Author Topic: MCT Speculation and Discussion Thread 4  (Read 570541 times)

Offline geza

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Re: MCT Speculation and Discussion Thread 4
« Reply #2280 on: 06/13/2016 11:55 AM »
How much is 6 or 8 years?

Kennedy challenge: May 25, 1961
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Online Semmel

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Re: MCT Speculation and Discussion Thread 4
« Reply #2281 on: 06/13/2016 12:05 PM »
Split out part thread on mini-BFR speculation:

Building a smaller BFR would require almost the same up front cost as the full thing. The factory, tooling, workforce, launch infrastructure, etc are at best slightly cheaper for a mini version but not by much. The cost of doing it twice (mini+full BFR) seems so prohibitive that investing only in the full size BFR seems much better in comparison. The length of BFR might change over time because it does not require a retooling of the factory.  I don't think the diameter will change once it is set though.
« Last Edit: 06/13/2016 09:42 PM by Chris Bergin »

Offline Robotbeat

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Re: MCT Speculation and Discussion Thread 4
« Reply #2282 on: 06/13/2016 12:56 PM »
Your only allowing 900 m/s for all drag and gravity losses which looks to be too low by around 300 m/s as I can't find any vehicle with total losses of less then around 1200 m/s.  That would drop your mass in LEO by about 25 mt and put the flight total at an even 8 which is roughly meeting in the middle of our earlier estimates.  Alternatively stretching your vehicle will likely make up the difference.

I'm willing to accept 8 as the best estimate for refueling flights needed to perform this all-chemical brute-force mission architecture when performing a fast crew transfer.  Do you have an estimate for what could be sent on a slow cargo flight?

P.S.  Oops, 300 m/s is what you get from Earth rotation so looks good.
8 is too many.
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Online envy887

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Re: MCT Speculation and Discussion Thread 4
« Reply #2283 on: 06/13/2016 02:40 PM »
Phil, if I might ask, why do you have so much performance spec'ed into the upper stage?

A fast (80-120 day) transfer to Mars rarely requires more than 4.5 to 5 km/s from LEO, but your numbers (1604t wet, 225t dry, 380 ISP) give 7.32 km/s of total performance with a 100t payload. Mars EDL will add somewhere in the 1.2 to 1.5 km/s range (I don't see an estimate in your spreadsheet), but even with that requirement your margins run from 12.6% to 28.4%. Since it can do a fast transfer and Mars EDL while only partly fueled, I get 4 to 5.5 refueling launches per fast Mars transfer.

Earth return definitely can use that performance, but you don't include any numbers for that. By my estimates the poor alignments in the 2020's prevent even a 1604t wet, 125t dry vehicle from returning before the next synod's optimal launch window.

Offline Robotbeat

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Re: MCT Speculation and Discussion Thread 4
« Reply #2284 on: 06/13/2016 02:49 PM »
1.2-1.5km/s is on the high end for EDL except if you decide to do a braking burn before entry.
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Online envy887

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Re: MCT Speculation and Discussion Thread 4
« Reply #2285 on: 06/13/2016 03:02 PM »
1.2-1.5km/s is on the high end for EDL except if you decide to do a braking burn before entry.

That would indicate 30t of fuel is excessive for Earth EDL, as that's 0.85 to 1.0 km/s for a 100t tanker depending on the altitude/ISP. I don't anticipate anything with an orbital re-entry heatshield will do an entry burn (it's rather pointless in Earth's dense atmosphere), and terminal velocity on Earth will be under 200 m/s for a very large, relatively light vehicle.

Offline Robotbeat

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Re: MCT Speculation and Discussion Thread 4
« Reply #2286 on: 06/13/2016 03:03 PM »
1.2-1.5km/s is on the high end for EDL except if you decide to do a braking burn before entry.

That would indicate 30t of fuel is excessive for Earth EDL, as that's 0.85 to 1.0 km/s for a 100t tanker depending on the altitude/ISP. I don't anticipate anything with an orbital re-entry heatshield will do an entry burn (it's rather pointless in Earth's dense atmosphere), and terminal velocity on Earth will be under 200 m/s for a very large, relatively light vehicle.
Right. And you'll have enough thrust to do a high-thrust landing, which means low gravity losses.

Also, 100t tanker is much too small.
« Last Edit: 06/13/2016 03:04 PM by Robotbeat »
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Online envy887

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Re: MCT Speculation and Discussion Thread 4
« Reply #2287 on: 06/13/2016 03:08 PM »
1.2-1.5km/s is on the high end for EDL except if you decide to do a braking burn before entry.

That would indicate 30t of fuel is excessive for Earth EDL, as that's 0.85 to 1.0 km/s for a 100t tanker depending on the altitude/ISP. I don't anticipate anything with an orbital re-entry heatshield will do an entry burn (it's rather pointless in Earth's dense atmosphere), and terminal velocity on Earth will be under 200 m/s for a very large, relatively light vehicle.
Right. And you'll have enough thrust to do a high-thrust landing, which means low gravity losses.

Also, 100t tanker is much too small.

That's 100t dry. Would be 1200t or 1500t at launch.

Offline Robotbeat

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Re: MCT Speculation and Discussion Thread 4
« Reply #2288 on: 06/13/2016 03:10 PM »
1.2-1.5km/s is on the high end for EDL except if you decide to do a braking burn before entry.

That would indicate 30t of fuel is excessive for Earth EDL, as that's 0.85 to 1.0 km/s for a 100t tanker depending on the altitude/ISP. I don't anticipate anything with an orbital re-entry heatshield will do an entry burn (it's rather pointless in Earth's dense atmosphere), and terminal velocity on Earth will be under 200 m/s for a very large, relatively light vehicle.
Right. And you'll have enough thrust to do a high-thrust landing, which means low gravity losses.

Also, 100t tanker is much too small.

That's 100t dry. Would be 1200t or 1500t at launch.
100t dry is too high for a tanker, I think. And maybe try 1000t instead. And greater payload.
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Offline Robotbeat

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Re: MCT Speculation and Discussion Thread 4
« Reply #2289 on: 06/13/2016 03:17 PM »
SpaceX can do dry masses of about 25:1 (and I think they can do better, actually). Take away 10% for using methane (due to lower density), 10% for TPS (this is what a TPS expert told me), 10% for landing legs (that's very conservative for an optimized design, by the way), 10% for landing thrust (assumes 300m/s landing delta-v, so 200m/s terminal velocity with 50% gravity losses), you're left with a mass ratio of ~18.

18 mass ratio with 1000t is 56t on-orbit (with enough propellant to land) and about 50t totally dry.

And I think SpaceX can do much better using better materials. If you look at how good the later versions of the Shuttle external tank were, combined with SpaceX's T/W=200, and further materials improvements (by using state of the art composites instead of just al-li alloys), I can definitely see it being improved further, and not just incrementally but dramatically. Like 30-40t dry.

But assuming 100t dry is about twice as heavy as what I think is realistic for SpaceX to achieve. That's sandbagging, IMO.
« Last Edit: 06/13/2016 03:19 PM by Robotbeat »
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Online MikeAtkinson

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Re: MCT Speculation and Discussion Thread 4
« Reply #2290 on: 06/13/2016 04:01 PM »
100t dry is too high for a tanker, I think. And maybe try 1000t instead. And greater payload.
For a dedicated tanker which does not share the outer mould line of the BFS then 100 tonnes is too high (I reckon something like 70 tonnes, but SpaceX might be able to reduce that even more). However, the first tanker flights may not be a dedicated design, instead just standard BFS with extra fuel in their tanks and perhaps long duration equipment removed. Then 100 tonnes dry is probably a bit too little.

Although a dedicated tanker design with all the excess mass removed would be cheaper in the long run, during BFR/BFS development it would be another craft competing for development funds and effort.

Online philw1776

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Re: MCT Speculation and Discussion Thread 4
« Reply #2291 on: 06/13/2016 04:40 PM »
Phil, if I might ask, why do you have so much performance spec'ed into the upper stage?

A fast (80-120 day) transfer to Mars rarely requires more than 4.5 to 5 km/s from LEO, but your numbers (1604t wet, 225t dry, 380 ISP) give 7.32 km/s of total performance with a 100t payload. Mars EDL will add somewhere in the 1.2 to 1.5 km/s range (I don't see an estimate in your spreadsheet), but even with that requirement your margins run from 12.6% to 28.4%. Since it can do a fast transfer and Mars EDL while only partly fueled, I get 4 to 5.5 refueling launches per fast Mars transfer.

Earth return definitely can use that performance, but you don't include any numbers for that. By my estimates the poor alignments in the 2020's prevent even a 1604t wet, 125t dry vehicle from returning before the next synod's optimal launch window.

Nit, I used 1629 wet, but that's beside your point.

Here's what I was using for Mars transit...
"LEO esc 3.2Km/s + Fast transit ~1.7 Km/s + aerocapture + Mars landing 2Km/s ~ 7 Km/sec Delta V for 2nd stage MCT"  Since I was sloppy and forgot to include the URL on my spreadsheet I'm not sure of the source.  I was trying for the 7Km/sec to Mars and propellant tank volume for 8 something Km/sec for return from Mars with reduced payload, 25t, not 100t.
Hopefully I erroneously over required Km/sec for transit, which would lower the # of tanker flights needed.
I solicit better estimates of the delta V needed to go all chemical from LEO to Mars surface.
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Offline Robotbeat

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Re: MCT Speculation and Discussion Thread 4
« Reply #2292 on: 06/13/2016 04:42 PM »
100t dry is too high for a tanker, I think. And maybe try 1000t instead. And greater payload.
For a dedicated tanker which does not share the outer mould line of the BFS then 100 tonnes is too high (I reckon something like 70 tonnes, but SpaceX might be able to reduce that even more). However, the first tanker flights may not be a dedicated design, instead just standard BFS with extra fuel in their tanks and perhaps long duration equipment removed. Then 100 tonnes dry is probably a bit too little.

Although a dedicated tanker design with all the excess mass removed would be cheaper in the long run, during BFR/BFS development it would be another craft competing for development funds and effort.
I think the initial BFSes will be on the order of 80-120 tons dry, even with crew equipment.

At first, they won't need much equipment due to a small crew. I mean,  the entire Salyut 6 module was 20 tons and was not anywhere NEAR mass-optimized but still was enough for 2 crew for 100 days. So yeah, I think another 30-50 tons dry over the tanker mass would be more than enough (actually, overkill) for a small crew.

But there's tons of room for mass optimization. How lightweight can things really get? Incredibly lightweight. Think backpacking, but with materials and manufacturing capabilities available in the 2030s. Already, you can easily halve the mass of many living quarters-related items with cleverness. So you can start supporting larger crews, like 50 passengers, in that original 30-50 tons. I could even see 100 passengers in 50 tons, plus consumables (many of which could be recycled or available in ultra-dense forms, like 300-500 grams of low-glycemic food per day, just 3-5 tons for the transit). It might take decades to get to that level of optimization, but they have decades before they will be sending that many people at once.

So I think we should make the assumption that SpaceX will start with fairly low dry masses. Like perhaps as low as 80 tons dry for the initial crew vehicle. To support larger crews, the dry mass may increase slightly over time, but they'll likely be aggressively optimizing and reducing the dry mass per passenger, so it may not change too much.
« Last Edit: 06/13/2016 04:45 PM by Robotbeat »
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Offline Robotbeat

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Re: MCT Speculation and Discussion Thread 4
« Reply #2293 on: 06/13/2016 04:59 PM »
http://www.defense.gov/News/Contracts/Contract-View/Article/642983

Quote
This other transaction agreement requires shared cost investment with SpaceX for the development of a prototype of the Raptor engine for the upper stage of the Falcon 9 and Falcon Heavy launch vehicles.

Considering:
1) the MCT (or BFS) is essentially a big reusable upper stage
2) Raptor is being built primarily for MCT/BFS
3) It's doubtful SpaceX would be building a new /expendable/ upper stage based on Raptor when they have plenty of performance with Falcon Heavy for any commercial payloads

...this info suggests that SpaceX is considering building an almost-certainly-reusable upper stage based on Raptor. Since all info suggests such a stage would be VTVL, it'd essentially already BE a lander, and Earth-lander. Such a stage could also be used on Mars (especially if the upper stage has the sort of long-lifetime modifications you might want for direct GSO insertion), just like Red Dragon is.

So it's false to say there's "zero reason to suspect that a mini-MCT may exist." The USAF contract suggests it could be an option, so that's non-zero reason.

...doesn't have to be a terribly CONVINCING reason, but it's reason nonetheless, so your statement is false.
« Last Edit: 06/13/2016 09:43 PM by Chris Bergin »
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Offline Robotbeat

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Re: MCT Speculation and Discussion Thread 4
« Reply #2294 on: 06/13/2016 05:01 PM »
Note: Elon's recent announcement of a first possible flight to Mars of MCT in 2022 reduces the odds that SpaceX will be using a Raptor-based upper stage in that manner.

I wouldn't bet any money that SpaceX is developing the mini-MCT, but it still remains a possibility.
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Online envy887

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Re: MCT Speculation and Discussion Thread 4
« Reply #2295 on: 06/13/2016 05:26 PM »
...
"LEO esc 3.2Km/s + Fast transit ~1.7 Km/s + aerocapture + Mars landing 2Km/s ~ 7 Km/sec Delta V for 2nd stage MCT"  Since I was sloppy and forgot to include the URL on my spreadsheet I'm not sure of the source.  I was trying for the 7Km/sec to Mars and propellant tank volume for 8 something Km/sec for return from Mars with reduced payload, 25t, not 100t.
...
Thanks. Terminal velocity should be in the range of 500 to 1,250 m/s at Mars, depending on atmospheric density, drag coefficient, etc. 2 km/s for EDL is very conservative.
128 day transits require 3.75 to 4.87 km/s from 200km LEO for TMI (80 days can be done under 5 km/s sometimes):
Edit (other link was rather long) See NASA's trajectory browser: http://goo.gl/yHCwzE
« Last Edit: 06/13/2016 06:45 PM by envy887 »

Online docmordrid

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Re: MCT Speculation and Discussion Thread 4
« Reply #2296 on: 06/13/2016 05:46 PM »
Or, this stage is a SpaceX long duration stage for bidding on the same contracts as Vulcan ACES.
« Last Edit: 06/13/2016 05:50 PM by docmordrid »
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Offline Robotbeat

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Re: MCT Speculation and Discussion Thread 4
« Reply #2297 on: 06/13/2016 06:11 PM »
Or, this stage is a SpaceX long duration stage for bidding on the same contracts as Vulcan ACES.
Right, but that could be done with their current kerolox stage as well, you just need more batteries.

Musk said they decided to put off second stage reuse because it's not quite worth it yet with kerolox (and to high energy orbits). But he left open the use of methane/LOx.

This would be the PERFECT opportunity to develop a reusable upper stage, and I cannot imagine them spending so much money on a new upper stage with a new propellant combination and a new engine and likely much, much higher performance that was not intended to be reusable.
« Last Edit: 06/13/2016 06:12 PM by Robotbeat »
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Re: MCT Speculation and Discussion Thread 4
« Reply #2298 on: 06/13/2016 06:23 PM »
How about the reusable RaptorVac S2 first, likely 5+ meters, with a later Falcon-Raptor high performance S1 to replace the F9 Merlin cores? That eliminates duplicious ground systems and possibly having 2 launchers.
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Offline Robotbeat

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Re: MCT Speculation and Discussion Thread 4
« Reply #2299 on: 06/13/2016 07:08 PM »
2km/s is ridiculously high for EDL unless you're doing a large braking burn.

Sandbagging stuff is not helpful and will mislead you.
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