is 10m^3 requiring 11.839kg of air to fill. Without an airlock we may wish to cycle that a few times, so let's say 118kg total. Next we need a one week supply of oxygen candles at 25.83kg per person, and LiOH to scrub the CO2 at 52.71kg per person. Finally there's food and water at 45kg per person. For a total of 488kg for a crew of three. Too easy! This leaves 1708kg for spacesuits and equipment.

Once on the surface, the crew would vent the chamber, get out and refill the fuel tanks. Having gravity, transferring the propellant is well understood. Return to Earth would be direct, with no need to enter lunar orbit or perform a rendezvous. As no parts fall off the Dragon-lander on the way it could be fully reusable, providing a stepping stone to in-situ produced propellants.

I estimate a Falcon 9 Heavy / Dragon-lander cargo configuration would cost around $40k/kg to the lunar surface. With the two fuel emplacement flights, this makes crew transport something like $130M/seat for crews of 3, but you could conceivably get that down to $55M/seat if you were delivering 7 at a time - most likely to some kind of base as they would have reduced volume for equipment.

Most of my calculations were done with this rocket equation calculator and I used an inert mass fraction of 0.15 for the lander.

11 comments:

  1. While it might be possible, I would think that the smarter thing is to use a Blue Origin or even Armadillo type VTVL craft to take up and down a capsule. The advantage of that, is that it protects the heat shield on the bottom of the capsule. The LAST thing that you want damaged in ANYWAY is that shield. Also, it would be nice to have a craft devoted to dealing with the lunar arch. esp since that is what we are spending the money on them for.

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  2. Anonymous8:53 AM

    Windbourne... very well put. Personally, I am
    hoping for the day that SpaceX, ULA, Bigelow,
    and the other new space orgs decide to pool their resources for a completely commercial manned lunar landing. Maybe they get together, have a few beers, watch "Destination Moon", and say "Hell yes!... Let's go to the moon!"
    Now THAT would get the public's attention!

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  3. Anonymous,

    I'll host and buy the beer. Pizza too, if they let me listen in...

    Trent, great article. I really like the way you think.

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  4. Anonymous12:26 PM

    It is a very bad idea to use the Dragon Capsule as a lunar lander. Its structure would weigh more than twice than other options. You also have to lift that heavy structure from the Moon all the way back to Earth. Look at the open cockpit landers and the TransHab (kind of like Bigelow) lunar landers from NASA's 1996 Manned Lunar study.

    The Falcon 9H with Raptor LH2 upper stage can place 50-tons into LEO and 20-tons into Lunar Transfer Orbit (LTO) which is sufficient for Dragon and a separate 4.5-ton Lunar Lander (i.e. from the 1996 NASA Lunar study) to land 2 astronauts on the Moon and to return to the Earth using the same method as Apollo.

    SpaceX has many low cost options to send Astronauts to the surface of the Moon.

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    Replies
    1. Falcon Heavy can only put about 10 tons to LTO, not 20 tons.

      Falcon Heavy has capasity of 12 tons to GTO, and LTO needs more delta-v than GTO.

      Delete
    2. The numbers haven't been published yet. In any case, I've grown apathetic to direct throw capabilities of the launcher. Just separate from the upper stage in LEO and use your own propulsion to get to LTO.

      Delete
  5. uh huh. The Dragon capsule has the interesting benefit of *actually existing*. It will be capable of the mission about the same time the Falcon 9 Heavy (no raptor upper stage) *actually exists*. That's two wins over a nonsense paper study that would cost billions only to be cancelled a few years later by Congress.

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  6. G'day,

    Dragon does *not* exist, at least as a flight capable vehicle. Only a bare bones prototype has flown so far and it will be the cargo version which will fly first. If and when a crewed version will ever fly depends on the market.

    What certainly does exist is a trillion dollar US deficit. Sooner or latter the axe will fall on all government programs including NASA. The discussion which needs to be had is how the US can streamline its space program into something leaner and far cheaper, because if it can't civilian space programs are dead.

    ta

    Ralph

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  7. Anonymous8:33 AM

    Maybe yes, maybe no.

    Not all countries are as dumb as the US appears to be.

    As far as civilian space programs are concerned I keep thinking "Colts". It would be so easy be be somewhere else over night that it is not funny.

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  8. Good article. Thanks for directing me here Trent.

    Windbourne, the heat shield isn't an issue if the selected Dragon is for moon use only. Remove the PicaX shield and add legs (already planned.)

    Even though the Dragon may not have been designed to be a lunar lander, it may be useful to have the potential for emergency use.

    Lunar property rights could be a step to making a lunar colony financially viable.

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  9. Anonymous4:43 AM

    If we want to return to the Moon, we need safety that we didn't have for Apollo.

    We were willing to risk lives so Russia couldn't claim and possibly legally own the Moon.

    Today, that risk is gone. I don't know that we can justify these single-ship missions where if anything goes wrong, they're sitting on the surface or orbiting forever.

    SpaceX or whoever goes is going to have to have safety that was unheard of back then. Multiple redundancy in everything from environment to main engines.

    ReplyDelete