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.In spacecraft going to mars we have a huge savings in propellants if we go slow but this is deleterious to human health so we send them fast and freight slow, this is all the incentive we need to segregate passengers and freight trips. And their is almost certainly a reduction in marginal mass needed as passenger counts grow due to the redundancy needed for probabilistic equipment failure and the ability to time-share public space and amenities, that's why the service is better on a 747 then on a piper-cub.
Quote from: Lobo on 10/02/2015 07:23 pmA top mounted thruster system like this would be inherrently stable, unlike landing on a tail engine (F9 booster)....like a helicopter is stable with it's thrust up on top and it's mass hanging below it.I'm rather surprised that you aren't aware of the Pendulum Fallacy: https://en.wikipedia.org/wiki/Pendulum_rocket_fallacy
A top mounted thruster system like this would be inherrently stable, unlike landing on a tail engine (F9 booster)....like a helicopter is stable with it's thrust up on top and it's mass hanging below it.
Quote from: Semmel on 09/30/2015 08:51 pmNice concept. But quite frankly, I like the concept developed in L2 better.If you aren't willing to show/share it, don't bring it up here. This kind of information sand boxing is why I'm not joining L2 anytime soon. Just my personal opinion...
Nice concept. But quite frankly, I like the concept developed in L2 better.
Lobo: Is that L2 vehicle concept posted anywhere where us plebs can read it?As I'm sure you remember I favor the 2nd of your scenarios, "Another is launching MCT dry on a 2nd stage rather than the integrated design, with just enough propellant to power it's LAS engines. So the whole dry MCT is aborted and lands. For a nominal launch, it docks with the depot, and the 2nd stage does it's own EDL and returns to launch site."
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?
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.
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.
[LAS lifeboat]
The difference is, that in an abort, the MCT (assuming it is its own second stage) has to be abortet including its entire mass of fuel. I think I remember that the dry mass of the MCT is about 20 % of its wett mass. So the 3g in normal operation would be reached near dry mass. The 5 g abort would be reached at wett mass. The force on the metal structure is F=m*a, mass times acceleration. Wett mass is 5 times dry mass, acceleration is higher, and air resistance is higher. So during about, the structure of MCT must take I would guess more than 10 times the force of normal operation. The metal structure must be sized accordingly.
In the one stage BFR your looking at the combo 2nd stage MCT vehicle is going to be considerably more massive then the smaller and un-fueled one I'm looking at, that will mean probably 3x more thrust needed for separation. Rather then the 9 million N I'm estimating for a 200 mT MCT your looking at 27 million N, the thrust of 12 raptor engines. I see a problem with rapidly initiating that much thrust INSIDE the interstate when the vehicle is still assembled, it would be like setting off a bomb. So the thrust must be vented laterally with Dragon like canted rocket nozzles along the perimeter of the inter-stage. With the cosine loss were looking at something like 400 Super Draco equivalent rockets. Do able probably by having multiple bands of nozzles such that the entire outer surface of the inter-stage is nozzles. But we should be aware that were increasing the number of parasitic engines, though they parasatizing the first stage rather then the 2nd which means they count for only 1/10th as much in final payload.
The decisive factor I think is that in a 2 stage BFR with the abort engines atop the 2nd stage and below a smaller MCT is that you can abort during both first AND second stage burn. You only give up the abort motor once full Earth orbit has been achieved. Where as with a one stage BFR your in a black zone after first stage separation and yet your still attached to a potential bomb which has 6-7 raptor engines which might set the thing off.
6 posts in a row? Is there some obscure forum record you are trying to break, Lobo?
Ok let me just touch base on terminology and what looks to be a set of acronyms that will describe them.Super Dragon Mars Colonial Transport (SDMCT): A dragon shaped vehicle which lands with a bottom side heat-shield and is powered by either side-wall engines or a heat-shield penetrating central engine. It launches on a 2 stage rocket which is essentially an enormous F9 colloquially known as the BFR. Least imaginative design, initially popular with many but now falling out of favor. Would abort as Dragon dose by pushing away the whole vehicle from the 2nd stage.Separate Bi-conic Mars Colonial Transport (SBMCT): A obviously bi-conic vehicle with bottom mounted engines and a horizontal high lift entry orientation that keeps engines and other systems in the rear. Propellant would be in the nose of the vehicle and cargo in the base just above engines. Launches on top of a conventional 2 stage rocket comparable to the SDMCT. Would abort similar to SDMCT by pushing the whole vehicle away from the 2nd stage. Designed around single stage to Low Mars orbit and a 'semi-direct' like architecture. My currently preferred solution.Integrated Bi-conic Mars Colonial Transport (IBMCT) A modification of the SBMCT which 'integrates' it with the 2nd stage of the rocket it would launch on such that their is a bi-conic nose and a cylindrical aft body, the tip of the nose is an abort capsule, the next section down is cargo and propellant fills the cylinder. Launches on a single stage rocket which is similar to the first stages of the alternative configurations. Designed specifically for Single stage Direct Earth return. Lobo and others currently preferred solution.Dose that look like an accurate depiction of the current positions?
We can all agree that MCT is SSTO on MARS, and that means the only means of abort would be disassembly of the vehicle via something like a nose-cone escape crew capsule.This should be avoided because we shouldn't even HAVE an integrated crew cabin, if the MCT simply has a large cargo hold in which cargo or crew modules are interchangeably placed then it becomes possible to make a powered escape module and place this into the vehicle when ever crew are on board.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.Also note that when aborting to the Mars surface your possible landing sights are very nearly a great circle around Mars as your point of landing varies becomes a track of the landing point of a sub-orbital flight, if it is late in the MCT assent phase and thrust is lost then your looking at a landing sight possibly on the opposite side of the planet from where you launched. Not a feasible distance from a base for rescue to pick up survivors even if they were landed without a scratch, an extremely difficult matter when your basically performing full EDL which is a process which requires considerable heat-shields and retro-propulsion.
No one is going to be unloading cargo by HAND, your going to be using forklifts, scissor-lifts movers and other wheeled equipment to take out large containers which would then be taken into garages or coupled to pressurized areas, this is what we do already on ISS for Christ sake. The lack of basic knowledge of logistical functions here is maddening sometimes.MCT doesn't need life-support because their is only a need for ONE vehicle variant which is a universal carrying shell, you put pressurized habitat modules (which has all the life-support equipment in it) in when you want to carry passengers and you put cargo containers in it when you want to carry cargo. Why is this so hard to understand? It is infinity more flexible and efficient then any other configuration.
The returning MCT should only have a small crew though, as most would be colonists left there.
The working idea is that all concepts would have a Dragon 2 like nose hatch (maybe about 5m wide or so) under which could be a docking collar. Tanker and depot dock in LEO and proceed to do a slow spin to settle propellants in the bottom of the tanks where it can then be pumped from the tanker to the depot, and later from the depot to MCT.
That last is where a separable lifeboat LAS has an advantage. Since MCT is a SSTO for Mars ascent, if there's an explosive problem with the main propulsion system, the crew is lost. That'd be the case for full vehicle abort of the SDMCT and NIBMCT. The LAS lifeboat can get away and then "fly" down range to a landing spot within it's ascent trajectory. What they do when they land safely is another story.If in the early exploration phase, the returning crew is small. If the lifeboat is of sufficient size to support them until they can get to a backup vehicle somewhere else on the surface, then that would be good. If later in colonization phase, there'd be a full colony with presumably long range rovers that could be dispatched for rescue. The returning MCT should only have a small crew though, as most would be colonists left there. So you really shouldnt' have to rescue 100 people, unless there's some abort from orbit and the lifeboat lands somewhere far away form the intended LZ The IBMCT would have 3 Raptors, but only two are needed for Mars ascent (maybe only one). It has 3 because 3 are needed to get it from booster staging to LEO. So it has engine-out backup for Mars EDL (with LAS assist) and it has engine out backup for Mars ascent. And if the MPS exploded, it's LAS can abort the lifeboat away.The SDMCT and NIBMCT have only two engines, as they don't do Earth ascent. If there's an engine out at Mars ascent, there's some issue of how to handle it. One engine thrusting at full throttle should keep it aloft long enough to burn off enough propellant so the LAS system can land it again for an emergency landing. It might not be able to get to orbit, but it should be able to do a controlled emergency landing. And it depends when the engine were to fail, far enough up just one engine should still get it home.
And you have nothing in it to be a tanker (perhaps stretched tanks within the same Outer Mold Lines). Just a big empty volume.And you have some cryo-refrigeration equipment in it as a depot.And you put a payload bay behind the nose to deploy FH and D4H class satellites to GTO. The vehicle then comes back around on the GTO eliptical orbit to Earth, where it does a small deorbit burn and does Earth EDL. Not expendable PLF required.The working idea is that all concepts would have a Dragon 2 like nose hatch (maybe about 5m wide or so) under which could be a docking collar. Tanker and depot dock in LEO and proceed to do a slow spin to settle propellants in the bottom of the tanks where it can then be pumped from the tanker to the depot, and later from the depot to MCT. Propellant lines would run from the nose to the tanks. (unless there's a way to pump liquids in zero g without using centrifugal forces? That's a bit of a gray area.)The crew MCT would also have an access tunnel for access to the flight deck and on through to the hab area below it.The Sat launcher would have a payload bay under the nose cap instead of a docking collar or access tunnel. Something like Rocketplane Kistler K-1 (below). The sat launcher has no need to refuel, so no need for a docking collar installed.OML's are the same.This is where I prefer the IBMCT. Just one vehicle, rather than than two including the dedicated SII. It's not an expendable "dumb" stage, it has to be it's own LEO reusable spacecraft in it's own right. And may or may not share much with the booster and MCT. There's extra cost in that.The IBMCT would have the same tank tooling, bulkheads, etc as the booster. So just 2 pieces with a lot of commonality. That wouldn't be the case for the Super Capsule. And may or may not be for NIBMCT depending on it's design. If a full biconic, then it won't share much with anything else. If a cylinder/biconic then it might, but it will run into issues that using booster sized tank tooling and domes will make the LOX tanks too big using 12m domes, because the tanks are smaller than IBMCT. So you might need custom tankage anyway.