Author Topic: BFR with expendable upper stage  (Read 3834 times)

Offline Michael Bloxham

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BFR with expendable upper stage
« on: 02/13/2018 02:41 AM »
Hey guys,

I'm wondering what the LEO and TMI performance of the BFR would be with an expendable version of the BFS acting as an upper stage? Assume no delta wings, no TPS, and deleted landing engines (so just 4x vacuum Raptors).

The scenario I'm thinking about is if reusable BFS is put in the too-hard basket, or is found suitable for LEO ops only. Or the economics of large-scale fuel manufacturing on Mars doesn't work out (at least for initial missions!)...

The idea of an expendable upper could be quite compelling given the extra TMI performance out of it. With a hammer-head PLF you could fit some very large diameter Mars landers in there, thereby keeping lander density low, easing EDL requirements.

I also found a Reddit thread talking about Falcon Heavy TMI performance with expendable rather than reusable central booster. It implied costs per kg were lower for the expendable case, because of the extra performance.

Cheers,

- Mike

Offline Robotbeat

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Re: BFR with expendable upper stage
« Reply #1 on: 02/13/2018 02:53 AM »
Boo. Theyíll keep trying reusable upper stage until they go bankrupt.
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Offline Michael Bloxham

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Re: BFR with expendable upper stage
« Reply #2 on: 02/13/2018 02:57 AM »
Iím sure theyíll have the reusable upper stage eventually, for LEO and maybe even GEO payloads. What Iím not sure about is the economics for the whole Mars thing.

Offline Prettz

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Re: BFR with expendable upper stage
« Reply #3 on: 02/13/2018 03:00 AM »
If it's expendable they can't afford to launch it in the first place, so what's the point?

Offline Robotbeat

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Re: BFR with expendable upper stage
« Reply #4 on: 02/13/2018 03:06 AM »
Iím sure theyíll have the reusable upper stage eventually, for LEO and maybe even GEO payloads. What Iím not sure about is the economics for the whole Mars thing.
You're confused because you're using direct-launch performance numbers to guide your intution. You basically can't actually direct-launch anything to Mars with a reusable two stage rocket. But that's irrelevant because you refuel the reusable upper stage in orbit. That is way more efficient anyway.

Don't ignore refueling. Refueling changes everything.
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Offline Michael Bloxham

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Re: BFR with expendable upper stage
« Reply #5 on: 02/13/2018 03:17 AM »
Iím not a fan of orbital-refuelling when it comes to Mars missions. With Mars EDL you want a low-density lander, so low surface mass and wide aeroshell diameters. Getting through the bottleneck of EDL is hard. With orbital refuelling, you just end up increasing the density of your payload. It doesnít help you at all. Iím not sure if SpaceX will achieve their EDL goals with BFS.

In a lot of Mars mission studies, the upper stage of the launcher also serves as the TMI stage, so I donít see why I canít assume the same with BFR.

Offline Michael Bloxham

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Re: BFR with expendable upper stage
« Reply #6 on: 02/13/2018 03:20 AM »
If it's expendable they can't afford to launch it in the first place, so what's the point?

If you assume very expensive payloads (as in a Mars mission), then expending the upper stage could make sense, given the increased performance. Youíre still reusing the whole first stage.

Online RonM

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Re: BFR with expendable upper stage
« Reply #7 on: 02/13/2018 03:25 AM »
Iím not a fan of orbital-refuelling when it comes to Mars missions.

That's the key step in the BFR/BFS architecture. Obviously, Elon Musk likes orbital refueling.

Offline Robotbeat

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Re: BFR with expendable upper stage
« Reply #8 on: 02/13/2018 03:31 AM »
I’m not a fan of orbital-refuelling when it comes to Mars missions. With Mars EDL you want a low-density lander, so low surface mass and wide aeroshell diameters. Getting through the bottleneck of EDL is hard. With orbital refuelling, you just end up increasing the density of your payload. ...
I can't see how you arrive at that conclusion. Nearly all the propellant you're refuelling with orbital refueling is burned doing TMI, so you end up with a low density "payload" (i.e. the entire BFS) with a high effective aeroshell area.

It seems you're not familiar with how BFS is planned to work. Well, you're in for a treat! Here's the 2016 video. BFR/BFS is essentially the same concept, just with a slightly smaller vehicle. Please watch the whole thing.

« Last Edit: 02/13/2018 03:32 AM by Robotbeat »
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Offline Robotbeat

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Re: BFR with expendable upper stage
« Reply #9 on: 02/13/2018 03:33 AM »
If you can't make the reusable upper stage work, then you don't have a Mars lander or aeroshell, either. They're the same vehicle. This is a key innovation in the BFR/ITS architecture. Please watch the above video.
« Last Edit: 02/13/2018 03:34 AM by Robotbeat »
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Offline Michael Bloxham

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Re: BFR with expendable upper stage
« Reply #10 on: 02/13/2018 03:40 AM »
But then you end up with this huge expensive BFS on Mars surface, which you need to refuel to send back. The whole idea of reusing the BFS for Mars relies on huge refuelling infrastructure on Mars. If this ends up being uneconomic, then the benefit of BFS for Mars goes away (still useful for LEO tho). Also, even with successful reuse, youíre still looking at the expense of setting up the fuel manufacturing on Mars in the first place. Youíre also taking the productive asset that is BFS and tying it up with incredibly infrequent Mars missions (one launch every 2.5 years). It might be better to use that asset for LEO work, where it could be flown hundreds of times per year. Thereís always a trade off. Iím suggesting it might make sense to go the expendable route for missions beyond LEO.

Online AC in NC

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Re: BFR with expendable upper stage
« Reply #11 on: 02/13/2018 03:42 AM »
Pretty sure they know how, by this point, to have run the numbers and decided you're wrong.

Offline Robotbeat

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Re: BFR with expendable upper stage
« Reply #12 on: 02/13/2018 03:46 AM »
Having a big refueling station on Mars isn’t that hard if you can land huge payloads on Mars cheaply. And BFS isn’t very expensive, on the order of $120m, still much cheaper than any comparable expendable lander.

The dream of a Mars City disappears without upper stage reuse and without reusable Mars landers. Full stop.
« Last Edit: 02/13/2018 03:48 AM by Robotbeat »
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Offline Michael Bloxham

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Re: BFR with expendable upper stage
« Reply #13 on: 02/13/2018 03:48 AM »
Pretty sure they know how, by this point, to have run the numbers and decided you're wrong.

Iím sure theyíre still working on their numbers. We havenít even seen their proposal for setting up the fuel manufacturing on Mars surface yet.
« Last Edit: 02/13/2018 03:50 AM by Michael Bloxham »

Offline Michael Bloxham

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Re: BFR with expendable upper stage
« Reply #14 on: 02/13/2018 03:55 AM »
The dream of a Mars City disappears without upper stage reuse and without reusable Mars landers. Full stop.

I disagree. You could build a nice Mars city using expendable landers. Itís down-mass (and volume) that you want. Which fits quite nicely with the idea of using large low-density aeroshells for surface payloads. BFS is volume restricted.

Offline Robotbeat

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Re: BFR with expendable upper stage
« Reply #15 on: 02/13/2018 03:59 AM »
The dream of a Mars City disappears without upper stage reuse and without reusable Mars landers. Full stop.

I disagree. You could build a nice Mars city using expendable landers. It’s down-mass (and volume) that you want. Which fits quite nicely with the idea of using large low-density aeroshells for surface payloads. BFS is volume restricted.
Nope. Can’t afford the downmass with expendable. Cannot and will not happen.

Luckily, there’s no magical law of physics preventing reusable upper stages and landers from happening. We already know it’s possible.
« Last Edit: 02/13/2018 03:59 AM by Robotbeat »
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Offline Michael Bloxham

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Re: BFR with expendable upper stage
« Reply #16 on: 02/13/2018 04:01 AM »
It comes down to the cost of setting up the fuel manufacturing and launch ops infrastructure.

Offline Robotbeat

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Re: BFR with expendable upper stage
« Reply #17 on: 02/13/2018 04:07 AM »
It comes down to the cost of setting up the fuel manufacturing and launch ops infrastructure.
...all of which is cheaper if you have the capability to land large payloads.

Oh, and if you don’t like that BFS is only used once per 26 months (although in several different roles, so effectively reused several times within 26 months), then there are ways to launch several payloads to Mars ballistically, then capture them at Mars and bring them down with a refueled BFS. So the one use per synod is not at all a hard and fast rule.
« Last Edit: 02/13/2018 04:07 AM by Robotbeat »
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Offline Michael Bloxham

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Re: BFR with expendable upper stage
« Reply #18 on: 02/13/2018 04:09 AM »
If you have to do 20+ flights of BFR/BFS to LEO to set up the infrastructure to return one BFS from Mars surface... Maybe it could work out cheaper to just to do one flight of BFR and an expendable BFS derived upper stage with a big dumb lander. The big dumb lander could help you set up the infrastructure more cheaply. Full reusability could come later, if itís desirable at all by that stage. You could just get straight into base-building with a big dumb lander.

Offline Robotbeat

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Re: BFR with expendable upper stage
« Reply #19 on: 02/13/2018 04:11 AM »
Without upper stage reuse, launch costs will never approach the low levels needed for mass settlement. With partial reuse we can get down to $1000/kg. We might need to get nearly $10/kg (consistent with spaceXís numbers for ITS/BFR) to really make mass settlement feasible. Just not possible without upper stage reuse.
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Offline Robotbeat

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Re: BFR with expendable upper stage
« Reply #20 on: 02/13/2018 04:13 AM »
If you have to do 20+ flights of BFR/BFS to LEO to set up the infrastructure to return one BFS from Mars surface...
No, you just need one booster, one BFS, and maybe another tanker version (which can be used for other tasks) to setup the required infrastructure. The launches themselves are cheap.
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Offline Michael Bloxham

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Re: BFR with expendable upper stage
« Reply #21 on: 02/13/2018 04:17 AM »
Without upper stage reuse, launch costs will never approach the low levels needed for mass settlement. With partial reuse we can get down to $1000/kg. We might need to get nearly $10/kg (consistent with spaceXís numbers for ITS/BFR) to really make mass settlement feasible. Just not possible without upper stage reuse.

You're implying you're not interested in anything less than the economic settlement of Mars being opened up by BFS. I don't see things that way... It could be that BFS will just not be able to make things as easy and as cheaply as Musk envisions. This stuff is hard. So what I'm asking (the performance of a BFR with expendable BFS derived upper stage) is maybe to look at plan B.

Offline Michael Bloxham

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Re: BFR with expendable upper stage
« Reply #22 on: 02/13/2018 04:28 AM »
With Plan B you could concentrate on an exploration phase of Mars, before committing to settlement. You could also look at cooperating more with NASA, and other potential stakeholders. The empty shells of the big dumb landers could also become very useful when you do commit to the base building phase. But anyway... All I want to know is the performance of BFR with an expendable upper stage! :-P

Offline Robotbeat

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Re: BFR with expendable upper stage
« Reply #23 on: 02/13/2018 04:32 AM »
You still donít seem to have a good grasp of the situation, though. Performance to Mars isnít actually lower because of reuse. No significant mass is wasted on the BFS. Everything you might need for reuse is also what you need to land on Mars.

A large expendable Mars lander still needs a huge heatshield. It still needs supersonic retropropulsion. It still needs landing thrusters. It still needs landing legs. You even still need large tanks on your trans Mars insertion stage.

You might save a little bit of IMLEO by staging off pieces (like tanks, heatshield, etc) as they are used, but that also increases complication and costs in its own way. How much did the EDL system for Curiosity cost? I bet itís still about the same as a BFS, even though itís purely expendable and can only land a hundredth of the mass.
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Offline Michael Bloxham

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Re: BFR with expendable upper stage
« Reply #24 on: 02/13/2018 04:41 AM »
Okay, I get it. But there still may be road-stoppers ahead. I'm curious about a Plan B. Okay? :-P

Offline Michael Bloxham

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Re: BFR with expendable upper stage
« Reply #25 on: 02/13/2018 04:46 AM »
One of the potential problems cited with BFS was the high g-loads during Mars EDL. That's because it's still a high-density design even with mostly empty tanks. That could be a problem considering your human cargo - and there's no provision for artificial gravity with BFS. So I was thinking about very low-density high-volume landers to keep g-loads as low as possible. But that question warrants a different thread, and after I'd gotten the answer to this one!

Offline Robotbeat

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Re: BFR with expendable upper stage
« Reply #26 on: 02/13/2018 04:50 AM »
High density design? citation badly needed.

The gee loads are pretty low since the body generates significant lift. The only reason they might look fairly high is the fast transit, which also minimizes microgravity exposure. If you wanted, you could always choose a slower trajectory which would have lower fee loads.
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Offline Michael Bloxham

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Re: BFR with expendable upper stage
« Reply #27 on: 02/13/2018 05:17 AM »
Ah okay. Well how about for a one-off exploration probe to say Titan, for example? How much extra performance could you get out of an expendable upper stage?

Offline Valerij

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Re: BFR with expendable upper stage
« Reply #28 on: 02/13/2018 09:21 AM »
Ah okay. Well how about for a one-off exploration probe to say Titan, for example? How much extra performance could you get out of an expendable upper stage?
Very small. The point is that the efficiency of a two-stage rocket starting from Earth, with the deltaV necessary for a flight to Mars, is very small.
   
For some specific missions, it may be advantageous to use disposable, consumable upper stages on the BFR. First of all, to create a depot of fuel in orbit. It is also possible to deliver the first elements of the fuel complex to Mars. But without refueling this stage on LEO you will not be able to deliver large modules not to the Moon, nor to Mars.
   
You will still need reusable upper stages as tankers for refueling the stage sent to Mars.

Offline DJPledger

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Re: BFR with expendable upper stage
« Reply #29 on: 02/13/2018 09:33 AM »
Expendable US for BFR defeats the objective as BFR is being designed for fully reusability. Expending any BFR hardware will dramatically increase it's cost per launch. Full reusability is what will give BFR extremely low launch costs.

Offline uhuznaa

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Re: BFR with expendable upper stage
« Reply #30 on: 02/13/2018 11:55 AM »
Expendable US for BFR defeats the objective as BFR is being designed for fully reusability. Expending any BFR hardware will dramatically increase it's cost per launch. Full reusability is what will give BFR extremely low launch costs.

"Full reusability" does not necessarily mean to only and always fly reusable second stages on BFR. If what you want to launch is something like a big space station module or some large module to land on the Moon, building a one-off second stage from parts or from a BFS near its end of life would be totally reasonable.

For one-way missions dragging a reusable stage with heat shield, landing engines, legs etc. all the way to the destination and back would be pointless.

Offline Robotbeat

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Re: BFR with expendable upper stage
« Reply #31 on: 02/13/2018 12:44 PM »
Ah okay. Well how about for a one-off exploration probe to say Titan, for example? How much extra performance could you get out of an expendable upper stage?
An expendable kick stage for one-off high energy missions may make sense! But you wouldnít want to do that from LEO. Instead, youíd want to send the BFS (with payload) to a high, elliptic orbit, then the kick stage would start itís burn while already very close to escape velocity at perigee.

Youíd want basically the biggest Star motor in OrbitalATKís catalogue. It, thatíd be one of the cheapest options.
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Online acsawdey

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Re: BFR with expendable upper stage
« Reply #32 on: 02/13/2018 04:12 PM »
Ah okay. Well how about for a one-off exploration probe to say Titan, for example? How much extra performance could you get out of an expendable upper stage?
An expendable kick stage for one-off high energy missions may make sense! But you wouldnít want to do that from LEO. Instead, youíd want to send the BFS (with payload) to a high, elliptic orbit, then the kick stage would start itís burn while already very close to escape velocity at perigee.

Youíd want basically the biggest Star motor in OrbitalATKís catalogue. It, thatíd be one of the cheapest options.

The step beyond that is a departure stage assembled in orbit (or brought up in a BFS if it can fit). Consists of just a few elements:

* tanks
* BFS-compatible refueling connection
* one Raptor-vac, perhaps with oversize radiatively cooled nozzle extension

Fuel it up (perhaps in a highly elliptical orbit), stick an outer solar system probe on it, and send it on it's way.

Since high chamber pressure isn't all that necessary for vacuum engines, one could also imagine using a low pressure engine that is pressure fed at something like 50 psi so you don't need special tanks. Perhaps Raptor would be affordable enough that it wouldn't be worth bothering.

Online groundbound

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Re: BFR with expendable upper stage
« Reply #33 on: 02/13/2018 05:17 PM »
Ah okay. Well how about for a one-off exploration probe to say Titan, for example? How much extra performance could you get out of an expendable upper stage?
An expendable kick stage for one-off high energy missions may make sense! But you wouldnít want to do that from LEO. Instead, youíd want to send the BFS (with payload) to a high, elliptic orbit, then the kick stage would start itís burn while already very close to escape velocity at perigee.

Youíd want basically the biggest Star motor in OrbitalATKís catalogue. It, thatíd be one of the cheapest options.

The step beyond that is a departure stage assembled in orbit (or brought up in a BFS if it can fit). Consists of just a few elements:

* tanks
* BFS-compatible refueling connection
* one Raptor-vac, perhaps with oversize radiatively cooled nozzle extension

Fuel it up (perhaps in a highly elliptical orbit), stick an outer solar system probe on it, and send it on it's way.

Since high chamber pressure isn't all that necessary for vacuum engines, one could also imagine using a low pressure engine that is pressure fed at something like 50 psi so you don't need special tanks. Perhaps Raptor would be affordable enough that it wouldn't be worth bothering.

I've actually wondered if someone might come up with a kick stage / tug hybrid that used methalox boost and solar electric return. Cycle time might be a year or more until it could get back to some earth orbit where BFS could refuel it, but that roughly matches the target market -- outer planet exploration missions.


Offline Ludus

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Re: BFR with expendable upper stage
« Reply #34 on: 02/13/2018 06:40 PM »
But then you end up with this huge expensive BFS on Mars surface, which you need to refuel to send back. The whole idea of reusing the BFS for Mars relies on huge refuelling infrastructure on Mars. If this ends up being uneconomic, then the benefit of BFS for Mars goes away (still useful for LEO tho). Also, even with successful reuse, youíre still looking at the expense of setting up the fuel manufacturing on Mars in the first place. Youíre also taking the productive asset that is BFS and tying it up with incredibly infrequent Mars missions (one launch every 2.5 years). It might be better to use that asset for LEO work, where it could be flown hundreds of times per year. Thereís always a trade off. Iím suggesting it might make sense to go the expendable route for missions beyond LEO.

If you like the frugality of using the BFS hundreds of times near earth, do that first. Once youíve amortized and depreciated your BFS so it has little book value, send it to Mars and treat it as expendable.

Offline TrevorMonty

Re: BFR with expendable upper stage
« Reply #35 on: 02/13/2018 07:26 PM »
An expendable 3rd stage would be better option for BFR. This could be fully fuelled 100t 1xRaptor stage + payload, delivered to LEO inside BFR 2nd stage. Shouldn't cost much more than F9 2nd stage, ideally have life of few days so it can deliver payloads direct to anywhere in cis lunar space.

Why initially expendable could turn into reuseable OTV that returns to LEO for BFR delivered payload and fuel. Much like what ULA plan to do with ACES.

The long life and refuelling capabilities this OTV needs are also required by BFR, so good test bed.

In expendable mode can deliver about 40t to  LLO or 30t when reusable. 30t to LLO in single launch using fully reuseable vehicles    would make for cheap lunar missions.
« Last Edit: 02/13/2018 07:37 PM by TrevorMonty »

Offline spacenut

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Re: BFR with expendable upper stage
« Reply #36 on: 02/13/2018 09:02 PM »
Musk mentioned it would take 6 tanker launches to completely fill the BFS or second stage for transport to Mars.  He is also sending two cargo ships to Mars 2 years prior to the human trip.  These would have the equipment to make Methane and LOX for refueling the human lander for return to earth.  He also said for a base, it would probably take 10 cargo flights to Mars with all the equipment and food for one human ship.  He also said the trip will be during the time when Mars is closest to earth and only require about a 2 month trip, so no real worries about zero g for humans for two months.  The stay on Mars would be about 2 years for the crew, to then return or continue to stay longer, eventually setting up the colony.  SEP tugs could also bring cargo via slow route on a continuous basis. 

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Re: BFR with expendable upper stage
« Reply #37 on: 02/13/2018 09:44 PM »
This is Plan B:

The Hunt for Red October (1990)
Quotes
Captain Ramius: When he reached the New World, Cortez burned his ships. As a result his men were well motivated.

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Re: BFR with expendable upper stage
« Reply #38 on: 02/13/2018 09:51 PM »
Ah okay. Well how about for a one-off exploration probe to say Titan, for example? How much extra performance could you get out of an expendable upper stage?

Why?

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Re: BFR with expendable upper stage
« Reply #39 on: 02/13/2018 10:32 PM »
Ah okay. Well how about for a one-off exploration probe to say Titan, for example? How much extra performance could you get out of an expendable upper stage?

Why?

Because BFS has a dry mass that cripples it for launching things to extreme energy trajectories. I buy that it is useful to Mars and maybe a little beyond. More than that and the final boost stage needs to go on a diet.

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Re: BFR with expendable upper stage
« Reply #40 on: 02/13/2018 10:48 PM »
Because BFS has a dry mass that cripples it for launching things to extreme energy trajectories. I buy that it is useful to Mars and maybe a little beyond. More than that and the final boost stage needs to go on a diet.
I can grant that and will.  It's beyond my direct knowledge.  But I don't think that means anything close to what the OP is wedded to.

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Re: BFR with expendable upper stage
« Reply #41 on: 02/13/2018 11:21 PM »
This is one of those "I figured out something SpaceX didn't" threads in which OP takes on all comers and keeps mutating the idea, or the assumptions, to try to keep it alive.

Ultimately, boring.

Locked. Use the report to mod function. to appeal the decision, if you must.
"I think it would be great to be born on Earth and to die on Mars. Just hopefully not at the point of impact." -Elon Musk
"We're a little bit like the dog who caught the bus" - Musk after CRS-8 S1 successfully landed on ASDS OCISLY

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Re: BFR with expendable upper stage
« Reply #42 on: 02/14/2018 02:43 PM »
Based on a request from a respected elder member of the forums, reopening, let's see how it goes.
"I think it would be great to be born on Earth and to die on Mars. Just hopefully not at the point of impact." -Elon Musk
"We're a little bit like the dog who caught the bus" - Musk after CRS-8 S1 successfully landed on ASDS OCISLY

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Re: BFR with expendable upper stage
« Reply #43 on: 02/14/2018 02:49 PM »
Ah okay. Well how about for a one-off exploration probe to say Titan, for example? How much extra performance could you get out of an expendable upper stage?

Why?

Because BFS has a dry mass that cripples it for launching things to extreme energy trajectories. I buy that it is useful to Mars and maybe a little beyond. More than that and the final boost stage needs to go on a diet.
I think I agree with this. Even with launch costs of like, say, $3-5m for BFR, for very high energy trajectories that need a lot of refueling, a $10m solid kick stage could be cheaper. Also, some trajectories would be super hard to recover the BFS from, and you don't want to expend a BFS or send it on a 5 year mission just for a small outer planets orbiter or something.
Chris  Whoever loves correction loves knowledge, but he who hates reproof is stupid.

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Re: BFR with expendable upper stage
« Reply #44 on: 02/14/2018 03:51 PM »
My opinion...Once BFS comes on line...
The need for (like said above) a solid or IMHO Hypergolic (or hybrid with both working together) kick booster will better serve the needs of high energy missions...  ;)

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Re: BFR with expendable upper stage
« Reply #45 on: 02/14/2018 04:46 PM »
My view: The base BFS system can do a lot. It should (and will) be built first. As experience is gained, optimizations for certain types of missions will be found. These may be in the form of third stages carried in the cargo bay, or they may be in the form of on orbit vehicles like tugs, or in the form of specialty BFS (the tanker mk II is likely to be something weird looking, not sharing the OML of the rest of the BFS variants) . All of this specialization starts from the base BFS, not the other way round.

(the basic Model T was done first, and then Ford and others (aftermarket) elaborated it into trucks, buses, racing vehicles, the model TT heavy truck/bus chassis,  even tractors.. (that last was a rather poor idea but it WAS tried) )

Eventually we move from "missions" to "cargo runs" at least in the inner system, and missions don't always (or even usually) originate on earth, they start with cargo taken somewhere. Lewis and Clark didn't start from NYC.
« Last Edit: 02/14/2018 04:47 PM by Lar »
"I think it would be great to be born on Earth and to die on Mars. Just hopefully not at the point of impact." -Elon Musk
"We're a little bit like the dog who caught the bus" - Musk after CRS-8 S1 successfully landed on ASDS OCISLY

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Re: BFR with expendable upper stage
« Reply #46 on: 02/14/2018 05:40 PM »
It seems to me that many alternatives and requirements are being conflated here.  I would very much appreciate if we could differentiate what we are talking about.

BFR with ExUS to me says the part bolted on BFR.  I just don't ever see this being expendable.  I can't imagine the cost/benefit ever working out vs. working within the limitations of what you've already got with some other approach (refueling, on-orbit assembly, etc..).  There are just too many options with planned marginal launch cost.

BFS-Cargo taking up some payload with an expendable stage is a different matter.

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Re: BFR with expendable upper stage
« Reply #47 on: 02/14/2018 05:59 PM »
What about for a BFS that has come to the end of its service life. Say itís done 4 round trips to Mars - the TPS has worn out. So itís stripped down and launched as an expendable...?

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Re: BFR with expendable upper stage
« Reply #48 on: 02/14/2018 06:05 PM »
Expendable US for BFR defeats the objective as BFR is being designed for fully reusability. Expending any BFR hardware will dramatically increase it's cost per launch. Full reusability is what will give BFR extremely low launch costs.

"Full reusability" does not necessarily mean to only and always fly reusable second stages on BFR. If what you want to launch is something like a big space station module or some large module to land on the Moon, building a one-off second stage from parts or from a BFS near its end of life would be totally reasonable.

For one-way missions dragging a reusable stage with heat shield, landing engines, legs etc. all the way to the destination and back would be pointless.

Exactly! I could also imagine SpaceX wanting to retire their Block 1ís & 2ís this way, once they start flying Block 3ís & 4ís. Even 747ís retire eventually ;-)

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Re: BFR with expendable upper stage
« Reply #49 on: 02/14/2018 06:39 PM »
What about for a BFS that has come to the end of its service life. Say itís done 4 round trips to Mars - the TPS has worn out. So itís stripped down and launched as an expendable...?

You're probably better off landing it one last time and using it as a permanent habitat module on Mars; or even just for spare parts. I think one of Robobeat's previous points is worth exploring in greater detail.

A large expendable Mars lander still needs a huge heatshield. It still needs supersonic retropropulsion. It still needs landing thrusters. It still needs landing legs. You even still need large tanks on your trans Mars insertion stage.

Let's look at a breakdown of a couple recent missions. From wiki:

Mars Science Laboratory:
 -Cruise Stage: 539kg
 -Curiosity Rover: 889 kg
 -Entry-Decent-Landing system: 2401 kg
 
 -Total mass: 3893 kg

Mars Exploration Rovers:
 -Instruments: 5 kg
 -Propellant: 50 kg
 -Heat Shield: 78 kg
 -Rover: 185 kg
 -Cruise Stage: 193 kg
 -Backshell / Parachute: 209 kg
 -Lander: 348 kg
 
 -Total Mass: 1,063 kg

Schiaparelli:
 -rear heat shield: 20 kg
 -hydrazine: 45 kg
 -main heat shield: 80 kg
 -lander: 280 kg
 
 -Total Mass: 577 kg


Even for a relatively "simple" missions, it's clear that incorporating EDL gear comes at great cost to your mission payload, for lack of a better phrase. At best, if you abandon BFS's built-in EDL capabilityes, one half if your total payload will be "useful". At worst, perhaps one fifth. Expending BFS would need to result in at least twice as much payload as a refeuled BFS. Likely better to use an end-of-life BFS as payload.
   

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Re: BFR with expendable upper stage
« Reply #50 on: 02/19/2018 02:39 AM »
One of the main concerns made by the OP on the first page is large scale fuel production on Mars.  But there's no avoiding that. ISRU is needed to get crew back. We certainly won't be landing return fuel with expendables.
So there's little point coming up with Plan Bs for that. There is no plan B to large scale ISRU.

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Re: BFR with expendable upper stage
« Reply #51 on: 02/19/2018 02:46 AM »
One of the main concerns made by the OP on the first page is large scale fuel production on Mars.  But there's no avoiding that. ISRU is needed to get crew back. We certainly won't be landing return fuel with expendables.
So there's little point coming up with Plan Bs for that. There is no plan B to large scale ISRU.

Pretty sure the OP wasn't interested in returning crew with that wide diameter low density lander.

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Re: BFR with expendable upper stage
« Reply #52 on: 02/19/2018 12:22 PM »
Is there a payload that needs that new lander?  Without crew, all we need for science is some orbiters and Curiosity sized rovers.

Also, I'm not worried about the first few BFS not coming back. It's not a waste, they're prototypes and SpaceX likes to iterate. The first few BFS build up the base. They're the ISRU plant, tanks, supplies, shelter. That's 4 ships right there that don't need return fuel, but do need to be the full configuration to test everything out. Using a new expendable lander configuration doesn't get us anywhere.
« Last Edit: 02/19/2018 02:21 PM by Norm38 »

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