Author Topic: F9 Second Stage Reusability  (Read 217473 times)

Offline Lars-J

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Re: F9 Second Stage Reusability
« Reply #920 on: 09/14/2018 06:50 AM »
With the announcement of the BFS with slight revisions for the circumlunar flight. Could you stick 3 to 5 Merlin SL engines to the F9 upper stage and use the external exhaust nozzle like on the revised BFS? At least as a prototype for the BFS.

"external exhaust nozzle"? Major BFS conjecture alert, here.

Raptor is a significantly higher performing engine (more like SSME than M1D), which may make losing the extended nozzle less of an issue than it would be for Merlin.

Besides, putting 3-5 merlins on a F9 upper stage... The single M-Vac already has to throttle down before burnout to reduce G-loads on payloads. Now imagine 4x more power. No.

Offline john smith 19

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Re: F9 Second Stage Reusability
« Reply #921 on: 09/23/2018 09:27 AM »
That depends on the design.
If you increase the area enough, you can end up with the deceleration happening very very high
And you need a very small amount of gas to inflate it. However it will collapse at a relatively low increase in outside pressure.
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There are so many ways inflatables can help with reentry varying from complex fragile designs to really quite dumb, and we have almost no indication what SpaceX may be trying.
True.

I suspect some people are imagining an F9 upper stage dangling from a large Helium balloon.
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Offline speedevil

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Re: F9 Second Stage Reusability
« Reply #922 on: 09/23/2018 09:43 AM »
With the announcement of the BFS with slight revisions for the circumlunar flight. Could you stick 3 to 5 Merlin SL engines to the F9 upper stage and use the external exhaust nozzle like on the revised BFS? At least as a prototype for the BFS.
'yes'.
And there is a thread for that!
If the aimed at Mars date is in fact 2022, the value of prototypes may be small, as it's very much not a gradual program.

A crash program on BFR, plus a backup program to make second stages reusable may make more sense than trying to build a vehicle whose purpose will go away.
BFS can be quite bad and still get to Mars


Offline Zed_Noir

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Re: F9 Second Stage Reusability
« Reply #923 on: 09/26/2018 03:40 PM »
With the announcement of the BFS with slight revisions for the circumlunar flight. Could you stick 3 to 5 Merlin SL engines to the F9 upper stage and use the external exhaust nozzle like on the revised BFS? At least as a prototype for the BFS.

"external exhaust nozzle"? Major BFS conjecture alert, here.

Raptor is a significantly higher performing engine (more like SSME than M1D), which may make losing the extended nozzle less of an issue than it would be for Merlin.

Besides, putting 3-5 merlins on a F9 upper stage... The single M-Vac already has to throttle down before burnout to reduce G-loads on payloads. Now imagine 4x more power. No.

Of course the overall thrust of the upper stage have to throttle down near burnout. But if you have like 3 Merlins when the upper stage ignites. Then it will only be about 3G acceleration to began with. So a shorter burn time and shorter transit time to the targeted orbit. Presuming there is some method to maintain the engine exhaust ISP near the Merlin Vac's ISP.

With the announcement of the BFS with slight revisions for the circumlunar flight. Could you stick 3 to 5 Merlin SL engines to the F9 upper stage and use the external exhaust nozzle like on the revised BFS? At least as a prototype for the BFS.
'yes'.
And there is a thread for that!
If the aimed at Mars date is in fact 2022, the value of prototypes may be small, as it's very much not a gradual program.

A crash program on BFR, plus a backup program to make second stages reusable may make more sense than trying to build a vehicle whose purpose will go away.
BFS can be quite bad and still get to Mars


My previous post was referring to a modified F9 upper stage not a SFS or a mini-BFS.



Offline Lars-J

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Re: F9 Second Stage Reusability
« Reply #924 on: 09/26/2018 04:19 PM »
Besides, putting 3-5 merlins on a F9 upper stage... The single M-Vac already has to throttle down before burnout to reduce G-loads on payloads. Now imagine 4x more power. No.

Of course the overall thrust of the upper stage have to throttle down near burnout. But if you have like 3 Merlins when the upper stage ignites. Then it will only be about 3G acceleration to began with. So a shorter burn time and shorter transit time to the targeted orbit. Presuming there is some method to maintain the engine exhaust ISP near the Merlin Vac's ISP.

I... don't think you understand how this works. 3G might be the starting acceleration, but it would go up quickly from there. A shorter burn time to get the same delta-V means higher G forces. FULL STOP. This is simple physics.

Online ChrisWilson68

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Re: F9 Second Stage Reusability
« Reply #925 on: 09/26/2018 06:17 PM »
With the announcement of the BFS with slight revisions for the circumlunar flight. Could you stick 3 to 5 Merlin SL engines to the F9 upper stage and use the external exhaust nozzle like on the revised BFS? At least as a prototype for the BFS.

I really don't get it.  What benefit are they supposed to get from that?

It seems like a distraction to me that would be an enormous amount of work (think Falcon Heavy -- it's just putting three first stages together, but it turned out to be far more work than even SpaceX had thought) and tell them almost nothing useful.

The best way to prototype to learn something to help BFS development is build the first BFS.  Anything else is just a distraction.

Offline wannamoonbase

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Re: F9 Second Stage Reusability
« Reply #926 on: 09/26/2018 07:26 PM »
With the announcement of the BFS with slight revisions for the circumlunar flight. Could you stick 3 to 5 Merlin SL engines to the F9 upper stage and use the external exhaust nozzle like on the revised BFS? At least as a prototype for the BFS.

I really don't get it.  What benefit are they supposed to get from that?

It seems like a distraction to me that would be an enormous amount of work (think Falcon Heavy -- it's just putting three first stages together, but it turned out to be far more work than even SpaceX had thought) and tell them almost nothing useful.

The best way to prototype to learn something to help BFS development is build the first BFS.  Anything else is just a distraction.

Given SpaceX's limited resources (relative term) I agree.  Start building and testing by flying.  This will also help with sales and finding investors.
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Offline geleto

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Re: F9 Second Stage Reusability
« Reply #927 on: 11/07/2018 07:21 PM »
https://twitter.com/elonmusk/status/1060253333116473344

https://twitter.com/elonmusk/status/1060253755315118080

This makes sense for two reasons:
1. FCC has denied a waiver for shorter deployment timeline of the Starlink constellation. So SpaceX has to launch thousands of satellites in the next 6 years. They urgently need reusable and more powerful(for FH) second stage.
2. They can use this as a BFS prototype.
« Last Edit: 11/07/2018 07:22 PM by geleto »

Offline speedevil

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Re: F9 Second Stage Reusability
« Reply #928 on: 11/07/2018 07:23 PM »
https://twitter.com/elonmusk/status/1060253333116473344

https://twitter.com/elonmusk/status/1060253755315118080

This makes sense for two reasons:
1. FCC has denied a waiver for shorter deployment timeline. So SpaceX has to launch thousands of satellites in the next 6 years. They need reusable second stage and more powerful stage for FH would be welcome.
2. They can use this as a BFS prototype.
Probably makes more sense to discuss this in the thread about it.

At least until it's clear it's more S2 derived than BFR derived.

Offline pathfinder_01

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Re: F9 Second Stage Reusability
« Reply #929 on: 11/08/2018 04:31 AM »
With the announcement of the BFS with slight revisions for the circumlunar flight. Could you stick 3 to 5 Merlin SL engines to the F9 upper stage and use the external exhaust nozzle like on the revised BFS? At least as a prototype for the BFS.

I really don't get it.  What benefit are they supposed to get from that?

It seems like a distraction to me that would be an enormous amount of work (think Falcon Heavy -- it's just putting three first stages together, but it turned out to be far more work than even SpaceX had thought) and tell them almost nothing useful.

The best way to prototype to learn something to help BFS development is build the first BFS.  Anything else is just a distraction.

FH was not a R/D project and by developing it they got experience with lighting almost as many engines as BFR and it told them which path not to take for the BFR booster.

I rather doubt that this would be an enormous amount of work compared to building a BFR and finding out that the aerodynamic or heating models were wrong and it needs extensive redesign. This is lowering risk a bit by getting some actual data. There is a danger that the scale being different might muck up things but that is a better risk than building it completely blind.

Online Slarty1080

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Re: F9 Second Stage Reusability
« Reply #930 on: 11/08/2018 06:37 PM »
With the announcement of the BFS with slight revisions for the circumlunar flight. Could you stick 3 to 5 Merlin SL engines to the F9 upper stage and use the external exhaust nozzle like on the revised BFS? At least as a prototype for the BFS.

I really don't get it.  What benefit are they supposed to get from that?

It seems like a distraction to me that would be an enormous amount of work (think Falcon Heavy -- it's just putting three first stages together, but it turned out to be far more work than even SpaceX had thought) and tell them almost nothing useful.

The best way to prototype to learn something to help BFS development is build the first BFS.  Anything else is just a distraction.
The first BFS prototype will teach SpaceX a lot about handling the BFS and raptors in the atmosphere and at supersonic and subsonic speeds. But its at orbital and hypersonic speeds that they have a serious lack of knowledge and it seems they consider it less risky to test the basics of the TPS and control surfaces on a disposable test bed at these speeds during entry before testing out a much more valuable BFR prototype.
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Online Robotbeat

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Re: F9 Second Stage Reusability
« Reply #931 on: 11/10/2018 12:34 AM »
...and can hedge against BFR hiccups by also developing a way to recover/reuse the upper stage of Falcon 9 in the process.
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