Author Topic: MCT Speculation and Discussion Thread 4  (Read 504040 times)

Offline baldusi

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Re: MCT Speculation and Discussion Thread 4
« Reply #2300 on: 06/13/2016 07:17 PM »
Upto now SpaceX modus operandi has been to under deliver initially and over deliver after a few iterations. Dragon is not a scaled down MCT. So they will probably start with something small to validate it and learn and only later do the huge stuff when everything else closes.
« Last Edit: 06/13/2016 09:43 PM by Chris Bergin »

Online philw1776

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Re: MCT Speculation and Discussion Thread 4
« Reply #2301 on: 06/13/2016 07:26 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.

...and reasonable Km/sec budget ranges from LEO to Mars surface are...?
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Online Robotbeat

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Re: MCT Speculation and Discussion Thread 4
« Reply #2302 on: 06/13/2016 07:38 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.

...and reasonable Km/sec budget ranges from LEO to Mars surface are...?
7km/s is reasonable for like 90-100 day transits and /especially/ bad transit opportunities. But I think 6km/s is enough for nominal mission.

...a big question is Mars to Earth.
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Offline envy887

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Re: MCT Speculation and Discussion Thread 4
« Reply #2303 on: 06/13/2016 08:02 PM »
7km/s is reasonable for like 90-100 day transits and /especially/ bad transit opportunities. But I think 6km/s is enough for nominal mission.

...a big question is Mars to Earth.
If you budget 6.3 km/s (5.0 km/s for TMI and 1.3 km/s for EDL), you will typically be able to make a ~100-day transit with plenty of margin. Fast TMI is typically 4.5 to 4.9 km/s:

Edit: fuel loads and payloads can always be tweaked for faster or slower transits, so tank size isn't really a limiting design factor. Design for a standard case, then optimize for other factors.
« Last Edit: 06/13/2016 08:06 PM by envy887 »

Online Robotbeat

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Re: MCT Speculation and Discussion Thread 4
« Reply #2304 on: 06/13/2016 08:10 PM »
7km/s is reasonable for like 90-100 day transits and /especially/ bad transit opportunities. But I think 6km/s is enough for nominal mission.

...a big question is Mars to Earth.
If you budget 6.3 km/s (5.0 km/s for TMI and 1.3 km/s for EDL), you will typically be able to make a ~100-day transit with plenty of margin. Fast TMI is typically 4.5 to 4.9 km/s:

Edit: fuel loads and payloads can always be tweaked for faster or slower transits, so tank size isn't really a limiting design factor. Design for a standard case, then optimize for other factors.
Right.

I think 6km/s is enough for LEO to Mars surface (I think 1km/s is more than enough for EDL, provided you can handle that fast of a reentry for gee-load reasons), but I wonder also about Mars to Earth.
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Offline MikeAtkinson

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Re: MCT Speculation and Discussion Thread 4
« Reply #2305 on: 06/13/2016 08:20 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.

...and reasonable Km/sec budget ranges from LEO to Mars surface are...?
7km/s is reasonable for like 90-100 day transits and /especially/ bad transit opportunities. But I think 6km/s is enough for nominal mission.

...a big question is Mars to Earth.

Yes, Mars to Earth is harder, especially with 25 tonnes of payload.

It would be instructive for someone to put together a table like that above showing transits in both directions.

I think it is not particularly easy to do, it needs to give a reasonable length of time on Mars (at least a week, more like a month, to allow cargo transfer/refueling/processing of multiple BFS) and also a reasonable length of time on Earth so that it can enable relaunched and refueling of the BFS fleet.

Some of these return transits may use longer, lower delta-v orbits, but at each synod there should also be a fast transit for crew (even if that transit does not allow one synod reuse).

I think returns in the 3-4 month duration are only possible if higher delta-v off-optimum Earth to Mars transits are made. It would be nice to have this confirmed. I've tried to do this with the data from NASA's trajectory browser, but have failed.

Offline mlindner

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Re: MCT Speculation and Discussion Thread 4
« Reply #2306 on: 06/13/2016 09:27 PM »
Upto now SpaceX modus operandi has been to under deliver initially and over deliver after a few iterations.

Since when?

Quote
Dragon is not a scaled down MCT. So they will probably start with something small to validate it and learn and only later do the huge stuff when everything else closes.

And Dragon is a scaled up version of nothing.

Why would they build something else when they've explicitly stated that they are not? The explicit statement is MCT first test launch in 2022. The only reason they've built anything smaller in the past is when they've been unable to build what they intended. Falcon 9 only happened because Falcon 5 wasn't big enough to carry Dragon and they'd already substantially designed Falcon 5. The rocket only got bigger because they found optimizations they could make to the architecture and the engines.

A mini-MCT would happen if they build the Raptor and then found it was underperforming and needed to scale back the MCT and it wouldn't be that much smaller and the goal wouldn't be the mini-MCT. I can't understand why an engineer would consider a mini-MCT as being preferable to a full-up MCT if the intention was ultimately to build an MCT. You should build the MCT first as you intend and then compromise if you have to and then work on fixing the bugs and refine your design over time. As it is, MCT is likely to scale up even further as they refine Raptor engine design.
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Offline Oersted

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Re: MCT Speculation and Discussion Thread 4
« Reply #2307 on: 06/13/2016 09:42 PM »
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.

We need to look no further than the ISS living quarters. Just little 'cocoons' with velcroed flaps. That should be just fine for intrepid spacefarers going to Mars.

Offline Impaler

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Re: MCT Speculation and Discussion Thread 4
« Reply #2308 on: 06/13/2016 10:23 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.

...and reasonable Km/sec budget ranges from LEO to Mars surface are...?

I estimate 800 m/s for Mars EDL which is consistent with Red Dragons simulations.  A decelerator in excess of 15m in diameter such as ADEPT or HIAD could lower this but at the cost of higher dry mass.

Departing Earth from EML1 with a lunar and Earth swing-buy can send you to a fast Mars transit can be around 1 km/s.

Offline envy887

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Re: MCT Speculation and Discussion Thread 4
« Reply #2309 on: 06/13/2016 11:31 PM »
EML1 is 3.77 km/s from 200 km LEO, so the total budget via that route is about the same as direct TMI from LEO. It's just broken into smaller steps.

Online philw1776

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Re: MCT Speculation and Discussion Thread 4
« Reply #2310 on: 06/14/2016 01:19 AM »
I think he's talking about starting at EML1, dropping to ~LEO and doing an Oberth burn
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Online Robotbeat

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Re: MCT Speculation and Discussion Thread 4
« Reply #2311 on: 06/14/2016 03:17 AM »
I think he's talking about starting at EML1, dropping to ~LEO and doing an Oberth burn
Indeed. And if you had a full propellant load at EML1/2, you could do even faster transits, though you'd have to reserve some of the propellant for slowing down. You're hitting diminishing returns if you have to use a lot of propellant to slow down, but it's still interesting to consider.
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Offline RobLynn

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Re: MCT Speculation and Discussion Thread 4
« Reply #2312 on: 06/14/2016 03:03 PM »
MCT booster design height:

An optimally over-expanded rocket (eg RD193) produces 56 tonnes thrust per square meter of nozzle area at sea level, won't be much difference between various hydrocarbon rockets as all have similar pressures and velocities at nozzle exit.

Packing of multiple nozzles will only cover about 60% of the rocket base area at best, probably more like 50% given allowances for gimballing.

Need about 1.3g acceleration at liftoff (or more for higher ISP)

So 56*.5/1.3= ~20-25 tonnes of mass per square meter of base area that can be supported by a rocket.  For LOX/CH4 bulk density of ~800kg/m that means that rockets can only support a column of fuel about 25-30m high in a prismatic form factor.

There are of course advantages to being thin for reduced aero losses - so maximum fuel column height within this constraint is likely to be the design chosen.

There will of course be substantial height taken by domed tank ends, 1st and 2nd stage engines, and finally cargo bay, but assuming a common bulkhead between LOX and CH4 and a single stick design I think could safely assume that will only be 30m+~1.4 stage diameter (4x domed ends for 2 stacked stages) + engine lengths (perhaps 4m per stage).

So overall I think 1st+2nd stage very unlikely to be taller than 50-60m.  Though payload might add 30m in some cases.  Ie rocket portion is pretty damned similar heights to Falcon9.  It will just be substantially bigger diameter.

100M+ tall rockets only make sense for LH2.

Given Musk's suggestions of 2-3MN thrust to weight optimum for SC engines, I think that the design starts to look quite constrained.  with perhaps the big choice being 2 rings (7-9 engines, 2000-3000 tonne GTOW) or 3 rings, 21-25 engines and 6-9000 tonne GTOW, and given development costs, launch site costs, and market it will almost certainly be the former.

Assuming 9x2.5MN engines each 4m long , that is about 9m diameter and total 50m tall (without cargo)
Assuming 25x2.5MN engines each 4m long, that is about 14m diameter and 58m tall (without cargo)
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Offline envy887

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Re: MCT Speculation and Discussion Thread 4
« Reply #2313 on: 06/14/2016 05:01 PM »
Given Musk's suggestions of 2-3MN thrust to weight optimum for SC engines, I think that the design starts to look quite constrained.  with perhaps the big choice being 2 rings (7-9 engines, 2000-3000 tonne GTOW) or 3 rings, 21-25 engines and 6-9000 tonne GTOW, and given development costs, launch site costs, and market it will almost certainly be the former.

Given that it needs to put 200-300 metric tons in orbit, it will almost certainly be the latter. SpaceX's goal is Mars in the 2020's, and a 9-Raptor booster that's only marginally more powerful than Falcon Heavy doesn't get them there.

Offline The Amazing Catstronaut

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Re: MCT Speculation and Discussion Thread 4
« Reply #2314 on: 06/14/2016 05:16 PM »
In addition, it needs to put 100 metric tons of payload, not just spacecraft mass, onto the Martian surface. It's it's go ginormous or go home.
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Offline docmordrid

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Re: MCT Speculation and Discussion Thread 4
« Reply #2315 on: 06/14/2016 05:16 PM »
Musk has stated 15,000,000 lbf of takeoff thrust, so take it from there. Go long or go home.

Edit: GMTA :)
« Last Edit: 06/14/2016 05:18 PM by docmordrid »
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Offline TomH

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Re: MCT Speculation and Discussion Thread 4
« Reply #2316 on: 06/14/2016 05:41 PM »
MCT booster design height:

An optimally over-expanded rocket (eg RD193) produces 56 tonnes thrust per square meter of nozzle area at sea level, won't be much difference between various hydrocarbon rockets as all have similar pressures and velocities at nozzle exit.

Packing of multiple nozzles will only cover about 60% of the rocket base area at best, probably more like 50% given allowances for gimballing.

Need about 1.3g acceleration at liftoff (or more for higher ISP)

So 56*.5/1.3= ~20-25 tonnes of mass per square meter of base area that can be supported by a rocket.  For LOX/CH4 bulk density of ~800kg/m that means that rockets can only support a column of fuel about 25-30m high in a prismatic form factor.

There are of course advantages to being thin for reduced aero losses - so maximum fuel column height within this constraint is likely to be the design chosen.

There will of course be substantial height taken by domed tank ends, 1st and 2nd stage engines, and finally cargo bay, but assuming a common bulkhead between LOX and CH4 and a single stick design I think could safely assume that will only be 30m+~1.4 stage diameter (4x domed ends for 2 stacked stages) + engine lengths (perhaps 4m per stage).

So overall I think 1st+2nd stage very unlikely to be taller than 50-60m.  Though payload might add 30m in some cases.  Ie rocket portion is pretty damned similar heights to Falcon9.  It will just be substantially bigger diameter.

100M+ tall rockets only make sense for LH2.

Given Musk's suggestions of 2-3MN thrust to weight optimum for SC engines, I think that the design starts to look quite constrained.  with perhaps the big choice being 2 rings (7-9 engines, 2000-3000 tonne GTOW) or 3 rings, 21-25 engines and 6-9000 tonne GTOW, and given development costs, launch site costs, and market it will almost certainly be the former.

Assuming 9x2.5MN engines each 4m long , that is about 9m diameter and total 50m tall (without cargo)
Assuming 25x2.5MN engines each 4m long, that is about 14m diameter and 58m tall (without cargo)

Well, your numbers differ astoundingly from Musk's numbers. And HE is the one building the rocket.

Online Robotbeat

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Re: MCT Speculation and Discussion Thread 4
« Reply #2317 on: 06/14/2016 06:43 PM »
We know from public info that BFR is supposed to be about 15 million lbf of thrust. That implies a mass of around 12E6lbm or 6000tons.

It's quite true that there's a limit to the height of the fueled portion of a rocket of constant diameter just simply due to what an efficient rocket engine can achieve.

But there will be space for plumbing and for an interstage and the payload (the cabin portion of the BFS) may be quite low density, so you could definitely imagine around 100m or more for the whole shebang.

...interestingly, try doing a similar calculation for Falcon 9, and you'll see that it should be much shorter than its 70m height.
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Offline Mongo62

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Re: MCT Speculation and Discussion Thread 4
« Reply #2318 on: 06/14/2016 09:49 PM »
IAC 2016 PLENARIES AND HIGHLIGHT LECTURES

Tuesday September 27
13:30-14:30 CDT = 14:30-15:30 EDT
Colonizing Mars -- A deep technical presentation on the space transport architecture needed to colonize Mars (SpaceX late breaking)

Is this in addition to the previously known Friday 8:30-10:30 CDT = 9:30-11:30 EDT session?
« Last Edit: 06/14/2016 09:59 PM by Mongo62 »

Offline envy887

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Re: MCT Speculation and Discussion Thread 4
« Reply #2319 on: 06/15/2016 04:20 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.

Musk is saying MCT will launch for Mars in 2022 if everything works right. I don't see any time to develop a ~5 meter Raptor booster before that, or any real reason to have it at all after BFR is flying. BFR will eventually service the common orbits (GTO, ISS, synchronous LEO, and all BEO), and other orbits will be serviced by Falcon, SEP from a BFR launch orbit, buying a whole BFR launch, or another launch provider.

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