Transterrestrial Musings

Comments

Rand,
While I agree that N2O4/Hydrazine (or MON/Hydrazine) are nasty combinations, they do have some big advantages when dealing with NASA. I would imagine that trying to do something new around the space station is a recipe for a much more expensive and difficult project. NASA's motto seems to be "better the devil you know..." Also, N2O4/Hydrazine are very high density, and can probably be stored in much lighter tankage than self-pressurizing nitrous and ethane, which might matter on what is probably a fairly weight constrained capsule. But then again, they are self-pressurizing, so that's one less system to fail, and maybe once you factor in the weight of the pressurization system its a wash. Dunno.

It would be nice to see them move away from hypergols. It would also be nice to see how well XCORs RCS engines actually function in a vacuum environment. My guess is that they'd be just fine, but it would help make their sale pitch a bit easier.

~Jon

Posted by Jonathan Goff at December 12, 2007 10:28 AM

Thanks for the support, Rand. That engine was developed primarily for RCS applications, just not that particular one. XCOR did talk to Space-X but they wanted something more mature. That's no problem; other applications will advance the maturity.

The current embodiment of that engine doesn't use LOX or any other cryogens. It burns ambi@nt temperature stored ethane and nitrous oxide, although if the ambi@nt drops below -10C, the tanks need to be either insulated or heated. That's because the self-pressurization pressures drop too low to run the system.

Jon: we've done some vacuum testing (all successful) and plan to do more.

And Rand, your site won't accept the correct spelling of the word word ambi@nt. It contains the name of a much-spammed pill.

Posted by Dan DeLong at December 12, 2007 10:42 AM

Dan,
Cool news. I didn't know if you guys had access to a vacuum test rig or not. I stand corrected.

~Jon

Posted by Jonathan Goff at December 12, 2007 11:05 AM

Given the apparent size of the Dragon capsule, it looks like it'd be very cramped if they carried 7 people to the ISS and took 2 days to get there like the Shuttle and Soyuz. Perhaps they can fly a faster rendezvous approach.

I wonder if they've considered offering their capsule as an emergency escape vehicle like the old CRV concept. If it could withstand long term exposure to space, you could have one attached to the ISS ready to serve as a lifeboat. That would allow for the ISS crew size to be increased and would generate some revenue for SpaceX.

Posted by Larry J at December 12, 2007 12:34 PM

Given the apparent size of the Dragon capsule, it looks like it'd be very cramped if they carried 7 people to the ISS and took 2 days to get there

For short orbital missions, "cramped" is not a bad thing. Spacesickness did not become a problem until the Apollo capsule allowed astronauts to get out of their seats. If astronauts want to float around, they'll have plenty of time to do that after they've reached ISS and gotten past the acclimation period.

Posted by Edward Wright at December 12, 2007 02:02 PM

Unfortunately, Elon is still repeating the myth that the Shuttle proves capsules are the only "reliable" way to get into orbit -- despite the fact that capsules have killed a higher percentage of their occupants than the Shuttle has.

He's also being less than candid when he says, "Finally, consider how, with years of Shuttle experience, NASA chose to return to a capsule architecture for the Orion lunar spacecraft. Thus, we favor the capsule design for reliable and economical transport to and from Earth orbit."

As Elon is well aware, SpaceX was working on Dragon well before NASA started to Orion. It was Elon and Frank Sietzen (then SpaceX's Washington lobbyist) who met with NASA Administrator Sean O'Keefe and sold him on the idea of replacing the Shuttle with space capsules and expendable rockets so NASA could go to the Moon, Mars, and Beyond. NASA did not come up with the idea independently.

Obviously, it was naive for Elon and Frank to think that NASA would allow SpaceX to build their exploration capsule, rather than Lockheed. And one can question whether we should sink more billions into the "spam in a can" approach which has proved so costly and (contrary to Elon) unreliable.

That said, however -- assuming NASA *is* going to use capsules for the next 40 years, as Mike Griffin says (or even for a fraction of that period), why not use the Dragon capsule as Elon originally intended, instead of spending $10 billion on the redundant Orion capsule?

Using Dragon, instead of Orion, NASA could begin manned lunar missions closer to 2010 rather than 2020. (Assuming Elon's schedule does not slip, which admittedly is possible. Of course, the Orion schedule is likely to slip also.) The $20 billion or so that NASA would have spent on Orion and Ares I could by a lot of Dragon missions.

They could land quite a few astronauts on the Moon in the 2010 decade, rather than a much smaller number in the following decade.

Posted by Edward Wright at December 12, 2007 02:40 PM

Would using the Dragon as a part of lunar missions necessitate an inbound rendezvous in Earth orbit before Earth descent?

The upside to that could be a greater likelihood of in-space infrastructure like a reusable lunar ferry and so on.

Posted by Habitat Hermit at December 13, 2007 02:33 AM

Personally, I like the idea of all missions outside of orbit stopping back at a station--you can specialize your vehicles somewhat, and don't have to lug around things like the heat shield, parachutes, high-G structure, etc. all over space. Unless there's a huge energy savings to be had by going directly in, why *not* make a pit stop in LEO?

Posted by Big D at December 13, 2007 06:41 AM

BigD,
In order to return to a LEO station from the Moon or some other destination, you need to provide just as much delta-V as it took to get to that trajectory in the first place. Ie if you want to go from LEO to a Lunar Transfer Orbit, it takes the same amount of delta-V to go from a Lunar Transfer Orbit back to LEO (~3km/s both ways IIRC). Now this can either be provided propulsively or using the atmosphere. Using the atmosphere requires a heat shield of some sort (preferably a reusable one so you're not having to replace it after every trip). Ironically IIRC, the peak heat flux and total heat load from coming back from Lunar Orbit to LEO is almost identical to the heat load and peak heat flux for returning to the earth's surface.

So, while I agree there are some real benefits to letting vehicles specialize a bit, there needs to be more work in aerobraking techniques, robust and reusable TPS, and such before this can really become a reality. With current technologies it does make more sense to just come back directly.

~Jon

Posted by Jonathan Goff at December 13, 2007 08:02 AM

Hmm I guess that probably rules out using the Dragon then or can their heat shield be upgraded that much without significant additional weight? Since it's PICA perhaps it could.

Jonathan does the general math look as bleak for higher Earth orbits? I mean if it's all simply a rendezvous (only to switch vehicles) with a mothballed capsule it might as well happen in GEO right? Or is a capsule return from GEO pretty much similar to one from LTO?

Orbital mechanics fascinate me, hand-holding book/study recommendations are welcome ^_^

Posted by Habitat Hermit at December 13, 2007 09:41 AM

The real problem is arriving in the right orbital plane on the return. You might have to wait quite a while for the node of the facility orbit to regress to the point at which you can arrive in it from where you are. Which means that you want multiple facilities, in different orbits.

Posted by Rand Simberg at December 13, 2007 10:09 AM

Hmm I guess that probably rules out using the Dragon then or can their heat shield be upgraded that much without significant additional weight?

Why do you assume that upgrading Dragon's heat shield would be a major problem?

Remember that Elon Musk founded SpaceX and started work on Dragon because he's a leader of the Mars Society and wants to go to Mars -- not ISS. Dragon was intended to *be* the CEV, before Orion was ever thought of. COTS is just a consolation prize.

Posted by Edward Wright at December 13, 2007 11:16 AM

In order to return to a LEO station from the Moon or some other destination, you need to provide just as much delta-V as it took to get to that trajectory in the first place... Now this can either be provided propulsively or using the atmosphere.

Don't forget Buzz Aldrin's cyclers, which eliminate the need to brake most of the return vehicle back into low Earth orbit.

Tethers might also be a possibility.

Ironically IIRC, the peak heat flux and total heat load from coming back from Lunar Orbit to LEO is almost identical to the heat load and peak heat flux for returning to the earth's surface.

However, at the end of the day, you have a spacecraft in LEO that can be used for another trip, rather than a spacecraft on the surface of the Earth. So, you do save the delta-vee needed to launch its replacement for the next trip.

Also, it might allow the use of advanced non-chemical propulsion system like nuclear engines that you wouldn't want to replace for each trip. (Not to mention the environmental problems of reentering a nuclear reactor on each trip.)

Posted by Edward Wright at December 13, 2007 11:45 AM

"Why do you assume that upgrading Dragon's heat shield would be a major problem?"

It wasn't a definite statement so much as it was a question. Based on the monster update the current heat shield is targeted at ISS reentry with a margin based on modeling accuracy that will be shaved off later. I know the PICA material is fairly lightweight as far as heat shields go but I don't know how much additional heat shield mass the Dragon or the bonding method can handle or how much it would need for a lunar return - hence the question.

As for Mars isn't that quite different and much more difficult considering the much thinner and less dense atmosphere? Would the Dragon design/shape/heat shield area in respect to mass/volume be suitable at all and especially with the mroe stringent requirements in respect to entry g-forces on the passengers? I have my doubts but I don't know, it would be great news if you or anyone else can point to some kind of proof or statement from SpaceX that the current Dragon design is intended to be able to handle that or be easily upgraded for it. But I don't think a professed general motivation of eventually reaching Mars is enough on its own.

Posted by Habitat Hermit at December 13, 2007 12:53 PM

Aerobraking to LEO, but not returning to earth, even if it keeps the beefy heatshield, would still be worthwhile specialization. Because you lose the complexity of terrestrial landing system ( parachutes, wings, retrorockets, ocean splashdowns ) from your lunar bus.
A combination of propulsive and aerobraking, like Jon discussed in one of his posts a while ago, would probably be ideal.

Posted by kert at December 13, 2007 01:13 PM

I'm wondering if a gravitational capture not discussed, in the context of making a pitstop on the return. It's all aerobraking or fuel burn (for example, Jon's answer to BigD). Is it because you have to end in a 23 degree orbit, which is harly ideal?

Posted by Pete Zaitcev at December 13, 2007 02:15 PM

I don't know how much additional heat shield mass the Dragon or the bonding method can handle or how much it would need for a lunar return - hence the question.

Why would that be more of a problem for Dragon than it is for Orion? NASA doesn't have a secret bonding method that's unknown to anyone else.

The CEV is meant for the return to Earth -- not to land on Mars.

I guess in the Zubrin Mars Direct architecture, it would land on Mars, but only as part of a much larger vehicle.

Posted by Edward Wright at December 13, 2007 02:44 PM

Interestingly enough, there's been a new "trip to Mars" miniseries running on Science lately. I missed the ep on rocket design, but it seemed fairly biased towards some of Mars Direct's "new conventional wisdom"--prepositioned supplies, artificial gravity (although the parts I saw used a long, rigid ship clearly built in space), 2-year stay, etc. It also included an idea from James Cameron--if your prepositioned hab is at a fixed site, make the rover into the lander, so that if you miss, you can still drive over there. I remember Zubrin putting wheels on the lander, but that's quite a step farther--and it raises a good question for why we couldn't do the same thing (lander=rover) on the moon.

Posted by Big D at December 13, 2007 08:21 PM

Big D, from the looks of the ATHLETE rover chassis, it could easily be adapted to be a lander. It is certainly worth a look:
http://www.youtube.com/watch?v=gDvoe091tk4

Posted by Ed Minchau at December 13, 2007 09:42 PM

Ed, that's very impressive. The wheels look a little small for dealing with rocks and boulders, but I suppose with those legs, most small obstacles can simply be avoided as they are driven over.

I do wonder about the top speed, however--for manned driving, you want to be able to move a number of miles a day, and a hab/lander would need its own reactor as a survival precaution, so you're talking about a good bit of weight there.

Posted by Big D at December 13, 2007 10:32 PM

Edward Wright wrote:
"Why would that be more of a problem for Dragon than it is for Orion? NASA doesn't have a secret bonding method that's unknown to anyone else."

You're reading far too much odd stuff into my questions that simply isn't there ^_^

I would still like something substantial affirming the SpaceX Dragon has been designed to cope with Earth re-entry from Luna or Mars, a theoretical possibility based on ultimate motivation (Mr. Musk wants to go to Mars) doesn't cut it even when I hope you're correct.

Posted by Habitat Hermit at December 14, 2007 07:09 AM

You're reading far too much odd stuff into my questions that simply isn't there ^_^

Since you won't answer my questions, that's all I can read.

I would still like something substantial affirming the SpaceX Dragon has been designed to cope with Earth re-entry from Luna or Mars, a theoretical possibility based on ultimate motivation

It was not the "ultimate" motivation. It was the original motivation. SpaceX originally intended Dragon to go to the Moon and Mars. The design was adapted to go to ISS. Elon has publicly stated that his BFR is designed for Mars missions and Dragon would be the capsule.

As to whether it's possible to build a heat shield that's light enough and a bonding mechanism that's strong enough -- well, Apollo and Zond did that 40 years ago, and Orion will have to do it, too. The laws of physics work the same way for SpaceX as they do for NASA.

Posted by Edward Wright at December 14, 2007 01:00 PM

Why should I bother giving detailed answers to strange questions you make up based on completely nonsensical assumptions you attribute to me? Straw-men on parade as far as I'm concerned; oddly entertaining in this case but still irrelevant to my questions and requests.

Really your assumptions from the very start run counter to what my comments contain. You might want to read them a bit more carefully or face condescending mirth - not much of a threat, I know ^_^

Since you continue to avoid backing up your claims (that would be something more than your memory, not that I don't find it interesting and welcome) on what the current Dragon design is or is not able to do it's my turn to assume that you have nothing more definitive beyond what you've already said on what kind of loads and margins, upgrades to the heat shield etc. the Dragon as it is currently designed can handle. That's ok and I still hope you're right about the Dragon capsule. All the miscellaneous trivia of past achievements or current aims of different capsule designs you've introduced is a conversation/argument you've kept going with someone in your head, not me, and for some strange reason you seem to think that all capsules are created equal. Meh, I digress.

Posted by Habitat Hermit at December 15, 2007 05:34 AM

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