Quote from: Robotbeat on 10/14/2015 01:41 amQuote from: Impaler on 10/13/2015 05:35 amQuote from: meekGee on 10/13/2015 05:07 amI'll also add that in addition to the trip duration, there's the "land the whole thing" mentality. If you're talking about ISS-esque solar array and radiator array, the idea of stowing them before EDL is not practical.If instead you have a "Hermes"-type orbit-to-orbit ship in mind, then now SEP starts making sense.But again, this is about MCT.We KNOW SpaceX is considering SEP for Mars, and as it is impossible to land a SEP, that mean IPSO FACTO that they are have ALSO considered a Semi-Direct architecture of a transit-vehicle and a separate landing vehicle. And that such a vehicle trades very well against a single massive direct vehicle.It's not, in fact, impossible to land an SEP on Mars. Just kind of odd and mass inefficient. But with a sufficiently good solar array, you could do it. I should point out that an "ISS-esque" solar array is, in fact, retractable!RB. You just used the word "in fact" along with stating you can build an engine with a T/W of 0.25, and an ISP in the thousands.Forget Mars... With your engine you can roam the solar system at will! You need a sanity check sometimes. You can't just add brochure numbers for this and that (solar panels, thruster) and arrive at meaningful numbers.
Quote from: Impaler on 10/13/2015 05:35 amQuote from: meekGee on 10/13/2015 05:07 amI'll also add that in addition to the trip duration, there's the "land the whole thing" mentality. If you're talking about ISS-esque solar array and radiator array, the idea of stowing them before EDL is not practical.If instead you have a "Hermes"-type orbit-to-orbit ship in mind, then now SEP starts making sense.But again, this is about MCT.We KNOW SpaceX is considering SEP for Mars, and as it is impossible to land a SEP, that mean IPSO FACTO that they are have ALSO considered a Semi-Direct architecture of a transit-vehicle and a separate landing vehicle. And that such a vehicle trades very well against a single massive direct vehicle.It's not, in fact, impossible to land an SEP on Mars. Just kind of odd and mass inefficient. But with a sufficiently good solar array, you could do it. I should point out that an "ISS-esque" solar array is, in fact, retractable!
Quote from: meekGee on 10/13/2015 05:07 amI'll also add that in addition to the trip duration, there's the "land the whole thing" mentality. If you're talking about ISS-esque solar array and radiator array, the idea of stowing them before EDL is not practical.If instead you have a "Hermes"-type orbit-to-orbit ship in mind, then now SEP starts making sense.But again, this is about MCT.We KNOW SpaceX is considering SEP for Mars, and as it is impossible to land a SEP, that mean IPSO FACTO that they are have ALSO considered a Semi-Direct architecture of a transit-vehicle and a separate landing vehicle. And that such a vehicle trades very well against a single massive direct vehicle.
I'll also add that in addition to the trip duration, there's the "land the whole thing" mentality. If you're talking about ISS-esque solar array and radiator array, the idea of stowing them before EDL is not practical.If instead you have a "Hermes"-type orbit-to-orbit ship in mind, then now SEP starts making sense.But again, this is about MCT.
Would anyone like to do the work of reversing that spreadsheet so we could look at more refined estimates of return delta V? I might try it at some point, but not today.
Quote from: Lars-J on 10/14/2015 11:15 pmThat has always been my assumption - that every new site would have not just one, but several unmanned MCT land to set up ISRU and other equipment. Most of those initial unmanned "pathfinder" MCT would not be returned, I would expect - They would instead become the first outpost habitats, storage sheds, and MCT spare part depots.Hmmm, in theory, only the first site or two would need that. Once you have the initial 3-6 MCTs left at those 1-2 sites, they could be used as suborbital hoppers for ferrying equipment (even people) to secondary sites to prep them for the incoming (fully reusable) MCTs.
That has always been my assumption - that every new site would have not just one, but several unmanned MCT land to set up ISRU and other equipment. Most of those initial unmanned "pathfinder" MCT would not be returned, I would expect - They would instead become the first outpost habitats, storage sheds, and MCT spare part depots.
Quote from: Lobo on 10/08/2015 11:13 pm[images of Apollo CM+SM and Dragon+trunk]Apollo didn't and Dragon doesn't do EDL with those modules attached. The level of integration your require is an order of magnitude more complex.
[images of Apollo CM+SM and Dragon+trunk]
Quote from: Rocket Surgeon on 10/14/2015 03:02 amJust on the Lifeboat idea, I know it was mentioned back-thread that it would be making things a lot more complex, but I think it would be worth it. Why? Not only can you Abort during Launch and EDL both at Earth AND Mars anywhere through the flight, Don't make the mistake of thinking that all an abort capsule needs to do is taking passengers away from an imminent explosion, to just survive the imminent danger. That works on Earth, because rescue is nearby, and the environment is survivable.But that is NOT enough for Mars. An abort capsule is USELESS if it just dooms you to death a couple of hours later. To be a proper abort/rescue vehicle it basically needs to be a fully independent spacecraft/lander, built into another spacecraft/lander. By adding enough rescue/survival capability, you get into a dark spiral that eventually leads you to conclude that your escape capsule needs another escape capsule.But there is another way. Build safety margins and redundancy into your MCT/lander and forego an abort system that is effectively useless.
Just on the Lifeboat idea, I know it was mentioned back-thread that it would be making things a lot more complex, but I think it would be worth it. Why? Not only can you Abort during Launch and EDL both at Earth AND Mars anywhere through the flight,
[Dragon's trunk] It is 2/3 of the way to a lifeboat on a biconic MCT.
So, how often have liquid boosters went "boom" in the US?How important is it to protect against that one failure mode?
Quote from: Burninate on 10/14/2015 11:14 pmWould anyone like to do the work of reversing that spreadsheet so we could look at more refined estimates of return delta V? I might try it at some point, but not today.Orbits are time reversible so don't we just need to look at the 'Vinf at mars' and calculate the DeltaV needed to archive that escape velocity from mars surface, then we would (if we pointed ourselves in the right direction) be headed back down the equivalent half of the outbound orbit and we should reach Earth in the specified time and with the specified Vinf so we know what we need to do to aerocapture at Earth as well.It would be nice to have this done for us on the spreadsheet though.
Quote from: Lobo on 10/15/2015 07:43 pmSo, how often have liquid boosters went "boom" in the US?How important is it to protect against that one failure mode?Antares, October 28, 2014.Falcon 9, June 28, 2015.That's two in the past 12 months.At least launching from Earth, it's pretty important. As you mentioned, for early MCT missions with a small crew, send the crew up on a Dragon 2.
Quote from: RonM on 10/15/2015 08:12 pmQuote from: Lobo on 10/15/2015 07:43 pmSo, how often have liquid boosters went "boom" in the US?How important is it to protect against that one failure mode?Antares, October 28, 2014.Falcon 9, June 28, 2015.That's two in the past 12 months.At least launching from Earth, it's pretty important. As you mentioned, for early MCT missions with a small crew, send the crew up on a Dragon 2.Not quite. I was referring to exploding boosters.As I understand, F9's was the 2nd stage which blew up causing the booster to then fail, which is different than the booster itself exploding, and trying to abort away form it. If that were an Integrated MCT, then the LAS lifeboat would be within a few meters of the exploding tank. Could an LAS could save it with such close proximity? (Which was one of the problems with putting Orion on a side mounted SDHLV if I recall correctly. Orion would be right next to the ET, so even with an LAS system, it's unlikely the crew could get away from a sudden ET explosion).The F9 v1.1 booster failure from a few years ago was an engine out, which as I said, would be accounted for.Antares was also an engine out. But with just 2 engines, there was no engine out redundancy so they terminated the LV, as I understand. Had Antares had engine out redundancy, there's not reason to think it wouldn't have made staging nominally.
But, yes, for that first several dozen flights of MCT, the crew can just go up on a simple F9/D2. There will be 4 active Falcon pads in operation after one, 3 on the East Coast. One of those launched per Mars mission seems pretty reasonable.By the time you'd be looking at putting 100 people on it, there'll be many exploration missions over decades. You'll probably have a pretty good idea of that point if it's reasonable to put people on MCT for launch, or go another route like a "Big Dragon" on a FH or something that can hold 100 people, and have a Earth LAS system. Or some other route.
Quote from: Lars-J on 10/14/2015 05:50 amQuote from: Rocket Surgeon on 10/14/2015 03:02 amJust on the Lifeboat idea, I know it was mentioned back-thread that it would be making things a lot more complex, but I think it would be worth it. Why? Not only can you Abort during Launch and EDL both at Earth AND Mars anywhere through the flight, Don't make the mistake of thinking that all an abort capsule needs to do is taking passengers away from an imminent explosion, to just survive the imminent danger. That works on Earth, because rescue is nearby, and the environment is survivable.But that is NOT enough for Mars. An abort capsule is USELESS if it just dooms you to death a couple of hours later. To be a proper abort/rescue vehicle it basically needs to be a fully independent spacecraft/lander, built into another spacecraft/lander. By adding enough rescue/survival capability, you get into a dark spiral that eventually leads you to conclude that your escape capsule needs another escape capsule.But there is another way. Build safety margins and redundancy into your MCT/lander and forego an abort system that is effectively useless. Lars,You make some good points here.My initial thought on the IBMCT is just that, to not have an LAS at all. To use Dragon 2 as a LEO taxi for exploration crews for the first couple of decades until MCT builds up a track record of reliability. Then go with the Space Shuttle or Airliner model when it's time to put a full 100 passengers on board. MCT should have dozens of launches under it's belt by then.It'd have full booster engine out capability, as well as full IBMCT engine out capability. So an engine out during ascent on either stage would still result in a nominal LEO insertion.It doesn't protect against one failure mode, an exploding booster. It's popular to look at the booster explosions on the N-1 tests and take that to mean you must protect against that, but what really needs to be looked at is how often to American liquid boosters actually explode once they are out of test phase and in to actual production? I don't know the answer to that exactly, but I don't think it's very often. Especially in modern times. The Challenger flight is one of the most famous examples, but that really wasn't a liquid booster failure. It was an o-ring joint failure from a segmented SRB than then caused a failure of the liquid booster. Had the O-ring failed on the outboard side so it didn't burn into the side of the ET, the stack likely wouldn't have exploded. Once detected they probably would have done an emergency orbiter abort and glided to a contingency landing site. Or the booster may have lasted to SRB staging.There was a Delta II that failed spectacularly not so long ago, but I think that was an SRB failure as well, not of the liquid core. I don't think there was a booster failure of the a production Saturn 1B or Saturn V, or Atlas II, III, or V, or Delta IV, III, or II outside of the SRB failure.And I think most of the Titan IV failures were SRB issues, and not the core (which wasn't really a booster, but a 2nd stage that ignited after SRB sep.)So, how often have liquid boosters went "boom" in the US?How important is it to protect against that one failure mode?
Quote from: Impaler on 10/15/2015 02:17 amQuote from: Burninate on 10/14/2015 11:14 pmWould anyone like to do the work of reversing that spreadsheet so we could look at more refined estimates of return delta V? I might try it at some point, but not today.Orbits are time reversible so don't we just need to look at the 'Vinf at mars' and calculate the DeltaV needed to archive that escape velocity from mars surface, then we would (if we pointed ourselves in the right direction) be headed back down the equivalent half of the outbound orbit and we should reach Earth in the specified time and with the specified Vinf so we know what we need to do to aerocapture at Earth as well.It would be nice to have this done for us on the spreadsheet though.They're reversible, but you're forgetting the Oberth effect: because on the way from Earth to Mars, you can dump your exhaust in a deeper gravity well than Mars to Earth, it takes less delta-v.
Impaler: I don't think Musk mentioned 100 day trajectory on the return trip, just on the way there. Also, exactly 100 is a little bit of, um, spurious precision. 102 days is essentially the same thing. (I know you know this, just want to point it out.)Not sure SpaceX would start at LEO. A high orbit seems more realistic, as it allows you to leverage SEP without actually including SEP on MCT directly. SEP would be just used to haul up propellant from LEO. This helps reduce IMLEO a LOT.But yeah, I still think MCT will start on the surface of Mars and go straight to Earth, with a much-longer-than-100-day trajectory.