Author Topic: BFR with expendable upper stage  (Read 11901 times)

Offline AC in NC

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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.

Offline Lar

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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.
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Offline Lar

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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

Online Robotbeat

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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.
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Offline John Alan

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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...  ;)

Offline Lar

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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

Offline AC in NC

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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.

Offline Michael Bloxham

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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...?

Offline Michael Bloxham

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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 ;-)

Offline 1

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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.
   

Offline Norm38

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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.

Offline AC in NC

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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.

Offline Norm38

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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 »

Online Ronsmytheiii

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Re: BFR with expendable upper stage
« Reply #53 on: 02/24/2018 12:11 PM »
A striped down version of BFR described by the OP (no heat shield, extra raptor engines removed) makes perfect sense as a lunar lander to me. You could use it as the BFS prototype, then rather than dispose of it after testing launch it as the first BFS orbital flight, then use early full up cargo BFRs to demonstrate in orbit refueling to send it to the moon.

That way you turn early test flights and equipment to deliver new operational capabilities!
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Offline Hotblack Desiato

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Re: BFR with expendable upper stage
« Reply #54 on: 03/20/2018 07:17 AM »
In my opinion, there is only one business case for an expendable upper stage for BFR:

A customer needs to get a massive monolithic payload into LEO (ballpark >300t), and is willing to pay for expending the stage. then a traditional upper stage makes sense.

But these customers will be very rare, especially since the BFR can already do monolithic payloads with 130-150t, and even if a 300t payload can't be split up in 2x150t, maybe it can be done at 3x150t.

So, SpaceX may not develop that stage because it's useless, and SpaceX already indicated that BFR will not be their biggest system. And suddenly, SX can do 300t reusable.

Offline rakaydos

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Re: BFR with expendable upper stage
« Reply #55 on: 03/20/2018 01:27 PM »
I wonder... there's been talk in other threads about a design for an orbital tug with a single raptor-vac and BFS-style refuelig pipes. Could a variant tug of this sort put itself into orbit "expendably" as a replacement second stage, under a fairing, and be fueled up in orbit for orbital operations?

Offline Slarty1080

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Re: BFR with expendable upper stage
« Reply #56 on: 03/20/2018 02:26 PM »
In my opinion, there is only one business case for an expendable upper stage for BFR:

A customer needs to get a massive monolithic payload into LEO (ballpark >300t), and is willing to pay for expending the stage. then a traditional upper stage makes sense.

But these customers will be very rare, especially since the BFR can already do monolithic payloads with 130-150t, and even if a 300t payload can't be split up in 2x150t, maybe it can be done at 3x150t.

So, SpaceX may not develop that stage because it's useless, and SpaceX already indicated that BFR will not be their biggest system. And suddenly, SX can do 300t reusable.

I think this has been asked before, but it is relevant here, so I will ask again, why are SpaceX building the BFS before the BFR? Seems to me that a BFR might be of great use on its own with the ability to launch 140mt to LEO (with a disposable adaptor and shroud attached).
 
If BFR could be launched repeatedly, rapidly and cheaply it could eat the space launch market alive, earning a lot of money to fund BFS development. But developing BFS first means itís going to sit around until BFR is ready to launch it, which seems a waste. Obviously SpaceX must have thought about this a lot more carefully than me; I'm just interested in their reasoning.


 
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Online RonM

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Re: BFR with expendable upper stage
« Reply #57 on: 03/20/2018 02:33 PM »
In my opinion, there is only one business case for an expendable upper stage for BFR:

A customer needs to get a massive monolithic payload into LEO (ballpark >300t), and is willing to pay for expending the stage. then a traditional upper stage makes sense.

But these customers will be very rare, especially since the BFR can already do monolithic payloads with 130-150t, and even if a 300t payload can't be split up in 2x150t, maybe it can be done at 3x150t.

So, SpaceX may not develop that stage because it's useless, and SpaceX already indicated that BFR will not be their biggest system. And suddenly, SX can do 300t reusable.

I think this has been asked before, but it is relevant here, so I will ask again, why are SpaceX building the BFS before the BFR? Seems to me that a BFR might be of great use on its own with the ability to launch 140mt to LEO (with a disposable adaptor and shroud attached).
 
If BFR could be launched repeatedly, rapidly and cheaply it could eat the space launch market alive, earning a lot of money to fund BFS development. But developing BFS first means itís going to sit around until BFR is ready to launch it, which seems a waste. Obviously SpaceX must have thought about this a lot more carefully than me; I'm just interested in their reasoning.

BFS is more of a technical challenge than BFR. If SpaceX can't get BFS to work the way they want then there is no need to build BFR.

Offline envy887

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Re: BFR with expendable upper stage
« Reply #58 on: 03/20/2018 02:45 PM »
In my opinion, there is only one business case for an expendable upper stage for BFR:

A customer needs to get a massive monolithic payload into LEO (ballpark >300t), and is willing to pay for expending the stage. then a traditional upper stage makes sense.

But these customers will be very rare, especially since the BFR can already do monolithic payloads with 130-150t, and even if a 300t payload can't be split up in 2x150t, maybe it can be done at 3x150t.

So, SpaceX may not develop that stage because it's useless, and SpaceX already indicated that BFR will not be their biggest system. And suddenly, SX can do 300t reusable.

I think this has been asked before, but it is relevant here, so I will ask again, why are SpaceX building the BFS before the BFR? Seems to me that a BFR might be of great use on its own with the ability to launch 140mt to LEO (with a disposable adaptor and shroud attached).
 
If BFR could be launched repeatedly, rapidly and cheaply it could eat the space launch market alive, earning a lot of money to fund BFS development. But developing BFS first means itís going to sit around until BFR is ready to launch it, which seems a waste. Obviously SpaceX must have thought about this a lot more carefully than me; I'm just interested in their reasoning.

BFR (the booster) can't launch 140 tonnes to orbit without an upper stage (like BFS). It could maybe launch 30 tonnes, but it would be expendable itself, and be far more expensive than a FH launch of the same mass.

BFR (the booster) with an expendable upper stage like the Falcon 9 upper stage could launch perhaps 100 tonnes to orbit, with a booster landing. However, this would require a lot of pad configuration for a system not intended to be operational for long.

BFR (the booster) with a large expendable Raptor upper stage (similar to New Glenn's 2nd stage) could launch perhaps 200 tonnes with booster reuse. But the expended upper stage would be quite expensive, and the overall utility of the system is questionable compared to the fully reusable BFR/BFS. There are no customers with 200 tonne payloads willing to pay that much, and FH can meet all existing payload requirements at a lower price.

The only system that is a notable improvement over F9/FH is one that is fully reusable.

Offline speedevil

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Re: BFR with expendable upper stage
« Reply #59 on: 03/20/2018 03:13 PM »
If BFR could be launched repeatedly, rapidly and cheaply it could eat the space launch market alive, earning a lot of money to fund BFS development. But developing BFS first means itís going to sit around until BFR is ready to launch it, which seems a waste. Obviously SpaceX must have thought about this a lot more carefully than me; I'm just interested in their reasoning.

The above assumes implicitly several things that may not be true.
* F9/H is not already capable of eating the market alive, especially with reduced costs with block 5, and reusability of fairings and other components.
The initial figure given way back when was that S1 was 60% of the stage cost, making S2 somewhere in the range of 20 million dollars without the fairing.

I am unsure if they have published more recently than that on the cost of S2s.
It's not unreasonable to suspect it might be considerably lower.

* There is no value in repeated F9/H launches that may transfer over to BFS/R operations.
At least some aspects - payload preparation, launch licences, ... are going to be similar, and these set a limiting cadence to BFR/S operations.
* BFS can't do anything without BFR.
Depending on questionable assumptions, it is at least somewhat plausible that BFS-SSTO can launch a large fraction of satellites, with the aid of in-orbit refuelling. BFS - without BFR eats the launch market.

* There is no 'political' value for SpaceX in delaying the apparent likely launch date of BFR/S.
Developing BFS up until they believe it is orbital-capable can be undersold as 'just tests', leading others to be able to insist to themselves it's not happening, and that full up-launches, for which they need to develop BFR anyway is still some years off.
Why they might be doing this is unclear. It could be for example that they want others considering entering the market to do so now, in the knowledge that they can kill them in several years, rather than to invent plans which might actually work in the face of BFR (I don't believe this one).

* More money would help the development effort.
At some point, if you've got a good team that works well together, and enough equipment, adding more people and equipment may not actually make stuff go faster - at least in a sustainable manner.
Being able to go out and buy subassemblies may mean you'd later need to develop that again internally, and face requalifying the systems they interact with.

* The architecture we saw at IAC2017 is still accurate.
Some of the speculation I did in the above mentioned thread was of a BFS-in-air refuelling. This is at least somewhat plausible, and would enable much, much greater capability for BFS without BFR.


Do I believe all of the above are false - no.
Do I know which - if any are actually true - no.

« Last Edit: 03/20/2018 03:39 PM by speedevil »

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