Author Topic: F9 Second Stage Reusability  (Read 213055 times)

Offline JamesH65

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Re: F9 Second Stage Reusability
« Reply #120 on: 04/03/2017 02:54 PM »
Maybe I am being a party pooper here, but I dont think the second stage will see any substantial design changes for the (if any) recovery attempt. And I think of things like heat shields, wings,  fold-up legs etc. that sit on the outside of the stage.
It would probably invalidate the use of the FH launch for the DOD certification process. Therefore, I suspect the recovery attempt will be something like a parachute mounted in the payload fairing adapter and have it reenter without burning up by spending fuel. How, I have no idea. But I think the most important thing is to get the DOD certification. Anything that would violate it is not going to happen.

Pretty much agree with this - it all sounds like Musk is going to give something that is right on the edge of possible a quick go to see what happens. It may not even be part of a longer term 'get the 2nd stage back` plan, but something they can try because they have a lot of spare payload mass (ie all of it) they can use.

If they do have more plans great, but I'm not holding my breath! It's not like he can get Scotty to change the laws of Physics and whack the warp core up to 11.

Offline Okie_Steve

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Re: F9 Second Stage Reusability
« Reply #121 on: 04/03/2017 03:25 PM »
Getting back the first S1 led to statement that they found something to make future flights more reliable.No reason that getting back a S2 would not be equally valuable even if it is a one time event because of FH demo extra fuel for S2/no payload.

Offline Rocket Science

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Re: F9 Second Stage Reusability
« Reply #122 on: 04/03/2017 04:41 PM »
For second stage reusability it all comes down to propellant and heat.

The stage is naturally bottom heavy. So the question is can the rear end and the nozzle survive the heating. If it can you don't need propellant (main or RCS) to keep the front end to bear the brunt of the heating through TPS.

2nd worst is if you have to run the engine to generate a "nose bubble" An interesting question would be could you run the engine quite fuel rich, as that's lighter. Keeping in mind that at these altitudes we're talking a few 10s of psi is 100x of (ambient) atmospheric pressure.

If that doesn't work then you have to keep the nose end on to the airstream, which suggests continuous movement of some kind of dynamic control surfaces, probably a set of grid of fins. SX are familiar with them and they operate over a wide range of Mach numbers, which will be even more important for US than booster stage recovery.

TBH doing SECO just below orbital velocity still seems like the best way to save a lot of propellant on the de-orbit burn. Just have the payload provide the last 10s of m/s delta V.  But as always the question is will it be enough?
Can side stepping needing a de-orbit burn at all, coupled with the the decelleration from control surfaces during a once around flight, coupled with thinner TPS due to the lower starting velocity lower the mass penalty for recovery enough?

This is an area where approximations don't really cut it and you're looking at fairly detailed simulation to see if something is at all possible. Slight mishaps don't add, they multiply.
Just a note John, the "shock-wave" interactions within the grid-fins "may" cause extreme temps even using titanium and even with an ablator...

Edit:typo
« Last Edit: 04/03/2017 09:22 PM by Rocket Science »
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Offline john smith 19

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Re: F9 Second Stage Reusability
« Reply #123 on: 04/03/2017 08:34 PM »
Just a note John, the "shock-wave" interactions within the grid-fins "may" cause extreme temps even using titanium and even with an abaltor...
I'd not considered Edney III and IV heating but I'm sure the SX engineers will have. This suggests the US grid fin sizing and grid spacing and layout would be different to that of the first stage.
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Offline hrissan

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Re: F9 Second Stage Reusability
« Reply #124 on: 04/03/2017 10:04 PM »
My bet is what user NovaSilisko suggested year ago.

https://forum.nasaspaceflight.com/index.php?topic=39314.msg1474695#msg1474695

His post has a nice picture, in a nutshell - like a bottom half of Dragon2 attached to the top of S2 legs forward.

P.S. If it adds 4000kg to the mass of S2, so be it. Good for experimenting when the payload to LEO allows it.
« Last Edit: 04/03/2017 10:08 PM by hrissan »

Offline dglow

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Re: F9 Second Stage Reusability
« Reply #125 on: 04/03/2017 11:28 PM »
FYI didn't see this Twitter exchange of Elon's noted here:

Quote from: Phil Plait‏Verified account @BadAstronomer
This is a very hard problem. Its moving at orbital velocity, will need leftover fuel, & some way to protect it from heat of re-entry.

Quote from: Elon Musk‏Verified account @elonmusk
Replying to @BadAstronomer
We can def bring it back like Dragon. Just a question of how much weight we need to add.

https://twitter.com/elonmusk/status/847958571895619584

Offline speedevil

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Re: F9 Second Stage Reusability
« Reply #126 on: 04/04/2017 01:14 PM »
There seem likely to be a large number of ex-stage-1 merlins, for the foreseeable future, sitting in warehouses.
How much is the performance hit of making a stage based on a reused engine, not vac optimised?

Offline spacenut

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Re: F9 Second Stage Reusability
« Reply #127 on: 04/04/2017 02:38 PM »
I've often wondered why SpaceX didn't go with a plug nozzle type engine for the second stage.  It would do double duty as the heat shield.  It was considered for the second stage of Saturn IB and Saturn V before they were cancelled.  It was based on the J2 engine at that time.  The Octoweb is almost like a plug nozzle using he engines as heat shield for the first stage.  I know it might be more expensive to develop, but in the long run to get 2nd stage reusable, it may be the best overall to do.  No need for heat resistant nose or sides, parachutes, etc.  Just landing legs like the first stage. 

Offline Jim

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Re: F9 Second Stage Reusability
« Reply #128 on: 04/04/2017 03:15 PM »
I've often wondered why SpaceX didn't go with a plug nozzle type engine for the second stage.  It would do double duty as the heat shield.  It was considered for the second stage of Saturn IB and Saturn V before they were cancelled.  It was based on the J2 engine at that time.  The Octoweb is almost like a plug nozzle using he engines as heat shield for the first stage.  I know it might be more expensive to develop, but in the long run to get 2nd stage reusable, it may be the best overall to do.  No need for heat resistant nose or sides, parachutes, etc.  Just landing legs like the first stage. 

Because it would not be a merlin then.

Offline spacenut

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Re: F9 Second Stage Reusability
« Reply #129 on: 04/04/2017 03:59 PM »
Couldn't the Merlin turbopump machinery be used to supply the plug nozzle? 

Offline sevenperforce

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Re: F9 Second Stage Reusability
« Reply #130 on: 04/04/2017 05:16 PM »
Couldn't the Merlin turbopump machinery be used to supply the plug nozzle? 
The turbopump would have be redesigned to fuel a series of individual chambers all opening onto the plug nozzle, driving up mass considerably. Multiple smaller chambers mean square-cube losses. The plug nozzle itself would be much heavier than the current nozzle and would have nowhere near the specific impulse. And the combustion chambers would not be sufficiently shielded from re-entry plasma, so there's not really much of an advantage there.

Not to say it couldn't be done; there are plenty of ways to make it work. But you're definitely no longer dealing with a Merlin in any form or fashion.

The Octoweb is almost like a plug nozzle using he engines as heat shield for the first stage.
I'm afraid the Octaweb is nothing like a plug nozzle at all.

There seem likely to be a large number of ex-stage-1 merlins, for the foreseeable future, sitting in warehouses.
How much is the performance hit of making a stage based on a reused engine, not vac optimised?
Not vacuum-optimizing is a complete nonstarter. The performance hit is immense. With a notional payload of 10 tonnes, the second stage has about 7.4 km/s of dV; if it was using a SL Merlin, that drops to 6.6 km/s.

However, it may be possible to retrofit a used SL Merlin. The two differences I know of are larger engine bell and the injection of the gas generator exhaust back into the bell.

Offline saliva_sweet

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Re: F9 Second Stage Reusability
« Reply #131 on: 04/04/2017 05:33 PM »
My bet is what user NovaSilisko suggested year ago.

https://forum.nasaspaceflight.com/index.php?topic=39314.msg1474695#msg1474695

His post has a nice picture, in a nutshell - like a bottom half of Dragon2 attached to the top of S2 legs forward.

P.S. If it adds 4000kg to the mass of S2, so be it. Good for experimenting when the payload to LEO allows it.

I think even that is too ambitious for this flight. Superdracos are tricky. I'm thinking just a dragon1 heat shield as a payload along with enough ballast to make it top heavy. Everything is payload and zero changes to the second stage proper. What burns burns. They may include parachutes if they have the time and resources, but they could also let it crash to the ocean and optionally fish it out later from the bottom.

Online guckyfan

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Re: F9 Second Stage Reusability
« Reply #132 on: 04/04/2017 05:40 PM »
They are designing pressure fed methalox RCS-thrusters for ITS. I wonder if it is possible to modify the design for kerolox and use those. It is experimental, does not need to be super efficient.

Offline GWH

Re: F9 Second Stage Reusability
« Reply #133 on: 04/04/2017 05:55 PM »
I think even that is too ambitious for this flight. Superdracos are tricky. I'm thinking just a dragon1 heat shield as a payload along with enough ballast to make it top heavy. Everything is payload and zero changes to the second stage proper. What burns burns. They may include parachutes if they have the time and resources, but they could also let it crash to the ocean and optionally fish it out later from the bottom.

The dragon heatshield/trunk interface could be repurposed at least so they can place loads above the head shield without dealing with numerous perforations for structure.

Offline john smith 19

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Re: F9 Second Stage Reusability
« Reply #134 on: 04/04/2017 08:49 PM »
My bet is what user NovaSilisko suggested year ago.

https://forum.nasaspaceflight.com/index.php?topic=39314.msg1474695#msg1474695

His post has a nice picture, in a nutshell - like a bottom half of Dragon2 attached to the top of S2 legs forward.

P.S. If it adds 4000kg to the mass of S2, so be it. Good for experimenting when the payload to LEO allows it.
Those people who've read "Frontiers of Space" by Bono & Gatland will recognize Bono's idea for an early PoC of Saturn 2nd stage reuse, including the nose landing scheme.

The problem remains how to you ensure that a rear heavy cylinder stays nose down when every slightest disturbance wants to push it nose up, if not into an active end over end tumble.
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Offline GWH

Re: F9 Second Stage Reusability
« Reply #135 on: 04/04/2017 09:23 PM »
The problem remains how to you ensure that a rear heavy cylinder stays nose down when every slightest disturbance wants to push it nose up, if not into an active end over end tumble.

"Drag" something behind it.

Offline sevenperforce

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Re: F9 Second Stage Reusability
« Reply #136 on: 04/04/2017 09:28 PM »
They are designing pressure fed methalox RCS-thrusters for ITS. I wonder if it is possible to modify the design for kerolox and use those. It is experimental, does not need to be super efficient.
RP-1 boils at 147 C, so...maybe?

Offline sevenperforce

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Re: F9 Second Stage Reusability
« Reply #137 on: 04/04/2017 09:28 PM »
The problem remains how to you ensure that a rear heavy cylinder stays nose down when every slightest disturbance wants to push it nose up, if not into an active end over end tumble.

"Drag" something behind it.
Like grid fins.

Offline Barrie

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Re: F9 Second Stage Reusability
« Reply #138 on: 04/04/2017 09:34 PM »
The problem remains how to you ensure that a rear heavy cylinder stays nose down when every slightest disturbance wants to push it nose up, if not into an active end over end tumble.

"Drag" something behind it.

Like a really big nozzle extension?

Offline cambrianera

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Re: F9 Second Stage Reusability
« Reply #139 on: 04/04/2017 09:35 PM »
A couple years ago there was a thread around here speculating about landing the second stage as a lifting body, with some kind of stub wings and landing skids. I thought it sounded like an interesting idea, but never heard any more in that direction, or any talk of scouting for landing sites.

This?
Happy you found it interesting.
Old Lobo idea, I did some rendering and dimensioning.
Reentering at high angle of attack (like Max Faget's shuttle) heatshield could be thin (and light).
But protection of nozzle is an issue, and SpaceX seems not found of Carbon-Carbon nozzles.
Oh to be young again. . .

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