Author Topic: MCT Speculation and Discussion Thread 4  (Read 617870 times)

Offline R7

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Re: MCT Speculation and Discussion Thread 4
« Reply #160 on: 06/18/2015 06:55 PM »
As a note, it occured to me where Rocketplane Kistler must have gotten then idea for the shape of the K-1.

;-)

George Mueller has been around for a long time (96yrs old!). He was there to manage things when Saturn V was being stacked before becoming Kistler Aerospace CEO. Also many other guys in Kistler had roots back to Redstone age. It was the newoldspace :)
« Last Edit: 06/18/2015 06:58 PM by R7 »
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Offline JasonAW3

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Re: MCT Speculation and Discussion Thread 4
« Reply #161 on: 06/18/2015 07:10 PM »
Mars surface to earth is much less than 9km/s. More like 7-7.5km/s.

But still the largest delta v requirement. At some point for growing vehicle dry weight other delta v requirements become larger. A 90mt vehicle weight may even be beyond that point. But leaving Mars surface it would be prudent to have significantly more capability than just the minimum. The item here is that use as 2nd stage to LEO, use as EDS and the use as direct return all have close to the same propellant requirements. The vehicle dry weight minus the payload weight drives which one is the driver for the amount the MCT must hold.

My values gives the information that it is doable and the reasoning behind some of the speculation on what the MCT would do.  For Earth departure you just keep filling the tanks until you get enough prop. Note here is that you need also prop for the landing phase on Mars of ~1km/s or more based on the vehicle shape and weight.

Edit: Difference between 2nd stage or EDS and Mars liftoff is payload size 100mt vs 25mt.

     I'm thinking that there may be more involved with the Fairing recovery concept than simply holding down launch costs.  It could be that any fuel tankage launched into orbit by SpaceX would be designed with minimal RCS, TPS, avionics and recovery equipment (parachute) for reuse.

    I'm also wondering about their choice to launch the pad abort with the trunk still attached.  Stability could be the reason, but one would think that with computer controlled throttling, that really shouldn't be an issue.  If the MCT lander is essentially a scaled up Crew Dragon, it could be that the main cargo and habitat for the passengers would be carried in a "trunk" style structure, to be released to it's own soft landing after atmospheric interface.  A strictly cargo and habitat module could take a rougher ride to landing, using Soyuz style retrothrust in the last seconds before landing, while the MCT itself would accomplish a propulsive landing after seperating from the Hab/cargo trunk.

     The habitat/cargo section can be made lighter than the MCT itself, as it is designed to be kept on Mars and won't have to go through launch and landing stresses more than one time.  (It may also be made as a specialized Bigelow Olympus class inflatible with seperate modules that could be linked to it as well).     
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Offline Lobo

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Re: MCT Speculation and Discussion Thread 4
« Reply #162 on: 06/18/2015 07:17 PM »
MCT will only be reused 15-30 times (once each for Mars entry and Earth entry), so for the crew version, PICA-X may be fine, perhaps with TPS replacement every decade or so. Metallic TPS probably doesn't make sense for MCT, except for perhaps a tanker or cargo version that would just travel to LEO but could be reused hundreds or even thousands of times and which isn't as mass-sensitive.

PICA-X should be just fine for MCT, though, if it works as good as SpaceX says it does. metallic would be suboptimal except for tanker/cargo duty.

So how would the PICA-X be arranged?  Just on the nose like F9USR and K-1?  Or on the nose and along one half of the cylinder like most biconic concepts?
And how long would/could the PICA-X be made to last?  Assuming two EDL's per round trip, and no more than 1 trip every 2 years, would mean 5 trips per decade max per MCT, and 10 EDL's?  I think Dragon's PICA-X is supposedly good for 10 EDL's, but that's from LEO reentry speeds. 
The idea is to not have to replace the whole TPS system too often.  If it only needed replaced once a decade, that wouldn't seem to be too cumbersome.

A tanker MCT could do several LEO EDL's per year though.  Then again, from LEO, metallic TPS should be adequate without the overheating issues of interplanetary return?

But do they want two different TPS configurations?  I would think they might rather go with a common platform.  Would replacing a PICA-X TPS on a cargo/tanker MCT be too cumbersome to do every 10 missions? (which could be within one year theoretically)  Can PICA-X tiles be made to last more than 10 LEO reentries?  Or would that be overly heavy?

Maybe a cargo/tanker MCT could have the PICA-X just on the nose like F9US-R and K-1, as it would be coming back only from LEO speeds like them, and would be uncrewed, so fairly high g-loading for that reentry profile wouldn't be a problem for humans.  That's be a pretty ballistic reentry without much lift vs. a belly entry.  But SpaceX and Rocketplace Kistler seemed to think it would work without crushing their beer cans.


« Last Edit: 06/18/2015 07:20 PM by Lobo »

Offline Lobo

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Re: MCT Speculation and Discussion Thread 4
« Reply #163 on: 06/18/2015 07:28 PM »
From Statements made by SpaceX representatives:
- 100mt payload delivery to Mars
- 1/4 payload SSTO return to Earth from mars surface
- prop density 1m^3 for 1mt (LOX and CH4)
- 15m diameter vehicle (this was hited at not actually specified by SpaceX
- Raptor engines 380-385 vacuum ISP 500klbf

A vehicle like this results:
- Vehicle structure+engines+ shield =40mt
- Max propellant load 900mt
- propulsion section (engines and tanks) cylindircal or nearly cylindrical section at base 15m diameter and 6m tall
- bi-conal payload section (first section 15m to 10m diameter 10m tall) (second section 10m to 0m 10m tall) ~1800m^3 volume
-MCT can be its own 2nd stage on the BFR (BFR is basically just the 1st stage) would have ~7.5km/s delta v capability with a 100mt payload+40mt vehicle dry weight +900mt propellant load
-An MCT tanker variant would be a Cargo MCT without any cargo which could deliver ~150mt of propellant to LEO would have 6km/s delta v capability

In order to get to Mars 6-9 tankers docking in LEO-MEO are required

Edit Added: BTW An MCT cargo used as the 2nd stage going just to LEO would be capable of delivering 180mt of payload. Note the 1st stage needs to be capable of ~3km/s delta v with a fully loaded MCT + 180mt of payload on top ~1120mt MCT+payload GLOW
15m diameter and other such details have not been mentioned. Please cite your sources and put the source quote in the MCT source thread so we know exactly what was said: http://forum.nasaspaceflight.com/index.php?topic=37839.0

I remember Shotwell I think saying something about 15m.  I think it was in the context of a question comparing it's scale to Saturn V.  I think she said it'll be wider than Saturn V, maybe as much as 15m wide.

But the reference was from awhile ago, if memory serves, before she made the comment about it being too big for pad 39A even.  And as we know, MCT's design appears to be very much in flux, so it may not be relevant any more, but I Think that's what Atlas was referring to by 15m being "hinted at". 

If I can dig up her actual quote somewhere, I'll post it on your other thread.

Online spacenut

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Re: MCT Speculation and Discussion Thread 4
« Reply #164 on: 06/18/2015 07:47 PM »
I really don't understand why SpaceX would not limit the BFR to 12 million lbs thrust to take advantage of the Kennedy, then so what if they can only get 75 tons to Mars on MCT instead of 100.  Once this BFR/MCT gets going, it will put SLS out of business since it will cost less and the entire Kennedy facility with 4 high bay doors and room for 6 BFR/MCT rockets at a time.  That would save SpaceX a ton of money, it seems to me, building the infrastructure for such a large rocket.  What would Kennedy be limited to 14m in diameter and 12 million lbs thrust?

Offline Robotbeat

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Re: MCT Speculation and Discussion Thread 4
« Reply #165 on: 06/18/2015 07:55 PM »
MCT will only be reused 15-30 times (once each for Mars entry and Earth entry), so for the crew version, PICA-X may be fine, perhaps with TPS replacement every decade or so. Metallic TPS probably doesn't make sense for MCT, except for perhaps a tanker or cargo version that would just travel to LEO but could be reused hundreds or even thousands of times and which isn't as mass-sensitive.

PICA-X should be just fine for MCT, though, if it works as good as SpaceX says it does. metallic would be suboptimal except for tanker/cargo duty.

So how would the PICA-X be arranged?  Just on the nose like F9USR and K-1?  Or on the nose and along one half of the cylinder like most biconic concepts?
And how long would/could the PICA-X be made to last?  Assuming two EDL's per round trip, and no more than 1 trip every 2 years, would mean 5 trips per decade max per MCT, and 10 EDL's?  I think Dragon's PICA-X is supposedly good for 10 EDL's, but that's from LEO reentry speeds. 
The idea is to not have to replace the whole TPS system too often.  If it only needed replaced once a decade, that wouldn't seem to be too cumbersome.

A tanker MCT could do several LEO EDL's per year though.  Then again, from LEO, metallic TPS should be adequate without the overheating issues of interplanetary return?

But do they want two different TPS configurations?  I would think they might rather go with a common platform.  Would replacing a PICA-X TPS on a cargo/tanker MCT be too cumbersome to do every 10 missions? (which could be within one year theoretically)  Can PICA-X tiles be made to last more than 10 LEO reentries?  Or would that be overly heavy?

Maybe a cargo/tanker MCT could have the PICA-X just on the nose like F9US-R and K-1, as it would be coming back only from LEO speeds like them, and would be uncrewed, so fairly high g-loading for that reentry profile wouldn't be a problem for humans.  That's be a pretty ballistic reentry without much lift vs. a belly entry.  But SpaceX and Rocketplace Kistler seemed to think it would work without crushing their beer cans.
Basically, yes, this is my thought process.
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Offline Robotbeat

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Re: MCT Speculation and Discussion Thread 4
« Reply #166 on: 06/18/2015 08:00 PM »
I really don't understand why SpaceX would not limit the BFR to 12 million lbs thrust to take advantage of the Kennedy, then so what if they can only get 75 tons to Mars on MCT instead of 100.  Once this BFR/MCT gets going, it will put SLS out of business since it will cost less and the entire Kennedy facility with 4 high bay doors and room for 6 BFR/MCT rockets at a time.  That would save SpaceX a ton of money, it seems to me, building the infrastructure for such a large rocket.  What would Kennedy be limited to 14m in diameter and 12 million lbs thrust?
They may have some clever ideas about how're get beyond usual limits, maybe how to reduce acoustic loads, as we already heard from what's his name's LC39A pad tour. Additionally, they could operate at lower thrust initially.

But only for initial flights. They will want their own launch facility.
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Offline Impaler

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Re: MCT Speculation and Discussion Thread 4
« Reply #167 on: 06/18/2015 11:41 PM »
A small forward facing heat-shield won't present enough surface area to slow down on Mars, you would probably impact the surface at high speed, also the L/D ratio would be very low or possibly nill in this configuration which results in even worse heat and g-forces.  A smaller ballistic coefficient works on Earth because we have a much deeper atmosphere to slow down in.

I don't know  how to calculate the ballistic coefficient of a bi-conic though, I think it is more complex then simply the cross-sectional area which is itself highly dependent on angle of attack.

I have to agree with earlier statements that a 40 mT MCT that dose single stage Earth return is not realistic. The 'long poles' are 7.5 kms DeltaV (btw did anyone account for landing on Earth in this math) and 14 kms Entry velocity both of which are HUGE.

My own conservative thought is for 5.1 kms DeltaV (launch and land on Mars) and 3.5 kms Entry velocity, both radically lower demands and I'd allocate 75-100 mT for it to allow for generous mass growth.

If SpaceX could build the first vehicle for 40 then they can certainly do the latter for even less which translates to even more cargo, safety margin and lower cost.
« Last Edit: 06/19/2015 12:30 AM by Impaler »

Offline Lobo

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Re: MCT Speculation and Discussion Thread 4
« Reply #168 on: 06/19/2015 12:19 AM »
A small forward facing heat-shield won't present enough surface area to slow down on Mars, you would probably impact the surface at high speed.  A smaller ballistic coefficient works on Earth because we have a much deeper atmosphere to slow down in.

My original thought was such a shield could do the initial deceleration and ablate enough of the thermal energy that the cylinder wrapped in a metallic TPS could then take the remaining heat load while MCT were to transition from ballistic entry to "belly" entry.  It could then present enough surface area to get down to supersonic terminal velocity.  Essentially the PICA-X nose would shunt enough heat that the metallic TPS wouldn't get over heated, as you were saying it likely would if coming directly in with only metallic TPS.  I'm not sure it would work, but was just thinking aloud.  In this way, only the relatively small and simple geometry of the nose would need to be periodically replaced, rather than a whole half of a cylinder or biconic.

But, as Robot pointed out, MCT heading to Mars physically could never do more than two EDL's every two years.  Maybe more like four if they need to be staged Mars Direct style where the arriving crew transfer to an MCT already on the surface and fueled, while theirs begins fueling itself for the next crew.  Depnding on refueling times and mission options.
So an ablative full belly/nose TPS could potentially go for a decade before being replaced if it could handle 6 or 8 EDL's before it ablated too far.

MCT's (or dedicated upper stages in your scenario) only going back and forth to LEO could then use just a metallic TPS, as they'll only see LEO reentry speeds, but could go through 10-12 reentries per year potentially. 
Or, theoretically, it could have just an ablator on the nose as it'll only be coming back from LEO speeds.  RPK and SpaceX seemed to think a ballistic entry like that would work, rather than needing to to a belly entry.

So some interesting options, if LEO MCT's/upper stages and Mars MCT's were to have a bit different TPS configuration from each other.

Offline Impaler

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Re: MCT Speculation and Discussion Thread 4
« Reply #169 on: 06/19/2015 12:45 AM »
Peak heat is also peak deceleration, small forward face delays deceleration in time and in depth into the atmosphere so the time remaining to do any other deceleration is reduced even if your able to increese your cross-sectional area and create more drag.  It's the same principle as opening a parachute on Mars, if your too late you impact the surface so I favor presenting maximum surface area as soon as possible.

And even under my lower velocity entry assumption something like ceramic tiles may be needed on the nose and control flaps, stagnation point temperatures can still be quite high. 

I don't favor any kind of ablative because I believe we can eventually do more then 2 EDL per synod by sending cargo containers outside of a vehicle and loading them into the lander in Low Mars orbit.  Then landing unloading refueling and returning to orbit with enough propellent to land again.  At a generous 1 week turn around one landing can bring down ~100 cargo loads per synod, so an indefinite lifespan TPS would be needed for that.  While only applicable to cargo this would be a huge improvement in efficiency over the baseline of 1 cargo in 1 lander in 1 synod.

Offline Impaler

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Re: MCT Speculation and Discussion Thread 4
« Reply #170 on: 06/19/2015 01:03 AM »

- propulsion section (engines and tanks) cylindircal or nearly cylindrical section at base 15m diameter and 6m tall
- bi-conal payload section (first section 15m to 10m diameter 10m tall) (second section 10m to 0m 10m tall) ~1800m^3 volume


Your volume estimate is badly off, I calculate your segments are from bottom to top 1060 m ^3 for the cylinder, 1244 m ^3 for the first frustum, 261 m^3 for the top cone (which would in reality need to be blunted to some degree).

Total 2565 m^3, a 60% increase over the vehicle size I'm proposing.

Keep in mind that a F9 first stage has a volume of 480 m^3 and has dry masses of 25 mT and holds just shy of 400 mT of propellents, this should really show how absurd a vehicle with 5 times the volume holding more then twice the propellent and with all the heat shields necessary to do high speed EDL could possibly have a mass only 15 mT more then a F9 first stage.
« Last Edit: 06/19/2015 01:14 AM by Impaler »

Offline Lobo

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Re: MCT Speculation and Discussion Thread 4
« Reply #171 on: 06/19/2015 07:19 PM »
Peak heat is also peak deceleration, small forward face delays deceleration in time and in depth into the atmosphere so the time remaining to do any other deceleration is reduced even if your able to increese your cross-sectional area and create more drag.  It's the same principle as opening a parachute on Mars, if your too late you impact the surface so I favor presenting maximum surface area as soon as possible.

You are probably right.  I'm no expert on Mars EDL.  My thought was mainly an easily replaceable ablator nose cap used in conjunction with metallic TPS behind it could perhaps operate as a hybrid system that would be easier to maintain than all ablative, but not overheat at interplanetary entry speeds as metallic might.  I don't know how long it would take from the time a conical shape presented it's full cross section during EDL until it reaches it's terminal velocity, which is what's needed before the final retro propulsion and landing.  If it could reach terminal velocity after presenting just it's nose, then transition to belly presenting it's full cross section and also reach terminal velocity in time to do the retro burn and landing, then that was my thinking.
Of course, if it's still has not slowed down to terminal velocity by the time it needs to do the final burn and landing, then as you say, this won't work and will crash, or would need more retro propulsion, which defeats the purpose.
Again, just thinking aloud.

And even under my lower velocity entry assumption something like ceramic tiles may be needed on the nose and control flaps, stagnation point temperatures can still be quite high. 

The problem with ceramic, as with the shuttle as I understand, is they are relatively fragile and more subject to MMOD strikes than metallic or ablator.  MCT will have it's TPS exposed to open space far more than the Shuttle, as well as on the surface stay.  But if you kept the amount to a minimum, the dangers are more minimized.

I don't favor any kind of ablative because I believe we can eventually do more then 2 EDL per synod by sending cargo containers outside of a vehicle and loading them into the lander in Low Mars orbit.  Then landing unloading refueling and returning to orbit with enough propellent to land again.  At a generous 1 week turn around one landing can bring down ~100 cargo loads per synod, so an indefinite lifespan TPS would be needed for that.  While only applicable to cargo this would be a huge improvement in efficiency over the baseline of 1 cargo in 1 lander in 1 synod.

In your preferred method that'd be a valid concern. 
With the direct approach Robot and I have been preferring, MCT would only do 1 EDL every synoid.  (1 Mars EDL during one synoid, and 1 Earth EDL for the next synoid, then back to Mars the next synoid, repeat).
So that'd be just one EDL every 2 years.  Just 6 in 12 years/3 round trips, with only 3 of them being the hotter Earth EDL's.  So as an example, assuming a single PICA-X ablator TPS can withstand 3 Mars EDL's and 3 Earth EDL's from direct entry from interplanetary speeds, it would only need replaced every 12 years.  That Particular MCT may even be ready to be retired at that point just due to 12 years of improvements/developments, and just the riggers of such lone periods in space and on the Mars surface.  So depending on the situation, it may never need to be actually replaced, and would last the service life of the Mars-MCT.
I think SpaceX has said the Dragon ablator is good for 10 Earth EDL's form LEO.  So 3 Mars EDL's and 3 Earth EDL's from BLEO speeds should be plausible, with perhaps a bit more thickness if necessary.  Then again, it could be thinner than Dragon due to the much larger surface area of MCT and much more "fluffy" density than Dragon.  As mass will be a premium on a Mars MCT, they may make it only thick enough to handle one round trip, and then just plan to replace it every time it comes home, to save every kg on a Mars MCT.  This K-1/F9USR geometry I've mentioned would be beneficial in that case, as it seems like it'd be far easier to replace PICA-X panels on it than on a non-uniform geometry like the space shuttle belly or the belly/nose of a true biconic or triconic.  So it wouldn't necessarily have to be an expensive or laborious thing.

Now, for the direct approach Robot and I have been favoring, there would be a depot, with many tanker missions to fill it up in preparation for a Mars MCT launch.  So there'd be a valid issue about the TPS system on those, and if they'd be any different than Mars MCT?  Would it be better to have a common system, and just accept the maintenance of replacing that Ablator every 10 missions or so? (That could be once a year)  And if so, maybe they could put just an ablative nosecap like K-1 and F9USR, which would be easier than doing the whole belly of it like the Mars MCT would need.  Just pull the whole nose cap off and have a new one that drops right in.  So replacement nosecaps can be prepared ahead of time to maintain these tankers.  In this way it wouldn't be much different than replacing a Dv1 heat shield.

Or go a whole different approach like metallic TPS? where you'd never have to replace the whole TPS, just individual panels that might be damaged.  That would make the tanker MCT at least somewhat different externally than the Mars MCT.  And may or may not be worth the trouble to have two different TPS systems.



Offline Lobo

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Re: MCT Speculation and Discussion Thread 4
« Reply #172 on: 06/19/2015 10:38 PM »
Unfortunately, NASA's funding is not purely for exploration, science, tech, etc, it is also a regional development program for Dixie and a couple other places. So you can have the coolest spaceship ever imagined for super cheap and amazing capability, and it still won't get but a small fraction of funding from NASA. Additionally, NASA is so much more than human spaceflight and launch services. $4 billion annually is the max possible I can imagine anything like BFR/MCT ever getting from NASA (in the next half century), with maybe $1-2 billion being far more likely.
This is a good point.  But it's also of note that ~$2B/year flowing into the coffers of SpaceX would really help with expeneses.  If they made $50M profit per Falcon launch, $2B would be the equivalent of about 40 Falcon launches per year worth of profit.

So I think it'd certainly help things out to have that, and provide maybe 2 lunar missions per year intially with 1-2 Mars missions per synoid later that NASA has seats on and input into where they land, etc.

The Mars missions would really be the deal, as SpaceX would be going there anyway.  They just have say 4 NASA astronauts along with say 3 SpaceX astronauts/spacecraft specialists on the early missions (7 total, so that a single F9/Dv2 can bring the whole crew up in one launch after MCT has docked with the depot and is all fueled up and ready to depart.  I think most NASA DRM missions were 4-6 crew, so 7 should work pretty well for the initial missions, then maybe expanding to 14...two Dv2 crew launches...for phase 2 once they start to get things figured out better, and maybe want to try to start an outpost.).

So that would be pretty much just all profit for SpaceX.

$2B/year could go a long way to aid SpaceX.  In return, NASA not only gets a HLV, they get a spacecraft, and the door to the Moon and Mars opened up to them.  A pretty good bargain for them!




Offline guckyfan

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Re: MCT Speculation and Discussion Thread 4
« Reply #173 on: 06/20/2015 05:57 AM »
I put this here because it seems the thread where it is least OT.

Then there's the 1st stage with expen$ive 10-15 meter tooling

It is always argued that a new tooling for a large diameter would be very expensive. But is it really true? I understand the common method is first make a circular barrel of a very thick aluminium sheet and then machine the structure in to make it light and robust. Bending a very thick sheet I imagine is expensive indeed. However with friction stir welding the sheet will be quite thin and the structure to make it robust is then welded on. Still expensive but a lot less so.

Any comments?


Offline oldAtlas_Eguy

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Re: MCT Speculation and Discussion Thread 4
« Reply #174 on: 06/20/2015 02:25 PM »

- propulsion section (engines and tanks) cylindircal or nearly cylindrical section at base 15m diameter and 6m tall
- bi-conal payload section (first section 15m to 10m diameter 10m tall) (second section 10m to 0m 10m tall) ~1800m^3 volume


Your volume estimate is badly off, I calculate your segments are from bottom to top 1060 m ^3 for the cylinder, 1244 m ^3 for the first frustum, 261 m^3 for the top cone (which would in reality need to be blunted to some degree).

Total 2565 m^3, a 60% increase over the vehicle size I'm proposing.

Keep in mind that a F9 first stage has a volume of 480 m^3 and has dry masses of 25 mT and holds just shy of 400 mT of propellents, this should really show how absurd a vehicle with 5 times the volume holding more then twice the propellent and with all the heat shields necessary to do high speed EDL could possibly have a mass only 15 mT more then a F9 first stage.

Thank you for the correction I noticed the error in the calculation of the second section and have corrected it in my spreadsheet for later use.

On weights it would be better to use Dragon and scale it up to see where the maximum weight of MCT would end up. Actual could be a value more or less than that but this estimate would be a good place to start. The weight scales with surface area increase not volume increase. The surface area increase of the MCT over a Dragon is ~ 19=exp(ln(2565/30)*2/3)^10 [Dragon having a volume defined by the surface skin of ~30m^3 (3.6m diameter in a cone 6m tall)].

Dragon weighs ~4.2mt so scaled up dry weight for MCT comes to about 80mt.

Edit: corrected math
BTW a 480m^3 1st stage weighing 25mt would give a scaled up weight for MCT of 76mt an even lighter value.
« Last Edit: 06/20/2015 02:36 PM by oldAtlas_Eguy »

Offline Robotbeat

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Re: MCT Speculation and Discussion Thread 4
« Reply #175 on: 06/20/2015 02:31 PM »
It's a pressure vessel (and internal equipment needs internal supports) so weight scales with volume, not just area.
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Offline oldAtlas_Eguy

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Re: MCT Speculation and Discussion Thread 4
« Reply #176 on: 06/20/2015 02:53 PM »
It's a pressure vessel (and internal equipment needs internal supports) so weight scales with volume, not just area.

Almost half the volume of the MCT is the prop tanks. So the scaling is very complex. So if the prop tank 1000m^3 section is scaled using an F9 1st stage and the upper capsule part is scaled based on volume increase of that section over that of a Dragon the result is 41mt for the tank portion and 215mt for the other non prop section. 215mt is a very high value and the actual as you suggest will be greater than the surface area increase derived weight for the "crew" section of 58mt and probably less than this volume derived weight of 215mt. So weights using this divide and conquer method is a range of 99-256mt for the MCT weight range.

An actual vehicle weight still maybe less than the lower value so we have an upper limit but not really any lower limit except maybe the 76mt value as the absolute best possible. Also note is that the F9 1st stage weight also includes engine weight which is 9 engines at ~.8mt each or 7.2mt of that 25mt of engines and tank.

Online spacenut

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Re: MCT Speculation and Discussion Thread 4
« Reply #177 on: 06/20/2015 05:49 PM »
I've seen steel pipe being made from 1/4" thick steel plate.   The plate steel is a huge roll brought in by rail.  They have rollers that can bend this plate into a long tube for 16" diameter pipe.  It is electric resistance welded without a seam down the middle.  Then it is cut to 42' lengths, beveled on the ends.  Then they sand blast to clean the pipe and a powder vinyl coating is put on the pipe to protect it from corrosion.  Then it is shipped to the gas pipeline companies.  If they can bend 1/4" thick plate steel, surely they can bend softer aluminum into a 15m diameter tube, that is a lesser bend.  Even the mill making the Falcon 9 could probably used the same bending tools, just using larger rollers for a larger diameter.  After initial investment, they can crank out rocket tubes fairly fast.  SpaceX has proven they can make a rocket far cheaper than Atlas or Delta.  So I think the BFR/MCT can be made far cheaper than SLS and in far less time, since it isn't political.   

Offline Impaler

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Re: MCT Speculation and Discussion Thread 4
« Reply #178 on: 06/20/2015 06:05 PM »
Perhaps a mass estimate based on adding up the parts of the MCT would be more reasonable.

You'll need,

# of Raptors and mass of Raptor (lets assume 100:1 thrust:weight which would make them 2.3 mT each.
Thrust structure mass, probably proportional to thrust, few good examples to base a comparison on
Tank volume and tankage fraction, F9 tanks are reasonable basis for comparison
Surface Area and mass per unit area of Thermal protection
Structural mass, probably proportional to internal volume and peak g-forces.
Landing legs, I've read that these are generally 10% of touch down mass.
Auxiliary systems, solar panels, radiators, batteries, avionics etc etc, again hard to estimate.


Offline Robotbeat

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Re: MCT Speculation and Discussion Thread 4
« Reply #179 on: 06/20/2015 07:14 PM »
Raptors would likely have a thrust to weight ratio at least competitive with Merlin 1D. They will be high pressure staged combustion, so although they'll be methane-fueled (not kerosene), they should still get at least Merlin 1D's level of performance. Merlin 1D has a thrust-to-weight-ratio of about 150.

The first stage should have a mass ratio of about 25 (including the landing legs, assuming propellant densification).

Assuming 15 million pounds of thrust (source is here: http://forum.nasaspaceflight.com/index.php?topic=37839.msg1391925#msg1391925 ), and 500klbf thrust per Raptor (source: http://forum.nasaspaceflight.com/index.php?topic=37839.msg1390942#msg1390942 ), then 30 Raptors looks reasonable. Note that these two figures weren't given at the same time, so it's possible the thrust of BFR/MCT will be different (and also number of Raptors), but my bet is they're likely to be fairly close to what BFR/MCT was supposed to be at the time of the AMA, based on other things mentioned there.
Chris  Whoever loves correction loves knowledge, but he who hates reproof is stupid.

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