But why would the passenger version of MCT use the super heavy legs of the full payload version? F9R's legs are removable and added at the launch site, so it would make perfect sense to be able to use less massive legs if you aren't landing a huge payload and that was a big mass driver.
and you land on a prepared surface.
Quote from: Robotbeat on 11/03/2015 12:14 amBut why would the passenger version of MCT use the super heavy legs of the full payload version? F9R's legs are removable and added at the launch site, so it would make perfect sense to be able to use less massive legs if you aren't landing a huge payload and that was a big mass driver.I haven't noticed this distinction before. Dramatically different masses for passenger vs cargo MCT - is that what you meant?Earlier I'd raised this in a different way, saying that a cargo MCT would contain denser materials than a passenger MCT, and thus to have the same mass in the same volume would have "free space" that would be better shared with a small passenger group who would greatly appreciate free space. Some people responded that the cargo MCT would just be packed with extra space. But you seem to be saying they'd actually just have a heavier version. More fuel would be part of that too of course. A higher mass for cargo MCT would impact its velocity (is it a slow trajectory?) and EDL issues/differences as well as launch differences. Actually... perhaps you're saying the opposite. You'd make a regular cargo MCT, but there'd be a low-load light version too for passengers? I would think you wouldn't travel light in a system capable of carrying more.I'd like to read the arguments for this... where/when was it?
Quote from: Lars-J on 11/01/2015 03:47 amQuote from: Robotbeat on 10/31/2015 10:59 pmQuote from: Burninate on 10/31/2015 09:26 pmQuote from: Robotbeat on 10/31/2015 05:25 pmQuote from: Lars-J on 10/30/2015 03:52 am...The size of boulders that would pose a problem for an MCT landing would be faaar to large to handle with a small rover. And boulders are only part of the issue, dust blasting may be more of a problem, and that certainly cannot be addressed by a small rover....Boulders big enough to cause problems for MCT landing are easily spotted using MRO. You can just avoid them in the planning stages.1) MRO can resolve objects of "about a meter across"...MRO has a resolution of ~30cm per pixel. Multiple exposures of the same site from different angles and ESPECIALLY with the Sun at high angles (thus casting long shadows) can identify hazards.Perhaps, but that kind of coverage does not exist.What do you mean? You can, right now, go and request new images be taken of a certain area. There's plenty of "coverage" to take multiple images of the same small area at multiple times of day.
Quote from: Robotbeat on 10/31/2015 10:59 pmQuote from: Burninate on 10/31/2015 09:26 pmQuote from: Robotbeat on 10/31/2015 05:25 pmQuote from: Lars-J on 10/30/2015 03:52 am...The size of boulders that would pose a problem for an MCT landing would be faaar to large to handle with a small rover. And boulders are only part of the issue, dust blasting may be more of a problem, and that certainly cannot be addressed by a small rover....Boulders big enough to cause problems for MCT landing are easily spotted using MRO. You can just avoid them in the planning stages.1) MRO can resolve objects of "about a meter across"...MRO has a resolution of ~30cm per pixel. Multiple exposures of the same site from different angles and ESPECIALLY with the Sun at high angles (thus casting long shadows) can identify hazards.Perhaps, but that kind of coverage does not exist.
Quote from: Burninate on 10/31/2015 09:26 pmQuote from: Robotbeat on 10/31/2015 05:25 pmQuote from: Lars-J on 10/30/2015 03:52 am...The size of boulders that would pose a problem for an MCT landing would be faaar to large to handle with a small rover. And boulders are only part of the issue, dust blasting may be more of a problem, and that certainly cannot be addressed by a small rover....Boulders big enough to cause problems for MCT landing are easily spotted using MRO. You can just avoid them in the planning stages.1) MRO can resolve objects of "about a meter across"...MRO has a resolution of ~30cm per pixel. Multiple exposures of the same site from different angles and ESPECIALLY with the Sun at high angles (thus casting long shadows) can identify hazards.
Quote from: Robotbeat on 10/31/2015 05:25 pmQuote from: Lars-J on 10/30/2015 03:52 am...The size of boulders that would pose a problem for an MCT landing would be faaar to large to handle with a small rover. And boulders are only part of the issue, dust blasting may be more of a problem, and that certainly cannot be addressed by a small rover....Boulders big enough to cause problems for MCT landing are easily spotted using MRO. You can just avoid them in the planning stages.1) MRO can resolve objects of "about a meter across"...
Quote from: Lars-J on 10/30/2015 03:52 am...The size of boulders that would pose a problem for an MCT landing would be faaar to large to handle with a small rover. And boulders are only part of the issue, dust blasting may be more of a problem, and that certainly cannot be addressed by a small rover....Boulders big enough to cause problems for MCT landing are easily spotted using MRO. You can just avoid them in the planning stages.
...The size of boulders that would pose a problem for an MCT landing would be faaar to large to handle with a small rover. And boulders are only part of the issue, dust blasting may be more of a problem, and that certainly cannot be addressed by a small rover....
QuoteBut it is still irrelevant. MCT will need to be able to land on unprepared terrain, it will be necessary to allow of off-nominal EDL and abort scenarios. So it will need a sturdy gear, and you seem reluctant for some reason to admit that.I am in favor of actual abort capability, instead of super heavy landing gear. Why do I need to "admit" something that has no basis in actual SpaceX communications, just some people's opinion on the internet? My mental model of MCT most resembles DC-Y and DC-I, which used fairly stubby landing gear and had real abort capability. (And yes, you'd need Soyuz-style cushioning thrusters on Mars in addition to a parachute. But the actual delta-v for that cushioning thrust is VERY low.)
But it is still irrelevant. MCT will need to be able to land on unprepared terrain, it will be necessary to allow of off-nominal EDL and abort scenarios. So it will need a sturdy gear, and you seem reluctant for some reason to admit that.
....You may not recall this, but the MER scientists were *quite* surprised that the landscape was as flat and featureless as it turned out to be.
This is not what early MCT's will encounter. Even if your fantasy
of tiny ground-clearing robots delivered by Red Dragon's materialize,
the ground is not hard as diamond. It won't be even close to a prepared landing pad on Earth.
And you continue to dodge the fact that the MCT legs also need to support a full propellant load, this will require something significantly more than an "elephant on stilts" approach.
Quote from: Robotbeat on 11/03/2015 12:14 amand you land on a prepared surface.Robotbeat, you have mentioned this 'prepared surface' in several posts. How this would be accomplished?
Quote from: oiorionsbelt on 11/03/2015 12:38 amQuote from: Robotbeat on 11/03/2015 12:14 amand you land on a prepared surface.Robotbeat, you have mentioned this 'prepared surface' in several posts. How this would be accomplished?NASA has some ideas so maybe SpaceX will get some advice from from them.http://www.nasa.gov/content/landing-pads-being-designed-for-extraterrestrial-missions
It's as if you and Impaler want MCT to be a non-viable, heavy monstrosity that barely can push its own weight around. It's almost as if you're not trying to imagine how to reduce dry mass in meaningful ways that just about any aerospace undergrad would think of (let alone a team of seasoned professionals).
"Of all the substances we studied, ablative materials seem to work best," Metzger said.
Quote from: Robotbeat on 11/03/2015 02:32 amIt's as if you and Impaler want MCT to be a non-viable, heavy monstrosity that barely can push its own weight around. It's almost as if you're not trying to imagine how to reduce dry mass in meaningful ways that just about any aerospace undergrad would think of (let alone a team of seasoned professionals).I think it's about having "conservative" mass estimates instead of "optimistic" ones, a.k.a. why Mars Direct was revised (e.g. Semi-Direct, DRM 3.0).
Quote from: Pipcard on 11/03/2015 02:58 amQuote from: Robotbeat on 11/03/2015 02:32 amIt's as if you and Impaler want MCT to be a non-viable, heavy monstrosity that barely can push its own weight around. It's almost as if you're not trying to imagine how to reduce dry mass in meaningful ways that just about any aerospace undergrad would think of (let alone a team of seasoned professionals).I think it's about having "conservative" mass estimates instead of "optimistic" ones, a.k.a. why Mars Direct was revised (e.g. Semi-Direct, DRM 3.0).The problem with that is "conservative" mass fractions are NOT realistic!If NASA, for instance, were doing initial design of F9 v1.1 full thrust and Falcon Heavy, no way would they contemplate giving the boosters mass fractions of 25 and 30, respectively.Heck, when NASA was trading ULA's Centaur for ESAS, they literally sandbagged the dry mass of Centaur 50% more than it actually weighs today! How they managed to do that, I do not know.But "conservative" mass fractions are the least likely for a company that both prides itself in world-class mass-fractions and that can't afford building something much larger and with extra stages than necessary. SpaceX doesn't need to spread work amongst several Centers, there's no reason they'll want to expand the number of stages beyond what they think they can achieve. "Conservative" is not realistic when you don't have infinite money.
QuoteThis is not what early MCT's will encounter. Even if your fantasyGive me a break.Quoteof tiny ground-clearing robots delivered by Red Dragon's materialize,If you think a 10-ton bulldozer is "tiny"
Construction equipment taken to Mars will probably be made from aircraft aluminum or titanium to reduce weight.
"A rocket is typically tested only once with a dummy payload with subsequent launches being with paid customers who have satellites."
Quote from: meekGee on 11/02/2015 02:41 amBut we know that SpaceX, even in the fast build-up scenario, needs to send multiple unmanned payloads to Mars first.BFR is an Earth-to-orbit vehicle, and so you're not risking a two-year delay when you first launch it. Yes, the first launch may not carry an MCT, but by the same token, it won't carry any other one-of-a-kind payload. Being reusable, it's a complete non-brainer to fly a dummy payload. If it works, you you relaunch. It it doesn't, good thing you didn't.Actually, once you're shifted to a reusable rocket, test flights are cheap enough that you could do multiple dummy payloads before you put an MCT on top - and it's not like you wasted multiple rockets doing it.So I think the "build up flight history" argument doesn't apply.It will be cheaper per kg to LEO, but it is by no means going to be cheap enough to build an adequate flight history (~12 launches) with nothing but dummy payloads all at SpaceX's own expense. A rocket is typically tested only once with a dummy payload with subsequent launches being with paid customers who have satellites. I expect prices of ~200 million per launch even with recovery (about $1000 per kg, Musk's optimistic goal) so it would cost Billions to do these launches without customers.I expect that 1st stage recovered will be attempted on every single BFR flight with very likely full success from the start. If first stage recovery works on the dummy flight and is declared a solved problem then the price point will likely be set such that SpaceX is fully covering the cost of the 2nd stage in case it is lost as I expect 2nd stage recover to require a long campaign of attempts with lots of failures and redesigns as we have seen with F9, the customer will not care any more about the success or failure of these recovery attempts any more then they care about 1st stage recovery attempts now. By the time you have your 12 flight history your close to getting the 2nd stage to recover reliably and can drop the price to perhaps 100 million and try to get more volume.
But we know that SpaceX, even in the fast build-up scenario, needs to send multiple unmanned payloads to Mars first.BFR is an Earth-to-orbit vehicle, and so you're not risking a two-year delay when you first launch it. Yes, the first launch may not carry an MCT, but by the same token, it won't carry any other one-of-a-kind payload. Being reusable, it's a complete non-brainer to fly a dummy payload. If it works, you you relaunch. It it doesn't, good thing you didn't.Actually, once you're shifted to a reusable rocket, test flights are cheap enough that you could do multiple dummy payloads before you put an MCT on top - and it's not like you wasted multiple rockets doing it.So I think the "build up flight history" argument doesn't apply.