Author Topic: MCT Speculation and Discussion Thread 4  (Read 568250 times)

Offline GORDAP

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Re: MCT Speculation and Discussion Thread 4
« Reply #580 on: 10/03/2015 01:07 PM »
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.

Lars, glad you like the idea, but I'm not clear how it affects the thrusters and/or landing engine choice for the MCT itself.  Though I kinda expect, given the very long duration of end-to-end mission for the MCT, that you'd want to use the same storable, ISRU producible fuel combination (Methalox) for these things as you would for the main propulsion in any case.

Offline GORDAP

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Re: MCT Speculation and Discussion Thread 4
« Reply #581 on: 10/03/2015 01:25 PM »
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.

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.

Offline GORDAP

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Re: MCT Speculation and Discussion Thread 4
« Reply #582 on: 10/03/2015 01:37 PM »
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.

Semmel, not sure I'm following your arithmetic re loads.  Whether the BFR is one stage or two, I'd expect the MCT to see in the neighborhood of 3g's at burnout before separation.  In fact, I'd think the BFR will need to throttle down to stay under this limit to protect people and payloads.  That is the force that will be seen at the interstage/MCT interface.  If the LAS is in the interstage, and it gives a short burst of 5g's during an abort, that's all this interface will see, which is 67% more, not 10X.  What am I missing?

Agree that the LAS in my proposal would have to be significantly larger to abort the entire MCT, with it's payload and fuel, as opposed to just jettisoning a smallish crew cabin.  But the upside of course is that you don't lose the MCT and payload.  I have no idea if that will be worth it to the designers of this system.

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).

[All - sorry for my 3 posts in a row, but I haven't posted in a while and I need to get it out of my system  ;) ]
« Last Edit: 10/03/2015 01:38 PM by GORDAP »

Online Semmel

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Re: MCT Speculation and Discussion Thread 4
« Reply #583 on: 10/03/2015 03:15 PM »
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.

Offline Paul451

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Re: MCT Speculation and Discussion Thread 4
« Reply #584 on: 10/03/2015 03:45 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.
« Last Edit: 10/03/2015 03:46 PM by Paul451 »

Offline Burninate

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Re: MCT Speculation and Discussion Thread 4
« Reply #585 on: 10/03/2015 09:12 PM »
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?
The question is implicitly about the power to weight ratio of rocket engines.

Ejecting an MCT at somewhere between 500 and 5000 tons at 5-10 G's, even for a second or two, is nontrivial.

Offline Impaler

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Re: MCT Speculation and Discussion Thread 4
« Reply #586 on: 10/03/2015 09:20 PM »

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.

Glad you approve, as you've given me a major solution form the configuration I'm pursuing it seems the least I could do.  And as you say the same principle applies if your BFR is one stage or two, most or all of the propellents will actually cause no parasitic loss, though the engine hardware would the expellent T:W ratio of SpaceX engine technology minimizes this.

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.

Also with regard to g-forces on the structure of the MCT, I've described a ~5 g abort acceleration which is comparable to Dragon, this would certainly put a lot of stress of the structure but because the MCT has cargo and only minimal propellant it's mass is half of what it would be at Mars takeoff.  The arrangement of propellent over cargo that I'm assuming means that the thrust structure that supports the propellant is under comparable load and won't collapse onto the cargo.   Likewise when ascending on Mars the rising acceleration is balanced by the falling propellant mass to keep forces under control.

Offline Burninate

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Re: MCT Speculation and Discussion Thread 4
« Reply #587 on: 10/03/2015 09:23 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.
You come perilously close to one of my ideas: using said Super-Dragon for Earth Abort, Earth Return, as MAV, and as Mars landing propulsion, then returning the main habitat & cargo section of MCT a year later.  I am not yet decided on incorporating that element: It becomes favored if the ISRU equipment has a low propellant mass production ratio, and I don't have firm numbers for that.

Offline GregA

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Re: MCT Speculation and Discussion Thread 4
« Reply #588 on: 10/04/2015 08:22 AM »
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).
Remembering that the point of the MCT is that there IS a colony on Mars, one would assume that there will be a support and rescue service of some form. One of the side effects of "wild west" multi-company independent and only semi-regulated growth in a colony is that it will create a risk level for some colonists that NASA would never accept (and the implications of that will be important in many ways, but not for this thread). I actually suspect that the rescue service will be a private venture too, that's pretty well the point of this colonisation model.

So rescue from an abort capsule will be possible on Mars.

But I agree that the LAS on Mars is not a necessity. There are practical constraints that limit what can actually be achieved and there are some hefty goals here that we don't want thrown out (including affordable settlement generally). SpaceX intends for only small numbers of people to return, so numbers launching back should be low, and I suspect that the risk on the surface of Mars will be far higher than the risk of a launch failure anyway.

If it can be done they'll try though, and it'll be made more possible if it can easily share the same system used for the Earth LAS or an in-transit lifeboat or whatever.

Online Semmel

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Re: MCT Speculation and Discussion Thread 4
« Reply #589 on: 10/04/2015 07:46 PM »
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. How often do you suppose a return MCT can fail before people think that the Mars Colony is a death trap? I dont know how to come to a conclusion on that topic. Given how strong the influence of that topic is on the design of any MCT, it would be extremely helpful to settle the topic, one way or the other.

Offline Robotbeat

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Re: MCT Speculation and Discussion Thread 4
« Reply #590 on: 10/04/2015 09:29 PM »
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.
Of course once you get to 50km or so, you could flip around and use those belly thrusters to achieve Earth Orbit or use them for Mars injection. I mean, a little dumb, but still possible.
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Offline Impaler

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Re: MCT Speculation and Discussion Thread 4
« Reply #591 on: 10/05/2015 04:05 AM »
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.

Basically you want to stay WITH the MCT as long as possible as it by necessity is capable of EDL and should be loaded for more then enough propellant to do a landing until very near the point when orbit is achieved, so you may be able to transition directly from a once-around the planet controlled landing abort to an abort to orbit.  If the MCT blows up your just SOL, but the next most likely problem with be Engine-out so I recommend the MCT have abundant surplus engines such that it can sustain one or more engines being lost and still achieve either orbit OR a once-around abort.

A MCT with a 400 mT gross take of mass has a weight of ~1,500 kN which would allow it to ascend on just one Raptor engine, but a single central engine would make the failure of the single engine would leave you with no propulsion.  So I'm looking at two pairs of engines with nominal flight being to ascend on one member of each pair in a diagonal line, in the event of a failure the opposite diagonal  pair engages.

Offline GregA

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Re: MCT Speculation and Discussion Thread 4
« Reply #592 on: 10/05/2015 08:14 AM »
The 4 engine setup makes sense as a failsafe.

I think trying to model how an aircraft handles issues is worthwhile in a SSTO rocket ... expanding from methods for aborting before take off to the options they have if something goes wrong in the first minute of flight and so on (i.e. planes don't need a way of ejecting 100 passengers if something goes wrong). Have options which include aborting to preset locations around Mars, or a full orbit abort, abort to LMO (with a plan to land soon after?), even dropping power 5 seconds after take off to slow down and land (if feasible)... the point being a new set of options that increase safety.

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"?

Offline Impaler

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Re: MCT Speculation and Discussion Thread 4
« Reply #593 on: 10/05/2015 09:38 AM »

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.  I'm looking at a volume of 500 m^3 for the cargo hold so minimum allowable density is 200 kg/m^3 or about 12.5 lb/ft^3 a number I selected specifically to match the average density of airfreight cargo.

Now lets look at a reasonable habitat analog the ISS Destiny module which masses 14 mT and has a volume of 100 m^3, fill the cargo hold with something that density and your mass if 72 mT which is ball park what we can carry and I'm sure pushing the density up a bit is feasible particularly when a lot of it will be consumables like food and water and we don't need such a big corridor down the middle.

Offline GregA

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Re: MCT Speculation and Discussion Thread 4
« Reply #594 on: 10/05/2015 10:48 PM »
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.

If the ECLSS for 30 people is twice as big as the ECLSS for 15 people... which I'm not sure is true at all... then splitting the people between 2 MCTs allows the unoccupied volume to be used for personal space for the passengers.

That unoccupied space is very valuable for people, just not valuable for cargo. (I would say for initial missions there is value in pre-assembled Mars-Cars, but otherwise not).

(My question still ties back to the value of 9 cargo MCTs and 1 passenger MCT, vs 10 passenger+cargo MCTs)

Offline Impaler

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Re: MCT Speculation and Discussion Thread 4
« Reply #595 on: 10/06/2015 12:58 AM »
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.

Offline mme

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Re: MCT Speculation and Discussion Thread 4
« Reply #596 on: 10/06/2015 01:40 AM »
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.
...
Virtually all large passenger aircraft also carry cargo because it's nice source of income.  The logistics are not difficult.  Passengers are not inconvenienced by ULDs that contain cargo.  There are financial advantages to being a "cargo only" airline, dealing with humans is a hassle.  You need to provide food, flight attendants, etc.  But there is no financial advantage to being "passenger only" and the airline business is not one that can afford to leave money on the table.

There may be all sorts of reasons to separate cargo and humans when colonizing Mars, but the airline industry is not a model for doing so.
Space is not Highlander.  There can, and will, be more than one.

Offline Vultur

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Re: MCT Speculation and Discussion Thread 4
« Reply #597 on: 10/06/2015 01:52 AM »
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.

Well, airliners don't have a LAS, and they carry lots of passengers.

I don't think the shuttle lesson means all manned space systems, forever, must have an LAS.

(and I'd heard that it's questionable whether a practical LAS could have saved the crew in that case due to the speed ... could be wrong though.)


(EDIT for my illiteracy: "feel" and "speed" are different words.... :( )
« Last Edit: 10/06/2015 01:54 AM by Vultur »

Online Semmel

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Re: MCT Speculation and Discussion Thread 4
« Reply #598 on: 10/06/2015 06:32 AM »
Cargo MCTs don't need life support.

Offline Zed_Noir

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Re: MCT Speculation and Discussion Thread 4
« Reply #599 on: 10/06/2015 07:03 AM »
Cargo MCTs don't need life support.

Yes they do. Unless you going to unload and maintained the cargo MCT in pressure suits while on the Martian surface. Also that will excluded the cargo MCT as backup crew deep space habs.

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