Author Topic: MCT Speculation and Discussion Thread 4  (Read 624561 times)

Online sanman

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Re: MCT Speculation and Discussion Thread 4
« Reply #1980 on: 05/05/2016 06:51 AM »
Will MCT use methalox for all parts of its Earth-Mars trip? If so, then is that volatile methalox supposed to last all the way until the Mars EDL burn? Or will hypergolics be used for the ending burn instead?
Likewise, what about for return trip back to Earth? There likely won't be a way to manufacture traditional hypergolics on Mars for the return trip, so it sounds like Methalox will need to survive from Mars all the way to Earth.

How will that methalox be preserved across the entire journey?

Will MCT use the same very-low-temperature cryo approach as Falcon FT?



Offline TheTraveller

Re: MCT Speculation and Discussion Thread 4
« Reply #1981 on: 05/05/2016 08:53 AM »
Will MCT use methalox for all parts of its Earth-Mars trip? If so, then is that volatile methalox supposed to last all the way until the Mars EDL burn? Or will hypergolics be used for the ending burn instead?
Likewise, what about for return trip back to Earth? There likely won't be a way to manufacture traditional hypergolics on Mars for the return trip, so it sounds like Methalox will need to survive from Mars all the way to Earth.

How will that methalox be preserved across the entire journey?

Will MCT use the same very-low-temperature cryo approach as Falcon FT?

Have read no plans to manufacture hypergolics on Mars, plus those engines have low ISPs. Storing methalox is just an engineering issue in optimal insulation techniques.

Reddit info said BFS was 15 x 60m, so a big space for heavily insulated fuel tanks during the transit. Another thought is only the Mars descent & LEO insertion / Earth descent fuel needs to be long time insulated so maybe large low insulation TMI / TEI burn tanks and small high insulation Mars descent, LEO insertion / Earth descent burn tanks?
« Last Edit: 05/05/2016 08:54 AM by TheTraveller »
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Offline Space OurSoul

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Re: MCT Speculation and Discussion Thread 4
« Reply #1982 on: 05/05/2016 05:07 PM »
I think this counts as MCT speculation:

While reading this:
There is speculation that MCT might be built/tested/flown all at the Cape.

There is no way SpaceX would do that unless the range is ready to support daily flights.
It occurred to me that it's *technically feasible* to build BFR and BFS at the cape and have them both self-ferry to Brownsville... in fact this is feasible from any eastern-seaboard location.
Perhaps this side-steps the problem of needing a large skilled workforce in an out-of-the-way place like Brownsville.


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Offline envy887

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Re: MCT Speculation and Discussion Thread 4
« Reply #1983 on: 05/05/2016 05:17 PM »
I think this counts as MCT speculation:

While reading this:
There is speculation that MCT might be built/tested/flown all at the Cape.

There is no way SpaceX would do that unless the range is ready to support daily flights.
It occurred to me that it's *technically feasible* to build BFR and BFS at the cape and have them both self-ferry to Brownsville... in fact this is feasible from any eastern-seaboard location.
Perhaps this side-steps the problem of needing a large skilled workforce in an out-of-the-way place like Brownsville.

Shipping large rockets by sea is a problem that has been solved many times. It's not only *technically feasible* but SOP.

Offline TheTraveller

Re: MCT Speculation and Discussion Thread 4
« Reply #1984 on: 05/05/2016 06:26 PM »
I think this counts as MCT speculation:

While reading this:
There is speculation that MCT might be built/tested/flown all at the Cape.

There is no way SpaceX would do that unless the range is ready to support daily flights.
It occurred to me that it's *technically feasible* to build BFR and BFS at the cape and have them both self-ferry to Brownsville... in fact this is feasible from any eastern-seaboard location.
Perhaps this side-steps the problem of needing a large skilled workforce in an out-of-the-way place like Brownsville.

Shipping large rockets by sea is a problem that has been solved many times. It's not only *technically feasible* but SOP.

SpX / Elon has breadcrumbed for a dedicated island site for MCT.

Where else can SpX build a 15m diameter core that is maybe 60m or 120m long and transport it to the launch pad?
« Last Edit: 05/05/2016 06:35 PM by TheTraveller »
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Offline Vultur

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Re: MCT Speculation and Discussion Thread 4
« Reply #1985 on: 05/05/2016 10:45 PM »
(Note that all Musk has ever said about the Moon is, paraphrasing, "it's on the way, I guess we should look at how to get there, but it's not anything like a goal of the architecture to be able to go to the Moon".)

I, for one, would like to see a new interplanetary transportation architecture be designed to support more than one possible destination, I guess... ;)

Well, if MCT has the delta-v to SSTO from Mars and go to Earth, it should be able to land on and take off from the Moon just fine, even if it's not really designed for that.

Offline CyclerPilot

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Re: MCT Speculation and Discussion Thread 4
« Reply #1986 on: 05/06/2016 03:28 AM »
Regarding the BFS TPS, I always thought the best place to put it was on the bottom.  This way force is always in the same direction (same for launch and EDL) and it would enable to lowest possible structural mass and simplist cargo/passenger stowage.  This also means that the TPS will look like Swiss cheese to allow openings for the engines (or alternatively nacelles).

If we put the TPS on top, it solves some of those headaches but creates a few new ones.  Engines would be protected during descent and TPS would be protected during landing.  However,  would a blunt TPS designed for Mars EDL have too much drag during earth ascent?  Does anyone have a drag coefficient by mach number graph for a blunt body?  Things like a lofted trajectory and throttling near max Q would help, but would add to gravity losses.

Assuming it does create too much drag during Earth assent (likely), would the mass penalty of adding a fairing be worth it?  Although i couldnt find any historical precedent, I think it could even be inflatable.  Inflatables have been proposed for mars descent, which should have higher mechanical and thermal loads than earth ascent.  Being inflatable could also make recovery easier by passively surviving descent and splashdown (or make a big slow target for a helicopter mid air grab.)

The last paragraph is outside my area of expertise, so I thought I would bounce the idea off the NSF community.

Offline Impaler

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Re: MCT Speculation and Discussion Thread 4
« Reply #1987 on: 05/06/2016 07:24 AM »
Drag in these kinds of situations basically boils down to cross-sectional area.

The design I think would be most efficient is to use an ADEPT type system of carbon cloth on tensioned carbon-fiber arms on the top/sides of the vehicle which opens like an umbrella.  Entry would be nose first and the base of the vehicle is clear for engines and landing gear.  The opening and closing of the decelerator also acts as the door to the cargo hold as the vehicles outer skin segments are attached to the arms and are carried out to the perimeter of the umbrella shape.

Offline Paul451

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Re: MCT Speculation and Discussion Thread 4
« Reply #1988 on: 05/07/2016 08:29 AM »
Regarding the BFS TPS, I always thought the best place to put it was on the bottom.
[...] and it would enable [the] simplist cargo/passenger stowage.

What's the reasoning for that?

If we put the TPS on top,

I've assumed if the TPS isn't at the bottom, it'll be down the ventral side. With a biconic or lifting-body forward shape, allowing lift/drag type EDL, extending the deceleration, reducing the amount of supersonic retropropulsion required, and lowering the final speed before the landing burn, both reducing fuel demand.

It also allows a longer glide, improving safety for suboptimal entry timing. (For eg, uneven upper atmosphere means the BFS decelerates earlier than expected, losing speed before reaching the intended landing area. BFS lands short, necessitating a rescue scenario. With a longer glide, you can stretch the descent.)

I think it could even be inflatable.

Doubtful. It would need to be repacked on Mars to allow for the Earth-return EDL. Or you'd need to carry two expendable inflatable heat-shields (one for Mars, one for return to Earth.)

(It just doesn't fit the typical SpaceX architecture.)

(or make a big slow target for a helicopter mid air grab.)

Errr, no. IMO, ULA's engine pod for Vulcan will end up being be too large for helicopter aerial recovery, the BFS would be too large for capture even by a giant transport plane.

Offline envy887

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Re: MCT Speculation and Discussion Thread 4
« Reply #1989 on: 05/07/2016 04:19 PM »
I think it could even be inflatable.

Doubtful. It would need to be repacked on Mars to allow for the Earth-return EDL. Or you'd need to carry two expendable inflatable heat-shields (one for Mars, one for return to Earth.)

(It just doesn't fit the typical SpaceX architecture.)

Inflatables are preferable for Mars because of the very high terminal velocity (compared to the same mass and shape on Earth reentry), so it may have an inflatable for Mars entry but drop it at Mars and use a traditional heat shield for Earth return.

If the inflatable has a secondary use on Mars or enroute (Raw material? Habitat?) it could be considered payload and wouldn't be just so much dead weight.

Offline CyclerPilot

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Re: MCT Speculation and Discussion Thread 4
« Reply #1990 on: 05/07/2016 05:11 PM »
Regarding the BFS TPS, I always thought the best place to put it was on the bottom.
[...] and it would enable [the] simplist cargo/passenger stowage.
What's the reasoning for that?
If acceleration is always in the same direction, then you only need to support cargo from one direction.  If not, straps / bolts have to be beefier and more numerous.  Not a huge issue.  Slightly more problematic is passengers after the flip maneuver and landing.  Their high-g chairs need to flip around.   I think some kind of swinging hammock would work, but as I said, acceleration in only one direction is simpler to design for.
I've assumed if the TPS isn't at the bottom, it'll be down the ventral side. With a biconic or lifting-body forward shape, allowing lift/drag type EDL, extending the deceleration, reducing the amount of supersonic retropropulsion required, and lowering the final speed before the landing burn, both reducing fuel demand.

It also allows a longer glide, improving safety for suboptimal entry timing. (For eg, uneven upper atmosphere means the BFS decelerates earlier than expected, losing speed before reaching the intended landing area. BFS lands short, necessitating a rescue scenario. With a longer glide, you can stretch the descent.)
Certainly all valid points.  I think everyone agrees that lift is pretty much required to minimize prop.

The craft I am describing is very similar in shape to the F9 S2 in the S2 landing video. I didn't mention this in my original post in the interest of keeping it short, but during descent,  BFS would need to tilt slightly to provide lift and would therefore need to have at least a thin heat shield along one side to protect it.

I think our two proposals are similar, but just vary in shape of the craft and angle during descent.  The shape you describe certainly has the advantage of the nose of the craft being appropriate for both ascent and descent.

Doubtful. It would need to be repacked on Mars to allow for the Earth-return EDL. Or you'd need to carry two expendable inflatable heat-shields (one for Mars, one for return to Earth.)
...
(It just doesn't fit the typical SpaceX architecture.)
...
Errr, no. IMO, ULA's engine pod for Vulcan will end up being be too large for helicopter aerial recovery, the BFS would be too large for capture even by a giant transport plane.

Maybe I wasn't clear enough.  This fairing would be jettisoned right around S1 separation.  The TPS is a rigid blunt shape underneath.  Fairing is not required in mars ascent because the thin atmosphere creates little drag even for a blunt nose.   
BFS would land vertically at the earth launch site.

Thanks for the feedback.

Offline drzerg

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Re: MCT Speculation and Discussion Thread 4
« Reply #1991 on: 05/08/2016 10:37 AM »
what do you think about tripropellant engine for long duration missions?
i know that lox+RP+H2 gives some good results but could lox+methane+H2 do that?

for example (engine is the same):

for BFR first stage: start on methane 66-75% + 33-25% H2 for best thrust, then high isp flight on pure H2 and landing on pure methane (no need for high insulation for H2)

for MCT: start to mars on pure H2 and landing on mars on pure methane. start from mars on methane or mix if it possible to produce H2. landing on earth on pure methane.   

Offline DJPledger

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Re: MCT Speculation and Discussion Thread 4
« Reply #1992 on: 05/08/2016 06:52 PM »
what do you think about tripropellant engine for long duration missions?
i know that lox+RP+H2 gives some good results but could lox+methane+H2 do that?

for example (engine is the same):

for BFR first stage: start on methane 66-75% + 33-25% H2 for best thrust, then high isp flight on pure H2 and landing on pure methane (no need for high insulation for H2)

for MCT: start to mars on pure H2 and landing on mars on pure methane. start from mars on methane or mix if it possible to produce H2. landing on earth on pure methane.   
Ain't going to happen. BFR+MCT will be 100% LOx/LCH4. Elon is staying away from H2 because of it's difficulties.

Offline Vultur

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Re: MCT Speculation and Discussion Thread 4
« Reply #1993 on: 05/08/2016 07:18 PM »
I kind of doubt the BFR will require inflatable decelerators or anything like that. Looking at some delta-v maps, LEO to Mars transfer orbit is a lot less delta-v than Mars surface to Earth transfer, so a little extra propulsive landing delta-v probably will be fine.

Given SpaceX's good mass ratios, I think accepting a requirement for a bit more delta-v/performance for architecture simplicity is the way to go, anyway.

Offline envy887

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Re: MCT Speculation and Discussion Thread 4
« Reply #1994 on: 05/09/2016 03:32 AM »
Putting 100t from LEO through TMI is about the same as putting 25t from Mars surface to TEI (exact number depends on non payload masses). The extra Delta v comes out in the payload.

That said, I agree that inflatables don't seem to be a thing at SpaceX.

Online matthewkantar

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Re: MCT Speculation and Discussion Thread 4
« Reply #1995 on: 05/10/2016 07:49 PM »
This is more BFR than MCT speculation. I believe a three core arrangement uses about a third more metal to hold the same amount of propellant at about the same strength as a single core of the same capacity, so it is no surprise they are not considering multiple cores in this clean sheet design. If logistics of ground or sea transport are discarded, the remaining major trade would seem to be between flight dynamics and tank efficiency.

More squat designs hold more liquid with less metal. I was thinking of the growing waste of space/added structure  as tanks get more spherical and remain stacked. I then imagined the ideal would be a sphere of LOX inside a sphere of methane. Inner sphere situated at the bottom of the outer sphere.

Fooling around with various layouts, I am wondering if it would make sense to have the LOX tank be an uninsulated cylinder inside of a cylindrical liquid methane tank, both with normal domes. Would seem to have some structural and plumbing benefits.

I am sure this has been thought of in the past, but I have not found references to it. What are the downsides?

Enjoy, Matthew

Offline envy887

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Re: MCT Speculation and Discussion Thread 4
« Reply #1996 on: 05/10/2016 08:00 PM »
Matt, that's exactly what a common bulkhead propellent tank design does. Look up the Saturn V second stage tank common bulkhead.

http://www.alternatewars.com/Games/KSP/Tut2/KSP_Tutorial_2-6.htm

Offline TomH

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Re: MCT Speculation and Discussion Thread 4
« Reply #1997 on: 05/11/2016 12:08 AM »
Matt, that's exactly what a common bulkhead propellent tank design does. Look up the Saturn V second stage tank common bulkhead.

http://www.alternatewars.com/Games/KSP/Tut2/KSP_Tutorial_2-6.htm

Yup, common bulkheads have been around a long time. S-IVB (Saturn IB's second stage and Saturn V's third stage) had one too.

Offline Impaler

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Re: MCT Speculation and Discussion Thread 4
« Reply #1998 on: 05/11/2016 12:37 AM »
Pressure vessel mass to volume ratios don't change much with total volume, and narrower tanks give better airodynamics.  I think we will see BFR at around 10 m to 12.5 m in diameter and 90 m tall.

Online matthewkantar

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Re: MCT Speculation and Discussion Thread 4
« Reply #1999 on: 05/11/2016 01:08 AM »
I am familiar with common bulkheads, especially in the Saturn V, I am asking about nested cylinders. At very least it eliminates the plumbing from the top tank to the engines.

Matthew

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