Author Topic: BFR with expendable upper stage  (Read 24374 times)

Offline Slarty1080

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Re: BFR with expendable upper stage
« Reply #60 on: 03/20/2018 06:12 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 it’s 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.

that was an interesting thread.
People were talking about what BFS might be able to do on its own. BFR must be much more capable than BFS as it doesn't have all the cargo decks etc.

Also it mentioned that the IAC2017 architecture is no longer accurate (extra engine added to BFS):
https://www.reddit.com/r/space/comments/76e79c/i_am_elon_musk_ask_me_anything_about_bfr/dodcg22/
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Offline envy887

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Re: BFR with expendable upper stage
« Reply #61 on: 03/20/2018 06:28 pm »

Online JamesH65

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Re: BFR with expendable upper stage
« Reply #62 on: 03/21/2018 03:52 pm »
How can BFR refer to two distinct things?

Surely BFB and BFS as individual items, BFR (or even BFx) should refer to the whole stack?

Offline speedevil

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Re: BFR with expendable upper stage
« Reply #63 on: 03/21/2018 03:56 pm »
How can BFR refer to two distinct things?

Surely BFB and BFS as individual items, BFR (or even BFx) should refer to the whole stack?

Yes, it should.
It would be very nice if it did, however Elon does not use these terms precisely, and there is no better one that has come up.
Unfortunately, BFR is ambiguous, and may refer to both, or only the booster.
I tend to - when I remember use BFR (the booster) the first time in a post, then BFR/BFS.

Offline envy887

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Re: BFR with expendable upper stage
« Reply #64 on: 03/21/2018 04:37 pm »
How can BFR refer to two distinct things?

Surely BFB and BFS as individual items, BFR (or even BFx) should refer to the whole stack?

We're talking about SpaceX. Consistent naming convention is not their strong suit, unfortunately.

Offline alexterrell

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Re: BFR with expendable upper stage
« Reply #65 on: 03/21/2018 05:19 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.
If the customer wanted an all in one starter moonbase, they could launch a BFS without the wings and TPS, land it on the moon, and it stays there?

But I'm not really sure the benefits - saving 20-30 tons isn't really worth it. I suppose the Raptor engines could be unbolted and returned at some point in the future.

The only other monolithic payloads are pressure domes. The mass of these increases in proportion to the volume. If someone wants a 200m diameter pressure vessel to make a base, that could need 200 tons. But it would also be too large for the lunar lander.

So we're back to standard BFS, and the only issue I can see with that is the apparently small door - when many payloads launched these days are 5m in diameter.
« Last Edit: 03/21/2018 05:20 pm by alexterrell »

Offline TrevorMonty

Re: BFR with expendable upper stage
« Reply #66 on: 03/21/2018 05:33 pm »
A single raptor expendable US could be useful for BLEO missions, especially if is refuelled by single tanker launch. Spacex take on ULA DL. Should be good for about 50t, with 150t topup.

Offline groundbound

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Re: BFR with expendable upper stage
« Reply #67 on: 03/21/2018 06:52 pm »
A single raptor expendable US could be useful for BLEO missions, especially if is refuelled by single tanker launch. Spacex take on ULA DL. Should be good for about 50t, with 150t topup.

Yup. As much as the BFR architecture is being sold as "do anything, just refuel more," its biggest weakness is sending a few tonnes to very high delta-V trajectories. Something with much much less dry mass would be better.

I think the other weakness that has been minimized by a little Elon slight of hand is that the economics of reusability really only apply for earth orbit. Send a BFS beyond Mars orbit and you have basically expended it. By the time it comes back it will be obsolete.

Offline envy887

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Re: BFR with expendable upper stage
« Reply #68 on: 03/21/2018 07:12 pm »
A single raptor expendable US could be useful for BLEO missions, especially if is refuelled by single tanker launch. Spacex take on ULA DL. Should be good for about 50t, with 150t topup.

Yup. As much as the BFR architecture is being sold as "do anything, just refuel more," its biggest weakness is sending a few tonnes to very high delta-V trajectories. Something with much much less dry mass would be better.

I think the other weakness that has been minimized by a little Elon slight of hand is that the economics of reusability really only apply for earth orbit. Send a BFS beyond Mars orbit and you have basically expended it. By the time it comes back it will be obsolete.

If there is a demand for very high energy trajectories, it might be worthwhile to develop another stage. But BFS is designed to minimize costs to LEO, GTO, the Moon, and Mars - not high energy trajectories.

For the very low number of high energy payloads, the cost of expending a BFS (especially a flight proven one) is not a major barrier. Europa Clipper, for example, is expected to use SLS Block 1B, at around a billion dollars for the launch. A fully refueled, stripped down, and expended BFS would be a about a quarter of that and offer a much larger mass or a higher C3.

Alternately, there are several ways to put a payload on Earth escape trajectory without having the BFS itself escape, including lunar gravity assists and aerobraking.

Offline speedevil

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Re: BFR with expendable upper stage
« Reply #69 on: 03/21/2018 07:17 pm »
A single raptor expendable US could be useful for BLEO missions, especially if is refuelled by single tanker launch. Spacex take on ULA DL. Should be good for about 50t, with 150t topup.

Yup. As much as the BFR architecture is being sold as "do anything, just refuel more," its biggest weakness is sending a few tonnes to very high delta-V trajectories. Something with much much less dry mass would be better.
Depends how much you believe fuel is cheap.

Refuel in LEO, and you can throw some tens of tons to 6km/s over LEO (ending up in an orbit you have barely enough fuel to burn back to an earth capture orbit some hours later).
Start out in GTO, (requiring twelve launches, filling two upper stages in LEO), and it's more like 8.5.



Offline envy887

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Re: BFR with expendable upper stage
« Reply #70 on: 03/21/2018 07:40 pm »
A single raptor expendable US could be useful for BLEO missions, especially if is refuelled by single tanker launch. Spacex take on ULA DL. Should be good for about 50t, with 150t topup.

Yup. As much as the BFR architecture is being sold as "do anything, just refuel more," its biggest weakness is sending a few tonnes to very high delta-V trajectories. Something with much much less dry mass would be better.
Depends how much you believe fuel is cheap.

Refuel in LEO, and you can throw some tens of tons to 6km/s over LEO (ending up in an orbit you have barely enough fuel to burn back to an earth capture orbit some hours later).
Start out in GTO, (requiring twelve launches, filling two upper stages in LEO), and it's more like 8.5.

Or fully refuel in LEO, burn to elliptical Earth orbit (e.g. GTO or similar) with a ~120 km perigee, wait one orbit, do the escape burn just before perigee, then sep the payload and do a sideways burn to drop to 70 km where there is enough atmosphere to slow back down and not escape. This gets 15 tonnes to about LEO+8500 m/s (almost enough for solar escape) without the BFS ever getting any further away than GEO.
« Last Edit: 03/21/2018 07:40 pm by envy887 »

Offline philw1776

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Re: BFR with expendable upper stage
« Reply #71 on: 03/21/2018 07:43 pm »
A single raptor expendable US could be useful for BLEO missions, especially if is refuelled by single tanker launch. Spacex take on ULA DL. Should be good for about 50t, with 150t topup.

If SpaceX were to develop a single Raptor expendable US for BLEO, make it in the profile for launch on the F9 or FH.
A Europa or Titan mission would see that F9 Raptor US refueled in LEO by a BFS and the refueled US would outperform the SLS at a fraction of the cost.

Heading into Kerbal territory, the BFS could add on side strapped propellant tanks (crossfeed zombie rises again) for a real high C3 launch.

If you're gonna do expendable, do an inexpensive Falcon class launched Raptor expendable enhanced by LEO refueling.
« Last Edit: 03/21/2018 07:45 pm by philw1776 »
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Offline acsawdey

Re: BFR with expendable upper stage
« Reply #72 on: 03/21/2018 07:50 pm »
A single raptor expendable US could be useful for BLEO missions, especially if is refuelled by single tanker launch. Spacex take on ULA DL. Should be good for about 50t, with 150t topup.

If SpaceX were to develop a single Raptor expendable US for BLEO, make it in the profile for launch on the F9 or FH.
A Europa or Titan mission would see that F9 Raptor US refueled in LEO by a BFS and the refueled US would outperform the SLS at a fraction of the cost.

Heading into Kerbal territory, the BFS could add on side strapped propellant tanks (crossfeed zombie rises again) for a real high C3 launch.

If you're gonna do expendable, do an inexpensive Falcon class launched Raptor expendable enhanced by LEO refueling.

If you have BFR/BFS, why bother making a stage that can launch with F9S1? Why not just bring it up as BFS cargo unfueled, then refuel it and send it on it's way. Then you don't need aerodynamics at all and it can just be a bundle of tanks and struts with a raptor at the back, payload at the front, and a BFS compatible fueling port.

Offline Robotbeat

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Re: BFR with expendable upper stage
« Reply #73 on: 03/21/2018 11:18 pm »
How can BFR refer to two distinct things?

Surely BFB and BFS as individual items, BFR (or even BFx) should refer to the whole stack?

Yes, it should.
It would be very nice if it did, however Elon does not use these terms precisely, and there is no better one that has come up.
Unfortunately, BFR is ambiguous, and may refer to both, or only the booster.
I tend to - when I remember use BFR (the booster) the first time in a post, then BFR/BFS.
I don’t believe this is correct. You remember incorrectly. BFR has consistently referred to the stack, with BFB now being used for the booster, and BFS always for the spaceship.
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Offline QuantumG

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Re: BFR with expendable upper stage
« Reply #74 on: 03/21/2018 11:33 pm »
I don’t believe this is correct. You remember incorrectly. BFR has consistently referred to the stack, with BFB now being used for the booster, and BFS always for the spaceship.

Hey, where's the first use of BFB? I didn't see it in the 2007 IAC, was it in there?
Human spaceflight is basically just LARPing now.

Offline RonM

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Re: BFR with expendable upper stage
« Reply #75 on: 03/21/2018 11:51 pm »
I don’t believe this is correct. You remember incorrectly. BFR has consistently referred to the stack, with BFB now being used for the booster, and BFS always for the spaceship.

Hey, where's the first use of BFB? I didn't see it in the 2007 IAC, was it in there?

On this site, I think this week.

Offline Robotbeat

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Re: BFR with expendable upper stage
« Reply #76 on: 03/22/2018 01:15 am »
I don’t believe this is correct. You remember incorrectly. BFR has consistently referred to the stack, with BFB now being used for the booster, and BFS always for the spaceship.

Hey, where's the first use of BFB? I didn't see it in the 2007 IAC, was it in there?

On this site, I think this week.
No, earlier and by SpaceX, I believe. Definitely not this week.

EDIT: Here’s a post by Waitbutwhy using BFB in 2016. Not SpaceX, but did get correspondence (and maybe approval?) with Elon: https://waitbutwhy.com/2016/09/spacexs-big-fking-rocket-the-full-story.html

...and I do believe there is a more recent use of SpaceX themselves using it. Elon, I think.
« Last Edit: 03/22/2018 01:18 am by Robotbeat »
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Offline QuantumG

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Re: BFR with expendable upper stage
« Reply #77 on: 03/22/2018 02:35 am »
...and I do believe there is a more recent use of SpaceX themselves using it. Elon, I think.

If you can de-vague that I'll consider it canon (until Elon just makes up some other words), but I think most of us have been using BFS/BFR.
Human spaceflight is basically just LARPing now.

Offline Robotbeat

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Re: BFR with expendable upper stage
« Reply #78 on: 03/22/2018 03:47 am »
...and I do believe there is a more recent use of SpaceX themselves using it. Elon, I think.

If you can de-vague that I'll consider it canon (until Elon just makes up some other words), but I think most of us have been using BFS/BFR.
I think Musk said it in some video since the start of the year, but I am a little too intimidated to look through all of them. It also may have been Shotwell.

But i strongly disagree that /most/ people were calling just the /booster/ "BFR." I think most were referring to it by "booster."

BFB is definitely superior and clearer terminology than calling BFR just a piece of BFR. Like, that makes no sense. BFR is clearly the whole rocket, not just the booster.
« Last Edit: 03/22/2018 03:49 am by Robotbeat »
Chris  Whoever loves correction loves knowledge, but he who hates reproof is stupid.

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Offline Robotbeat

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Re: BFR with expendable upper stage
« Reply #79 on: 03/22/2018 03:57 am »
Or it could also be "BRB"...

...QuantumG is right that Elon sure likes to mess with terminology.

But can we all just call it: BFS + BFB = BFR for now??

EDIT: here's the evidence of Musk calling it BRB:
https://www.reddit.com/r/SpaceXLounge/comments/7vl7j4/qa_elon_musk_discusses_the_launch_and_flight_of/
« Last Edit: 03/22/2018 04:00 am by Robotbeat »
Chris  Whoever loves correction loves knowledge, but he who hates reproof is stupid.

To the maximum extent practicable, the Federal Government shall plan missions to accommodate the space transportation services capabilities of United States commercial providers. US law http://goo.gl/YZYNt0

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