GORDAP - I like it! If abort is only practical for Earth launch, it is an elegant solution.The MCT will somehow need to be able to use Methalox for its thrusters - not sure how that will work. Would there be smaller LOX and Methane containers that are pressurized at high pressure (continuously refilled from the main tanks) that feed the maneuvering thrusters? One could also imagine such a system could be scaled up for abort/landing thrusters.
Agreed, a pushing fast-start 1 second abort motor located in the interstate is a very clever solution.I think the solid booster solution is viable (though SpaceX seems to dislike all pyrotechnics). Also it doesn't need to really be the whole of the inter-stage, all you really need is a naked thrust-structure that connects the abort motor to the bottom of the MCT which can generate and transmit the ~9 million N of thrust. As that is a lot of force I would recommend just sticking cones into the nozzles of the Raptors and pushing on them directly (a bit like the new center-pusher stage separator) so your reusing the thrust structure of the vehicle itself as if the engines were running without having to create new hard-points that bypass the engines. As the Raptors start up they will simply push out the abort motors as they are burning out.Based on the total impulse of the Shuttle boosters and their total mass I would estimate that to deliver 9 million N-s of total impulse would require 15 mT of solid rocket motors, quite heavy. But total take off mass might be lessened by the fact that your putting less abort propellents in the MCT itself. Still it is a hit to take on returning the stage it is on. If that is a first stage it should be do able without much problem, but if it's a 2nd stage as I've proposed it looks like it could be problematic.So I propose to do the Dragon abort trick, use the abort engine to land the 2nd stage in the event that no abort happens. That means if their is an abort the 2nd stage is sacrificed even if the fault is in the first, but your probably fleeing an explosion that was going to destroy it anyway so no big loss. Now the abort system isn't as parasitic because we MUST have smaller vernier engines to land a second stage, the main propulsion system of Vacuum Raptor engines is not going to cut it as their are too few engines and they can't throttle low enough or safely at Sea Level. Though certainly the amount that we need for abort is over kill over what we would for just landing.If a Super-Draco like liquid engine in used you would need A LOT of them, like 120 to get the same thrust, around a 10 m core you should just manage to get that on the vehicle in a giant ring of engines packed side by side. In reality I think we will see some new engine several times more powerful then Super Draco (SuperDuber Dracos??) and the number will be more reasonable. Still a large number of engines will make touch-down very easy, no only can you come to halt really fast but with such a large depth of throttle it should be possible to put the stage down on very delicate tooth-pick landing gear which should save considerable mass.
I am not sure an interstage abort system would work. The mechanical loads on the MCT would be 10 or more times stronger during an abort than during normal operation. The MCT including fuel would need an acceleration of 5+gs as opposed to less than 1. Plus the force to overcome atmospheric pressure at max Q. The parasitic mass would be within the metal structure of MCT instead of fuel and abort angines. I am not sure that is lighter than a crew abort capsule. Also you can't abort from an exploding MCT during first ascend on earth or ascent on Mars. An ascent abort on Mars might be required when a colony is already established and an infrastructure for rescue on Mars is available.
[LAS lifeboat]
I'd like to offer a completely different MCT LAS for consideration:Considering that the MCT will certainly have high efficiency Methalox engine(s) for its main thrust (and perhaps landing?) the issue seems to be the delay time required to 'spin up' the turbines in the case of a mid flight abort. So this means that the MCT really only needs a short burst (<4-5 seconds?) of high thrust (5gs?) to pull it safely away from a disintegrating 1st stage. Correct?So, how about placing an LAS set of engines, with associated fuel, in the interstage between the booster and the MCT? Under normal conditions, the MCT would stage, leaving the interstage attached to the booster, and the booster plus interstage/LAS would RTLS and be completely reused. Under abort conditions, the LAS engines would fire and the interstage/LAS would separate from the booster and propel the MCT a distance away. The MCT main engine(s) (Raptors?) would spin up during this LAS firing period, then the MCT would detach from the interstage and proceed to do an abort landing.The LAS could be fast acting, high thrust (poor ISP) hypergols, or even (gulp) solids. This system wouldn't need to be very massive, given its short firing duration, but it would admittedly still be considerable parasitic mass hurting the RTLS effort.I like this idea because (1) no parasitic mass going to orbit and beyond, penalizing the whole system, (2) entire system is reusable, and (3) unlike 'puller' LAS systems, you don't require that the LAS system cleanly separate from the manned portion as an additional staging event, lest you have LOM.Thoughts?
Impaler, love the idea of using these abort engines as landing engines. That way, their fuel is not parasitic at all! There may be more engines than are needed for landing (so I guess the unused ones are 'parasitic'), but these can also just serve as redundant backups, as they are on the Dragon 2.I'm in the camp that's expecting the BFR to be a single stage booster, with the MCT serving as its own embedded second stage. If this is the case, then the LAS engines could be used on the BFR stage 1 to assist landing, and I'll bet their not much oversized for that mission. Hmm, I think the BFR would still use a center Raptor to do the big initial 'boostback' burn though.
Quote from: Lobo on 10/02/2015 07:23 pm[LAS lifeboat]Essentially you've added the cost of developing a 100-person Super-Dragon capsule, with the added cost of integrating it into MCT like a matryoshka doll. (And you rarely actually use it. Most of the time, all of its independent systems are just dead weight. And it won't have the develop-use-update-use pattern that SpaceX prefers.)If you're going to the expense of developing an extra vehicle, why not just use the 100-person "lifeboat" as a shuttle to ferry passengers to LEO when the MCTs are fuelled and ready to go? It doesn't need to go to Mars.
I agree that this leaves no LAS for the launch from Mars phase of the MCT. But as others have pointed out, it's debatable whether or not an LAS is needed/useful for an SSTO vehicle (which the MCT is at this point).
In a previous article, the same author, Richard Heidmann, seems to have fixed on the idea that BFR is an SSTO, that MCT has no main engine, just belly thrusters for landing. That's led to some weird conclusions, and hence the current article.
This won't weigh down the vehicle when it is serving in a cargo only flight (which will be probably x10 more numerous then crewed flights) and it allows us to change and modify the crew carrying modules for evolving needs and numbers, from small crews which will initially live inside the MCT to future high volume passenger counts which will immediately disembark.
Does cargo really have the same mass-volume relationship as human "cargo"?
So long as the cargo-hold volume is sufficient for the lowest density thing you want to transport then there's no problem, the denser items will just leave some volume unoccupied.
That's like asking why do we have airplanes that are full of only people and ones that are full of only air-freight. That's the nature of every mature transportation system to make a very sharp distinction of between a cargo carrying trip and a passenger trip, it makes the logistics simpler and maximizes passenger comfort....
The topic of a Mars ascend LAS is of course highly debatable. If there is no infrastructure to rescue people after an ascend abort, there is no point in aborting in the first place. But if there is already an established colony, that might be a different topic.If we learned anything from the first shuttle failure, it is that we need to have an abort mechanism. It is not possible to have a system "secure by design" it doesn't work that way.
Cargo MCTs don't need life support.