Author Topic: MCT Speculation and Discussion Thread 4  (Read 611270 times)

Offline Lobo

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Re: MCT Speculation and Discussion Thread 4
« Reply #960 on: 10/28/2015 08:46 PM »

For GEO satellite deployment MCT could be refuelled in LEO, transfer to GEO, drop off the satellites and return. Only one MCT per year is necessary for all the commercially competed GEO satellites, plus 1-3 tanker flights.


I think it better to just have it go to a GTO, and then do what's typically done today, which is drop it off there and have the payload place itself in the GEO orbit.  I think the propulsive hit becomes large when entering a GEO orbit to directly deploy the sat, and then have to deorbit from there.
A GTO will bring it right back to Earth, where it can do a small deorbit burn and come right home. 

Up to now I thought the same. But given the capabilities of a refuelled MCT GSO may well be the better option. No need to give com sats the capability to reach GSO on their own. GTO was so far the better option because the launch vehicle could be smaller and the upper stage can easily deorbit after placing the payload in GTO. None of the restrictions would be applicable for MCT, especially when refuelled but with few sats maybe even without refuelling.

Hmmm...interesting.

How difficult is it to put enough propellant on a sat for it to do it's own GEO insertion?  I assumed it was pretty easy/common, as that's usually how it's done now, isn't it?

The thing I'd think they'd want to stay away from is refueling, because that means another BFR/MCT launch is required to put the propellant in the depot so the MCT-sat launcher can tank up.  So that's two giant BFR launches for a sat that could otherwise go on an expendable FH.  I get it's fully reusable, but that just seems like a lot of overkill for a D4H/FH class payload.  There's something about the optics there that seems like it would make using BFR/MCT less economic than even an expendable FH.

Maybe that's an incorrect perception though.

But if it's typical/common/inexpensive to add propellant tanks to a sat so it can do it's own GEO insertion, it's probably a moot point.  Most GEO sats have a propulsion system anyway, don't they?  For orbital corrections and such?  And to deorbit at the end of their service life?  (I thought that was some new thing for sats to help cut down on the space clutter of dead sats).  So it's it just a matter of enlarging the tanks on the system the sat would have anyway?  That doesn't seem like a bid deal.  But it's not something I'm well versed in, so I may have an incorrect perception of it.

Offline Impaler

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Re: MCT Speculation and Discussion Thread 4
« Reply #961 on: 10/28/2015 08:46 PM »
Interpretation 2: 100 tonnes in addition to MCT dry-mass.

This is the correct interpretation, this is one of the few clear goal statements we have from Musk and while I think many folks over interpret many statements this one isn't being embellished.  At a minimum a vehicle holding 100 mT of cargo will land on mars, and most of us assume that it will be launched with the cargo inside, but most agree that significant amount of additional support launches will be necessary to get one lander onto mars.


Musk mentioned ten cargo MCTs per passenger MCT, so 11 MCT's per 100 colonists.

However, orbital refuelling means multiple launches in addition to the main launch. So if you need, say, three launches of fuel per MCT-to-Mars, then you've 44 launches per 100 colonists.

If the price per colonist is $500,000, that puts the price of a single launch at $1.14m (less for the launch itself, since the total price includes all the extra orbital handling for refuelling, the ECLSS for 100day flights, etc.
Sounds like a plan to me.

When you think about how expensive BFR would have to get before it's not a viable commercial launcher, it's a lot higher than the point where it's no longer a viable Mars colony vehicle.

I think your in error assuming that a passenger ticket comes with 10 mT of luggage at no additional cost.  SpaceX will likely be charging money for 'freight' flights in proportion to their cost and the passenger flight represents the cost of moving just a person with a suitcase to mars.  I anticipate a cost of around 200-300 million per launch of a BFR initially when it is in early deployment and reuses are limited (and occasionally cut short due to recovery failures), this would still be more then enough to push SLS completely to the side as NASA could afford 4-5 launches a year for what they are going to pay for 1 SLS launch.  Prices would gradually drop as scale increases, refurbishment is streamlined, reuse lifetime maximized and launch facility throughput is improved.

The efficiency of LEO->Mars surface mass transfer needs to improve too which is one of the major reasons to go SEP, if pushed to the extreme that ratio could be near 80%, with an all chemical propulsion system it won't get past 10% meaning you will always need large numbers of propellant launches per cargo or passenger launch.

Offline Lobo

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Re: MCT Speculation and Discussion Thread 4
« Reply #962 on: 10/28/2015 09:00 PM »
I think you are right Lobo.  SpaceX has the same diameter S2 on Falcon 9, and the same engine optimized for vacuum.  It makes sense to have the same Raptor engine second stage optimized for vacuum, same fuel as first stage, even the same landing legs.  Either a clamshell heat shield for the S2 engines that doubles as interstage, or the conical one side coated heat shield on the second stage/MCT for atmospheric entry.  Like you said, a dedicated S2 might through 20 more tons into orbit, but then you have three things going on.  S1, S2, MCT.  Cheaper would be S1, S2/MCT combo.  Both build robustly and reusable for cost reusability cost effectiveness.  Same with airlines.  Same planes are for either passengers or cargo.  Only the guppy versions for large diameter items are different.

That's the thought anyway.  The synergy between the F9 booster and S2 is one example, although a dedicated S2 can share the same tank tooling as the booster and a separate biconic MCT.   The Rocketplane Kistler K-1 is another, although that has different diameter stages.  It's be like if SpaceX mounted a Dragon 2 less the heat shield on an F9US, and then coat the nose and one side with your TPS.  Much thinner TPS tiles as it would have a much better L/D ratio.  And now you have a reusable crew capsule as well as a reusable upper stage, and the whole integrated assembly would weigh less than a separate reusable Dragon 2 and reusable F9US.
But this of course then runs into the issue of fast abort and pad abort options, just like it does for the integrated Biconic MCT concept. 

Putting a clamshell type heat shield on the bottom of a dedicated S2 means you have to launch it with the doors open, so they need to have a way to not interfere with the interstage until booster sep.  Then they need to close up again air tight or else you risk loosing the stage.   Which is why I prefer biconic concepts rather than Super Capsule concepts for MCT...you aren't trying to open and close large doors in your heat shield.  And for a dedicated S2 as well.  Biconic is probably the best bet. 

« Last Edit: 10/28/2015 09:02 PM by Lobo »

Offline Impaler

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Re: MCT Speculation and Discussion Thread 4
« Reply #963 on: 10/28/2015 10:22 PM »
I'm not saying a dedicated 2nd stage couldn't be made cheaper and easier than an integrated MCT basic platform stage.  It's criteria aren't as harsh as for MCT. 
But, that doesn't speak to your original comment about an integrated design being technically unfeasible.  There may be other reasons to go with a dedicated 2nd stage, but I don't think those are reasons because the integrated design is some sort of impossible or implausible concept.

Now, if the integrated design is feasible, that just leaves the question of if it's the better way to go or not?
Mike speaks well to that here:



So effectively MCT has a superset of the R2S requirements. Anything the R2S can do the MCT can do as well, so there is no need for the R2S.

For GEO satellite deployment MCT could be refuelled in LEO, transfer to GEO, drop off the satellites and return. Only one MCT per year is necessary for all the commercially competed GEO satellites, plus 1-3 tanker flights.

That's always been my point.  You have to have MCT anyway.  It already can do everything a cheaper/easier S2 can do anyway.  Why not just make it be the S2 then?

Will it be a little heavier than the dedicated S2?  Probably.  It probably won't have quote the payload capability of the integrated MCT platform.  But will that be a problem?  Probably not.  It's only going to be an issue for two potential MCT/BFR missions.

1)  Unmanned payload launching to space.
2)  Tanker service to depot.

The first shouldn't be much issue, because there's already nothing that needs 100mt of throw capacity (although it would likely be even more than that, because it would be stripped down to just the tanks, engines, and OML)  To my knowledge there's no current or near term future need for more capacity than D4H or FHE will have to LEO or to GTO.  And such an integrated BFR would have much more capacity than those.  So if you have 100mt to LEO instead of say 120mt to LEO for a dedicated lighter/cheaper S2, it doesn't hurt you from the commercial or government payload launching market.

As for a tanker, it probably wouldn't be able to get quite as much propellant to a depot per launch.  But then again, you have a reusable launch system.  So if it takes 5 BFR launches to fill up a depot prior to a Mars mission instead of 4, is that really a detriment? 
And again, when comparing the dedicated S2 to the basic integrated MCT platform...stripped of everything but the bare shell, tanks, engines, and landing gear, I doubt the difference would be all that much.

Also, as a satellite launcher, it wouldn't really need it's big MCT solar arrays.  And it would have large residuals of propellants.  So a little methalox fuel cell or IVF engine could generate the short term loiter power it needs, so it wouldn't even need to have the solar arrays.  A dedicated S2 would need something like that too....or a lot of batteries.  Either way that would be needed.

First off, NO it is not a simple super-set, it is clearly harsher in totality, but not all in the same ways.  Many avenues would be available for easing the design of one that aren't available to the other or to the composite.  Second your MCT hurdles are only that high when you DON'T have a 2nd stage.

I need to see some better figures from you on the total mass and thrust of your BFR concept and the actual mass of this integrated 2nd stage to even get an idea what your LEO performance is and what it's likely to cost, you really can't throw around generic statements like 'lighter' when I have SPECIFIC mass estimates and DeltaV goals.  You need to do more homework to demonstrate this (or show the homework from L2 as I suspect this is where it all is).  The launch vehicle performance calculator http://www.silverbirdastronautics.com/LVperform.html should be the basis for comparison as I've used it for my calculations (median performance estimates only).



Payload Carrying System:  R2S:  Payload adapter at top of tank     MCT:  Internal cargo bay with doors

You could either have an internal cargo bay for MCT...as there will be an empty volume between the nose and tanks, or have a payload adapter on the nose like you say for the S2.  Either set up will have a TPS on the nose, so there's not much difference in have a payload adapter on one vs. the other. 

Wait a sec, we are both in favor of Bi-conic entry vehicles for MCT (you convinced me of that), and you can't put anything on top of a bi-conic that is where the heat-shield is, you can't have exposed attachment point for payload, they would melt and I can't see retracting them into doors as being practical either.  The picture we were shown of a F9 reusable upper stage was fatally flawed when it depicted a heat shield on the top of the stage, it is both unstable as an entry vehicle and it provides no attachment, that's why I came up with the petal heat-shield/legs and bottom first entry.




The differences are almost endless and I foresee a very different 2nd stage that is a fairly normal cylindrical shape about 20 m tall and 10 m diameter holding around 1200 mT of propellants and equipped with 7 Raptor engines.  It would be recovered by using a petal segment heat-shield covering the engines (rather then the unstable head-first entry depicted in the old F9 reuse video).  The petals would then open and act as landing legs, the heat shield material can be a single use ablative (PICAX) that is attached to the structural leg/rib.  The tank sides would likely have a metallic sandwich TPS system to protect them as well.  The top of the vehicle would deploy parachutes, decelerators and other disposable systems from underneath the payload adapter.  Grid-fins and vernier engines for touchdown would likewise be positioned at the top.  Dry mass fraction of 6% would yield a 75 mT dry mass.


Yes, there can be differences, but it doesn't change that MCT would do everything S2 would do, and much more.  So the advantage of going that way is you only need to develop one spaceship, and all of them have the same basic common platform.  Where as the two spacecraft you've described appear to have different landing legs, different TPS systems, and different methods of landing...a lot of different duplicated systems that cost more to develop and support, and add extra mass to the stack when launching MCT.
 I think there's a plausible argument than the economic and logistic advantage...as well as mass advantage to LEO of have the one common integrated system outweigh loosing a little mass efficiency as a tanker or satellite launcher...especially when it could still be very capable at both.
which gets to the larger point, I think as others have pointed out, MCT/BFR will be designed to do specifically one thing primarily...serve as a Mars vehicle.  It'll be able to do other things of course, (which can generate money to help fund the Mars goals...definately a good thing for SpaceX) but the hardware won't be designed around those things primarily, it'll be designed around going to Mars as safely and economically and efficiently as possible. 
An integrated design also has better throw capacity to LEO when launching MCT...which would be it's primary designed function.  Whereas with the dedicated S2, it would have more throw capacity to LEO when launching payloads other than MCT...which isn't it's primary designed function.
So (IMHO) a dedicated S2 will probably only be part of the system if there's a Mars-centric reason for it.  Like a whole vehicle fast abort option for MCT or maybe they want the most efficient tanker possible, which would be a dedicated S2-tanker.  Or some other Mars related reason.

Your ignoring the fact that I'm calling for a MCT with STAGGERINGLY lower performance requirements then you are, your trying to pretend this a comparison between an RV towing a boat, vs your proposed RV only (cheap by 1 boat), but what you have is more like a hovercraft with a house on it which would be much harder to make and may not even be possible.  Two or more vehicles that split the missions performance hurdles between them are infinity easier to develop (and less risky) then one which shoulders all the burden.  Making vehicles reusable costs a considerable performance margin as well.

The differences between our MCT vehicle concepts is huge, your looking at 8 km/s DeltaV and (I think) full speed aerocapture at >12 km/s (I'm still not clear on your preferred trajectory and how aggressive your transit times are).  I've been trying to use every tool available to to lower the performance specs of the MCT lander down to 4 km/s DeltaV and EDL from just 4 km/s to make the 'long-pole' of the whole operation as short as possible.

A simpler lander is made possible BECAUSE of the 2nd stage and the use of SEP tugs as well.  And your even admitting that the 2nd stage makes a better LEO tanker, which will be launched 6-12 times more often then the lander so it is quite important. 

Your logic that the whole system will be designed with zero consideration for uses other then mars is deeply flawed, this will not lower the price, it will raise it because the history of vehicles of every kind is that lowest prices are achieved with highest volume.  If MCT can be used for a wider set of missions then volume goes up and price goes down.  And this is particularly important when laying down a big investment on a new vehicle it needs to cover as much market space as possible to ensure that it survives, the rocket that's a flop financially and becomes a millstone around your neck is infinity more damaging to Mars colonization then one that might be a little lower in performance but actually flies.
« Last Edit: 10/28/2015 10:27 PM by Impaler »

Online Robotbeat

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Re: MCT Speculation and Discussion Thread 4
« Reply #964 on: 10/28/2015 11:11 PM »
....
Lets look at all the ways they differ

Active time of flight:  R2S (Reusable 2nd Stage):  Hours   MCT:  Months
So basically you're saying SpaceX wouldn't want their 2nd stage to be capable of cislunar flights like Vulcan's 2nd stage will be capable of?

Quote
Entry Velocity:    R2S: 7.7 km/s    MCT:  >12 km/s
So basically you're saying BFR wouldn't be useful for anything beyond LEO? Interesting, as beyond-LEO is where the vast majority of the commercial and military launch market is (and commercial/military launch is basically the whole argument for why you'd want to release BFR before MCT). Anyway, SpaceX is probably going to use PICA-X, which works in both cases. A big reason why RLVs are normally considered for LEO is that they're either single stage (does not apply here) or use a low-temperature TPS (also does not apply).
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Aerocapture necessary:   R2S:  NO    MCT: YES
This is basically a subset of the above. Either could work with aerocapture.

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Landing Site:  R2S:  Spaceport concrete pad + support facilities    MCT:  Unprepared martian regolith surface
Citation needed. There's a pretty good chance SpaceX could be sending Dragons with equipment ahead of time. In either case, this would be irrelevant after the first MCT landing. You cannot say that the MCT will be landing on unprepared regolith.

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Landed Payload Mass:  R2S:  Self dry mass     MCT:  Self dry mass + 100 mT cargo (mars) 25 mT (Earth)
The first valid point!

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Payload Carrying System:  R2S:  Payload adapter at top of tank     MCT:  Internal cargo bay with doors
Yet another argument to skip straight to MCT, so they don't have to develop a payload adapter at the top of the tank.

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Payload Separation Conditions:  R2S:  Axial decoupling in zero-g    MCT:  Horizontal removal on mars surface
Meh. Horizontal removal works, we've done it before. And using MCT means you ALSO can recover payloads from orbit or potentially save expensive payloads in case of a first stage mishap. Additionally, if the payload bay is at the top, they still can do axial decoupling in zero-g just fine.

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Single use disposable Landing systems allowed (parachutes, airbags etc):   R2S:  YES    MCT:  NO
Not from what we saw from SpaceX's Falcon 9 upper stage reuse video. Additionally, SpaceX is moving /away/ from this sort of expensive disposable system. Why would they decide to reverse course and invest millions on a technology that for them is a dead-end?

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Abort system necessary:  R2S:  NO    MCT: YES
Only needed for crewed MCT flights. There are other ways to do abort for crewed flights, like the Delta Clipper concept.

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Take off Site:  R2S:  Upper atmosphere after stage separation    MCT:  Unprepared martian regolith surface
Again, you have NO citation for that, and there are very good arguments that MCT will be neither landing on nor taking off from unprepared Martian regolith.



Quote
The differences are almost endless and I foresee a very different 2nd stage that is a fairly normal cylindrical shape about 20 m tall and 10 m diameter holding around 1200 mT of propellants and equipped with 7 Raptor engines.  It would be recovered by using a petal segment heat-shield covering the engines (rather then the unstable head-first entry depicted in the old F9 reuse video).  The petals would then open and act as landing legs, the heat shield material can be a single use ablative (PICAX) that is attached to the structural leg/rib.  The tank sides would likely have a metallic sandwich TPS system to protect them as well.  The top of the vehicle would deploy parachutes, decelerators and other disposable systems from underneath the payload adapter.  Grid-fins and vernier engines for touchdown would likewise be positioned at the top.  Dry mass fraction of 6% would yield a 75 mT dry mass.
So in other words, MCT doesn't fit with what your ENTIRELY made-up reusable 2nd stage would look like, even though it probably fits pretty well with what SpaceX /has/ released so far with respect to a reusable 2nd stage, so obviously they're totally different. Uh huh.

The only real point you've got there is the requirements for landing a dry stage and landing a significant payload on Mars are different, so you'd probably need somewhat beefier legs on MCT than what would be absolutely essential for an empty upper stage. That's true, but not a good enough reason to develop a completely different vehicle. Heck, even if BFR/MCT DOES have 3 effective stages (big booster, reusable 2nd stage, and MCT), you'd still probably want to keep the 2nd stage and MCT /very/ similar in order to avoid having to pay for development of a completely different vehicle, totally different TPS, etc. (as well as the loss of safety since you aren't also testing MCT every time you launch another payload)

And things like an abort system or beefier legs could be developed at a later time. Sure as heck beats the cost of developing an /entirely/ different spacecraft from the 2nd stage, as you propose.

Money matters.
« Last Edit: 10/28/2015 11:48 PM by Robotbeat »
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Online Robotbeat

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Re: MCT Speculation and Discussion Thread 4
« Reply #965 on: 10/28/2015 11:47 PM »
Let me clarify.

Impaler, I don't think your ideas of how to build a reusable upper stage are technically bad, I'm just not at all convinced that that is what SpaceX has in mind, based on their F9 reusable upper stage, their work with Dragon, and their various statements about MCT/BFR. A more integrated system fits very well with statements like this from Musk:
"We’re looking at our Mars transporter being around 15 million pounds of thrust"
(from here: http://forum.nasaspaceflight.com/index.php?topic=37839.msg1391925#msg1391925 )
« Last Edit: 10/28/2015 11:47 PM by Robotbeat »
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Offline Pipcard

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Re: MCT Speculation and Discussion Thread 4
« Reply #966 on: 10/28/2015 11:57 PM »
I still don't get it, why is integration of systems not acceptable for this:

However, many viewed [the Mars Direct plan] as too technologically ambitious to be credible – citing a myriad of issues, such as hopelessly optimistic technology assumptions, and lack of adequate mass margin.

The Mars Semi-Direct architecture proposal that followed (also 1991) might be interpreted as a partial rebuttal to some of these concerns. Instead of specifying such a wildly ambitious “do it all” ERV vehicle, the function of ferrying the crew from Mars surface back to Earth was split up into two parts, to be performed by two separate vehicles: A Mars ascent vehicle (MAV), which needed only to generate enough fuel for the crew to ascend to Mars’ orbit, and the Earth return vehicle (ERV) which was pre-placed in Mars’ orbit – and which would perform the propulsive maneuver needed to send the crew back to Earth.

but is acceptable for MCT?
« Last Edit: 10/29/2015 12:00 AM by Pipcard »

Online Robotbeat

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Re: MCT Speculation and Discussion Thread 4
« Reply #967 on: 10/29/2015 12:40 AM »
Because:
1) SpaceX has demonstrated supersonic retropropulsion. That alone is HUGELY enabling for large payloads on Mars. Since you need rockets to land anyway, it also greatly simplifies the whole EDL process.

2) MCT assumes lots of equipment on the ground, not least of which extensive water/CO2 to Methane/O2 ISRU.

3) MCT assumes orbital refueling, at least in Earth orbit but that also leaves open the possibility of orbital refueling in Mars orbit which would allow propellant to be loaded on to MCT for Trans-Earth-injection and also for landing (on Earth or Mars). So if you think all those various roles means that SpaceX isn't able to get MCT up to 6.5-8km/s or so delta-v, they can use orbital refueling (on both sides) to fill that gap, at least until they're able to refine MCT.

4) Related to this is that MCT will likely experience refinement. If at first you end up with F9 v1.0, you can double the performance to LEO (and more to higher energy orbits) with some evolutionary refinement by the time you get to v1.1 full thrust. The /platform/ just has to allow the stated performance goals (which in the case of MCT are listed in terms of round numbers with low implied precision, like "100 tons useful payload" or "up to ~100 passengers" or "~100 day transit to Mars"), /eventually/.

These various factors are self-reinforcing: ballistic coefficient too low? Use more propellant. Supersonic retropropulsion is nicely scalable that way. Not good enough mass fraction so far? Refuel more often. After all, you're launching the propellant to Earth orbit using a large RLV, and you're producing the propellant on Mars with an expanding infrastructure (and also using the same large RLV). If you want better IMLEO, you can maneuver the propellant using SEP.

Because MCT is much bigger than the Mars Direct vehicle, it makes the other roles much easier and less marginal.

But just because people were skeptical about Mars Direct doesn't mean it wouldn't have worked. For all the skepticism it generated, its influence is largely mainstream now in NASA, and all subsequent DRMs bear the mark of Mars Direct.

 (BTW, refueling in general is incredibly important: it allows "staging" without needing to reconfigure your vehicle, thus cutting down on the analysis that needs to be done. Also, it means you don't need to design, build, test, and launch a new stage. Mars Direct goes partway by doing surface fueling, but by refueling multiple times in each mission cycle, you can hugely improve the mission cost. And it doesn't even have to require a lot of delta-v in your vehicle---maybe 5km/s--as you can add more delta-v just by refueling.)
« Last Edit: 10/29/2015 01:01 AM by Robotbeat »
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Offline Vultur

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Re: MCT Speculation and Discussion Thread 4
« Reply #968 on: 10/29/2015 12:55 AM »
Did you remember to factor in the ISP differences, all thouse SSTO vehicles were going to use Hydro-Lox at 450 ISP.  When you use use that kind of ISP the 9.5 km/s DeltaV requires an 8.62 ratio.  So yes they are quite equivalent.

Oh I think you'd use methane or propane for a pure-rocket (not Skylon) reusable SSTO in real life, liquid hydrogen just isn't worth it, especially since the greater losses on ascent (IIRC) and the huge bulky tankage largely wipe out the advantages.

Their are g-forces, entry corridor width and landing accuracy to consider as well as heating,

MCT is doing propulsive landing not parachute, so it should get a landing accuracy advantage for that.


Quote
if it has dropped enough to not completely consume the entire vehicle mass on it's own. 

Wait, what? Mars re-entry velocities aren't nearly THAT high! The Galileo probe did an entry at over 40 km/s and even it wasn't 100% heat-shield (about 50% IIRC) and that's like x10 the kinetic energy. So how could MCT possibly have to be all heat-shield?
« Last Edit: 10/29/2015 12:55 AM by Vultur »

Offline Impaler

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Re: MCT Speculation and Discussion Thread 4
« Reply #969 on: 10/29/2015 01:07 AM »
Let me clarify.

Impaler, I don't think your ideas of how to build a reusable upper stage are technically bad, I'm just not at all convinced that that is what SpaceX has in mind, based on their F9 reusable upper stage, their work with Dragon, and their various statements about MCT/BFR. A more integrated system fits very well with statements like this from Musk:
"We’re looking at our Mars transporter being around 15 million pounds of thrust"
(from here: http://forum.nasaspaceflight.com/index.php?topic=37839.msg1391925#msg1391925 )

This is an odd figure for you to bring up because it's not really very relevant to the Integrated vs non Integrated 2nd stage argument.  The thrust is clearly referring to the First stage boosters thrust which would not be all that different.  Lobo seems to be operating under the assumption that the first stage booster is identical in both cases (but I really want to see his numbers on the booster size).

I see a BFR with 12 million pounds of thrust in the core which can be augmented with pairs of raptorized FH boosters with each pair adding 3 million pounds.  That gives a 12, 15, 18 and 21 million pounds of thrust as options.  The no booster configuration would do all the propellant milk-runs, commercial launches etc etc, with the maxed out configurations being used for MCT.  LEO performance (28 degree inclination 200 km circular orbit) would be 110, 140, 168, 193 mT respectively as you add boosters (with full recovery of everything).  To GTO I estimate you would get around 30 after accounting for the boost back to LEO for the stage to be recovered. 

Offline Impaler

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Re: MCT Speculation and Discussion Thread 4
« Reply #970 on: 10/29/2015 01:24 AM »
Did you remember to factor in the ISP differences, all thouse SSTO vehicles were going to use Hydro-Lox at 450 ISP.  When you use use that kind of ISP the 9.5 km/s DeltaV requires an 8.62 ratio.  So yes they are quite equivalent.

Oh I think you'd use methane or propane for a pure-rocket (not Skylon) reusable SSTO in real life, liquid hydrogen just isn't worth it, especially since the greater losses on ascent (IIRC) and the huge bulky tankage largely wipe out the advantages.

Their are g-forces, entry corridor width and landing accuracy to consider as well as heating,

MCT is doing propulsive landing not parachute, so it should get a landing accuracy advantage for that.


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if it has dropped enough to not completely consume the entire vehicle mass on it's own. 

Wait, what? Mars re-entry velocities aren't nearly THAT high! The Galileo probe did an entry at over 40 km/s and even it wasn't 100% heat-shield (about 50% IIRC) and that's like x10 the kinetic energy. So how could MCT possibly have to be all heat-shield?

I get the sense that I am the only person who dose any research on EDL around here.  Super-sonic-retro-propulsion is not a magic wand that eliminates all your problems and make landing on mars like landing on the moon (aka a purely propulsive operation that you just throw propellant at).

SRP happens AFTER peak drag and heating after >90% of your speed has been dissipated, it basically substitutes for a parachute or SIAD.  It doesn't save you any heat-shield mass or structural mass, it just lets you break out of the parachute size constraint and makes landing safer because your using one system (rocket) for both supersonic and terminal landing rather then having two systems with must operate flawlessly to not crash.

All the thermal, and g-force problems are still present and the systems to mitigate them would be dangerously close to consuming everything that is not the payload and propellants which we need to carry for the later SRP and landing.

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Re: MCT Speculation and Discussion Thread 4
« Reply #971 on: 10/29/2015 01:28 AM »
Hypersonic retropropulsion is also possible.

Also, you seem to think that PICA sucks. It is, in fact, one of the highest performing TPSes that exist, used on the highest speed entries, used on Mars Curiosity Rover's hyperbolic entry, and a serious candidate for Orion's heatshield before they went to Avcoat (because they thought they could just do the Apollo-era solution, though now they're going with a tile-based solution anyway). It is very lightweight (I have a sample of it on my desk) and an excellent insulator. You could put it all around your vehicle if you wanted to (though that is not necessary, as even a metallic coating on the Apollo capsule's backshell survived). And SpaceX has made some improvements on it after flying it several times on its own Dragon.

You bring up entry accuracy as if it's still a problem. You read a paper from 1992 and then continue to rehash old problems that have already been solved for probes like MSL Curiosity. Modern tracking means we can get VERY high precision entry accuracy, and adding some spatial positioning satellites (as well as atmospheric monitoring) at Mars can enhance this even more. A huge amount of MSL's dispersion in landing position is due to its use of parachutes, by the way. And in principle, there's no reason a vertical landing rocket can't get within a meter or two of a landing spot (heck, even tens of centimeters is possible and has been demonstrated via GPS). And if SpaceX needs to do a burn before they aerocapture, or if the early missions need to be 120-180 day transits, then so be it. None of these things are the showstoppers you seem to think they are.
« Last Edit: 10/29/2015 02:00 AM by Robotbeat »
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Re: MCT Speculation and Discussion Thread 4
« Reply #972 on: 10/29/2015 02:03 AM »
I'm well aware that PICA is the best ablative we currently have but neither you nor anyone else who insists it will be used can give me a heat-shield mass for a prospective vehicle and entry profile.  And for the millionth time structural mass counts too and are of a greater concern then the thermal protection systems.

At entry the vehicle will have nearly 50% payload mass, just SRP will require ~20% propellant fraction.  That leaves something like 30% for all the structure, all the TPS, all the engines and tank mass, landing gear, systems used only in space like solar arrays and radiators that have been stowed etc etc.  This is not remotely easy even when you do as I've suggested and enter from mars orbit at just 4 km/s rather then twice as fast or more coming in on direct entry.

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Re: MCT Speculation and Discussion Thread 4
« Reply #973 on: 10/29/2015 02:51 AM »
I'm well aware that PICA is the best ablative we currently have but neither you nor anyone else who insists it will be used can give me a heat-shield mass for a prospective vehicle and entry profile.
And? Can you give me a heatshield mass estimate? I don't see your estimate. Previous times we estimated the math, we came up with something like:
Dragon has 8cm thick of PICA-X, and when it reentered, barely any of it had burned away. PICA-X is about .27grams/cc. So each square meter would weigh about 20kg. 15 meters by 20 meters of surface is about 6.5 tons of PICA-X. Dragon has a ballistic coefficient of about 1000kg/m^2 (let's arbitrarily set drag coefficient to 1 to allow easier comparison), but if MCT weighs 150-200 tons with payload, then MCT should have a superior ballistic cofficient, almost half as much as Dragon. Of course, there's structure underneath the heatshield to interface the heatshield to the rest of the structure, but I doubt it weighs more than the mass of the PICA-X itself. And if it does, it's a good target for mass optimization, so I'll say 50% of the PICA-X mass. So, about 10 tons for the TPS, another 2 tons for some SPAM on the backshell (if it needs more than just a nice reflective coating) for 12 tons total with an entry mass of 150-200 tons. And by the way, that's for the max 100 ton payload. Nowhere did Musk say 100 day transits for 100 ton payloads! So those are separate things. You'd have slow transits with high payload or fast transits with low payload.

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And for the millionth time structural mass counts too and are of a greater concern then the thermal protection systems.
For the millionth time, pressure stabilization will be used and can be used for the compressive loads. Additionally, tensile structures inside can be made out of high performance materials like Spectra (or certain types of carbon fiber), with a strength-to-weight ratio ten times that of high performance aluminum alloys as well as very high toughness.

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At entry the vehicle will have nearly 50% payload mass, just SRP will require ~20% propellant fraction.  That leaves something like 30% for all the structure, all the TPS, all the engines and tank mass, landing gear, systems used only in space like solar arrays and radiators that have been stowed etc etc.  This is not remotely easy even when you do as I've suggested and enter from mars orbit at just 4 km/s rather then twice as fast or more coming in on direct entry.
3.5km/s, actually, not 4km/s (actually, at the equator, it's more like 3.25km/s). Easy, no, but mars luckily has places with much lower terminal velocity. Valles Marineris is 5km below datum, and Hellas Basin can be 8km below datum. Terminal velocity is proportional to: e^(h/(2*10.8km)) since the scale height of Mars is 10.8km, so at -5km in Valles Marineris, you have a terminal velocity 79% of that at the datum.


Additionally, there's an important thing you have neglected to mention: If MCT has a very large delta-v capability, like 7km/s with a low payload, then you could do a fully propulsive landing with a HUGE payload from low Mars orbit, provided you use strong legs.
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Re: MCT Speculation and Discussion Thread 4
« Reply #974 on: 10/29/2015 04:20 AM »
My whole mass breakdown was pages ago for every part of the vehicle with the justification for each value.

http://forum.nasaspaceflight.com/index.php?topic=37808.msg1433370#msg1433370

This is actually the second time I posted them too.

My mass for metallic TPS that only need to handle the low heating rates of a LMO entry is just 5 kg/m^2 and the area is 650 m^2 (the entire surface excluding the bottom), which would be 3250 kg which I then padded to 5 mT even.  The skin is applied directly over the structural frame and doesn't have an additional backing requirements.


I'm not clear on the shape of your vehicle, is it a circular blunt body Dragon like capsule, why not express the area as a diameter then rather then an arbitrary rectangle.  A 15 m diameter Dragon (the Super Dragon concept) is only an area of 176 m^2 about half the area your specifying.


Pressure stabilization only keeps the propellant tanks from crumpling and their is zero evidence that technique can tolerate that much force and what it will cost in extra tank masses to withstand the pressure, the portion of the vehicle that carries cargo (actually ever portion of the vehicle) also needs to withstand high g-forces.

The altitude to which we need to land cargo has never been specified by Musk, I do not think the vehicle will be restricted to only landing at low altitudes.  Though it may have to lower the payload mass to achieve high altitude landings.  I think the most neutral interpretation is land 100 mT at the Datum but until Musk clarifies we can't say for sure.  Terminal velocity is rather meaningless on Mars as it is super-sonic and quite lethal and you never reach terminal velocity, you switch to propulsion long before you get close to terminal velocity and while still several km from the surface.  Past missions cared a lot about landing altitude because it was so important to parachutes, if we aren't using thouse it matters a lot less.

The last statement really makes no sense, how is MCT full of propellants at entry when you used them all to to depart Earth?  Or are you in favor of SEP tugs bringing it to LMO, a solution I'm considering for Earth return?  You can't bring propellants from the surface of mars, even after establishing a base because your landing solution involves burning ten times more to land then could possibly be delivered to orbit by the MCT's.  At most I could see a MCT bringing enough propellant to orbit to be able to land again say 800 m/s DeltaV which is still a considerable 40 mT by my estimate, which I'm aiming for in my design but we certainly can not do an all propulsive or even mostly propulsive landing.


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Re: MCT Speculation and Discussion Thread 4
« Reply #975 on: 10/29/2015 09:52 PM »
We don't know the exact shape of MCT, so I used a ballpark figure. I don't expect it to be better than +100%/-50% accurate, so sure, your 15m-diameter Dragon could work, too. It'd have about the same ballistic coefficient as regular Dragon (with largely-empty tanks).
« Last Edit: 10/29/2015 10:01 PM by Robotbeat »
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Re: MCT Speculation and Discussion Thread 4
« Reply #976 on: 10/29/2015 10:16 PM »
...
The last statement really makes no sense, how is MCT full of propellants at entry when you used them all to to depart Earth?  Or are you in favor of SEP tugs bringing it to LMO, a solution I'm considering for Earth return?  You can't bring propellants from the surface of mars...
How does it make no sense when you yourself thought of an acceptable solution? :)

Also, who says you have to use all the propellants to depart Earth? A 100mT uncrewed payload doesn't need to have an especially fast transit.

But yes, I had in mind that you would probably launch the propellant from Mars (especially as a colony is established, this will become increasingly more attractive versus launching it from the surface of Earth). From the surface of Mars to LMO is about 4.1km/s (you could do slightly better, let's leave it at that). To land would take, worst case, about 4.1km/s again (although realistically, about 1km/s less because aerodynamics is helping you, even without a need for significant TPS) for a total of 8.2km/s even with no significant TPS. A Raptor-based booster with a mass ratio of 21 (it would be ~30 for a Falcon Heavy booster, but I give 23.75 given the lower bulk density of methane/LOx, and 21 mass ratio to take into account the mass of the legs and stuff, though that can be less massive on Mars) can give you 11.3km/s, so you have plenty of margin to deliver lots and lots of propellant on the first 4.1km/s leg. I calculate that you can put about 20-25% of your lift-off mass to Mars orbit. But that's with basically zero TPS, so an ideal vehicle with TPS should be able to improve on that. With MCT's TPS (and everything else stripped off to save weight, and using lighter weight legs than regular MCT and maybe a couple less Raptors), you can probably do about that 20-25% figure.


...but you could also get that fuel from Earth. A slow-boat trajectory from Earth to Mars with modest aerocapture and then multi-pass aerobraking would allow a huge amount of propellant to be brought from Earth without needing to send a SEP tug to Mars (although you could have a cislunar SEP tug to improve the efficiency of LEO-to-Earth-Escape if you wanted to).

There's a lot of ways to skin that cat. We don't need to pick a favorite one here, just point out at least one that is acceptable. Relying on a heavily-propulsive-landing is one possible way to allow the occasional 100mT payload to the surface of Mars without that requirement unduly burdening the rest of the MCT requirements. Yet again, the power and versatility of refueling.
« Last Edit: 10/29/2015 10:22 PM by Robotbeat »
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Re: MCT Speculation and Discussion Thread 4
« Reply #977 on: 10/29/2015 10:57 PM »
The only real point you've got there is the requirements for landing a dry stage and landing a significant payload on Mars are different, so you'd probably need somewhat beefier legs on MCT than what would be absolutely essential for an empty upper stage. That's true, but not a good enough reason to develop a completely different vehicle

@this.  But I would think that'd be immaterial.  MCT would be landing with 100mt payload on Mars on unprepared regolith.
That would be in the 0.38g of Mars gravity...where the MCT plus payload will still weight less than a dry MCT back on Earth.  Probably weigh less than a dedicated dry S2 back on Earth. 

So the landing gear may not really have to be much different (or different at all).    Dragon 2 is being designed to land on a prepared pad on Earth.  Yet SpaceX seems to be thinking about trying to drop one on the unprepared regolith on Mars.  So the two may not really be different.  Maybe MCT would have larger foot pads and a means of auto-leveling the legs for the uneven surface where they won't need those on a prepared level pad.  But that's not too much difference.

Heck, even if BFR/MCT DOES have 3 effective stages (big booster, reusable 2nd stage, and MCT), you'd still probably want to keep the 2nd stage and MCT /very/ similar in order to avoid having to pay for development of a completely different vehicle, totally different TPS, etc. (as well as the loss of safety since you aren't also testing MCT every time you launch another payload)

Agreed.  I've mentioned this before.  If MCT were a non integrated biconic vehicle on an S2...so it could do full vehicle abort for example, then there's not much reason the S2 couldn't share as many systems with MCT as possible.  Same TPS panels, same landing gear, same landing systems (landing thrusters or a central Raptor capable of operating at sea level) and maybe even the same outer mold line.  If MCT is riding on top of an S2, then it's tanks will be smaller, although it will have volume inside for cargo and hab.  The S2 would have longer tanks, but no need of internal volume.  So they could have similar or the same OML, so that they still are only developing one spaceship, which will have just one EDL profile.  One "flight" trajectory during reentry, one system of landing, etc.  It would look a bit like two MCT's stacked on each other on the booster, with an interstage adaptor which would house the biconic nose of the S2.

Hard to say if that's what SpaceX would do...but it's seems like a way that could help to minimize development and maximize commonality.


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Re: MCT Speculation and Discussion Thread 4
« Reply #978 on: 10/29/2015 11:24 PM »

I need to see some better figures from you on the total mass and thrust of your BFR concept and the actual mass of this integrated 2nd stage to even get an idea what your LEO performance is and what it's likely to cost, you really can't throw around generic statements like 'lighter' when I have SPECIFIC mass estimates and DeltaV goals.  You need to do more homework to demonstrate this (or show the homework from L2 as I suspect this is where it all is).  The launch vehicle performance calculator http://www.silverbirdastronautics.com/LVperform.html should be the basis for comparison as I've used it for my calculations (median performance estimates only).

Well, that's kind of the problem.  I was told by Chris specifically that I cannot post any pictures or details of the project, since it's been posted to, and discussed on L2.  I checked with him after discussing it in general here, to see how much I could share or say.  Since other members of their group posted our work on L2, it's all pretty much sandboxed in there is what I was told (in no uncertain terms).  So What I do say is more my personal impressions of what we've worked on.  The Integrated biconic MCT was sort of my baby, but fleshed out a lot by the rest of the group, including a lot of performance the mass estimates by guys more knowledgible than me about it.   I'm more of a "concept" guy.  ;-)

I do apologize if I allude to anything I can't expand on.  I'm trying to stay away from doing that because it's poor form. 

But, in general, when I say that the IBMCT will mass less than an MCT + S2, and thus will have more gross LEO capability, that's simple logic.  You aren't duplicating your hardware and systems, like you are with a S2R + MCT.  You have one TPS, not two.  You have two tanks, with one common bulkhead and two domes instead of four tanks with 2 common bulkheads and 4 domes.  You have one set of Raptors rather than two.  You have no interstage adapter between MCT and the S2R.  One have one set of landings legs, rather than two.  So the mass sitting on top of the booster will be less with the IBMCT, just by logic.  So if you assume the same booster for either, then the LEO dry mass to LEO will be less for the integrated vs. non integrated.  You'd need to enlarge the booster for the non integrated design to get the two on par with each other.
IF you were doing a 3 stage to LEO, that might be different.  But you are talking TSTO either way, the only difference is how much total mass is sitting above the booster.  And that will be more with a non integrated system by necessity.


Wait a sec, we are both in favor of Bi-conic entry vehicles for MCT (you convinced me of that), and you can't put anything on top of a bi-conic that is where the heat-shield is, you can't have exposed attachment point for payload, they would melt and I can't see retracting them into doors as being practical either.  The picture we were shown of a F9 reusable upper stage was fatally flawed when it depicted a heat shield on the top of the stage, it is both unstable as an entry vehicle and it provides no attachment, that's why I came up with the petal heat-shield/legs and bottom first entry.

Oh, I think you can put something above a biconic nose.  But I'm not a rocket engineer, so it's above my pay grade to say -how- exactly it would be done.  But SpaceX seemed to think they could do it for their more blunted F9USR.  As far as attachment points in the TPS, didn't the Space Shuttle have 3 attachement points in it's TPS where the ET and propellant lines were attached?  What if you have had some sort of payload adaptor that used 4 such type attachments on the biconic nose?  Maybe something like the Apollo LAS attachment?  (Just with a payload carrier rather than solid LAS).
Dunno...it's just a guess.  But I think it's plausible that real rocket engineers could get it to work, as they got it to work in the 70's for the Space Shuttle.

I don't know about stability for reentry, but both SpaceX and Rocketplane Kistler thought that blunt nosed cylinder would work for reentry.  So I assume there's plausible ways to make that design work.

I'll address you last point when I get more time.  I gotta run now, sorry!



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Re: MCT Speculation and Discussion Thread 4
« Reply #979 on: 10/30/2015 01:36 AM »
And remember DC-X demonstrated the Swan Dive maneuver, a transition from slightly angled down nose-first entry to vertical landing. A small aerosurface or two might help if you wanted to go at an even greater angle, but I see nothing that suggests the transition is impossible.



BTW, I guess that conceptually, I think of MCT as a less-ambitious version of DC-Y/DeltaClipper/DC-I. Less ambitious because: It wouldn't need to be SSTO (6-8km/s is all that's needed, which makes a HUGE difference vs 9.5km/s... basically it means you can afford TWICE the dry mass including payload!), it uses methane (which in spite of the Isp hit probably would make SSTO easier due to the FAR higher bulk density) instead of hydrogen, and it'd basically always operate in vacuum except for final landing at Earth, thus making an aerospike nozzle unnecessary. But like DC-Y/DC-I, you would have very high delta-V, ability to land on just about any solid, flat surface (especially if prepared... and you can do the preparing via a rover), an option for a cargo bay, and ability to carry crew WITH any-altitude abort capability. If we can figure out how to put abort capability on DC-I's design, then we can put it on MCT (though Mars abort would require Soyuz-style or MER-style rockets for terminal landing of the abort pod, this actually could be done fairly compactly).


Things like a prepared surface or a crane to remove payloads, etc, should be secondary or tertiary concerns. They can be addressed without impacting the main MCT design itself. MCT will necessarily have to rely on some preplaced ground infrastructure. It will be incredibly foolish to hobble the fundamental design just because of the very first mission. If it is a problem, SpaceX can use Dragon to pre-land a rover or a crane, or some one-shot modifications made to MCT to allow it to land on an unprepared surface.

http://www.astronautix.com/lvs/dcy.htm



BTW, if you want an argument why methane (and higher hydrocarbons, like ethylene, which can be made exothermally from methane using the right catalyst) is better than hydrogen for a SSTO vehicle, see here:
http://web.archive.org/web/20130826050557/http://dunnspace.com/alternate_ssto_propellants.htm
« Last Edit: 10/30/2015 02:39 AM by Robotbeat »
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