Author Topic: Reusable Second Stage Configuration  (Read 381822 times)

Offline Mindbuilder

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Reusable Second Stage Configuration
« on: 06/20/2013 02:07 am »
Maybe stow the second stage inside the nose fairing for re-entry on the fully reusable Falcon. I had long struggled to think of a really good configuration for the fully reusable second stage. Unstable nose first or hard to shield engine end first. Then while I was watching Shotwell talk about the huge fairing test, it occurred to me that on a fully reusable second stage, the fairing would not be jettisoned like on an expendable launch of a satellite (though there would probably be no fairing on a Dragon launch). The fairing could fly around and swallow the second stage, engine end first, before re-entry.

 The heat shield could rest on the nice structure of the top of the fairing. Then the heat shield and engine would both be on the same heavy end for re-entry. There could be one very short leg, or an air bag, in the middle of the top of the fairing to take the blunt of the landing loads, and some very light legs extending down from the sides for little more than lateral stability. Alternatively, a truck towing a trampoline could maneuver to be under the stage at touchdown, in order to save the precious second stage weight of legs. The overall length of the structure at touchdown would be shorter, reducing tip over risk.

To minimize g forces on re-entry you don't want your heat shield diameter to be any larger than necessary. So you probably wouldn't want to go as big a diameter as their current biggest fairing. Maybe just barely large enough to slip over the second stage.

This arrangement might also reduce the amount of heat shielding needed, if any, on the sides of the second stage.

An interesting side effect of this arrangement is that it may allow second stage reusability to be developed almost purely as an add-on to the current Falcon 9v1.1 second stage.

Offline garidan

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Re: Reusable Second Stage Configuration
« Reply #1 on: 06/20/2013 08:23 pm »
Second stage should be reused in orbit: so much effort to put it up there, why bring it back just to relaunch? Carry a little more spare fuel and "park" it somewhere, then follow up with a second launch from earth with only payload and fuel, randez vous and join, you spare the engine and avionics, and their weight.
Do exist orbit and timings to do that ?  I don't know  ::)

Offline Jim

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Re: Reusable Second Stage Configuration
« Reply #2 on: 06/20/2013 08:28 pm »

Not really viable.  The loss in performance in bringing the fairing to orbit plus the weight from the mods to make it reuseable and to support the second stage for entry and landing would be greater than the vehicle payload capability.

A fairing is just a windshield.  It provides no support the payload.  It only has the minimum strength to support itself and aeroloads.
« Last Edit: 06/20/2013 08:29 pm by Jim »

Online guckyfan

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Re: Reusable Second Stage Configuration
« Reply #3 on: 06/20/2013 11:54 pm »
Second stage should be reused in orbit: so much effort to put it up there, why bring it back just to relaunch? Carry a little more spare fuel and "park" it somewhere, then follow up with a second launch from earth with only payload and fuel, randez vous and join, you spare the engine and avionics, and their weight.
Do exist orbit and timings to do that ?  I don't know  ::)


There is no demand for all those second stages in orbit. Some might be used that way but as has alredy mentioned they are not designed for an extended lifetime.
Most of the second stages would be needed for another launch from the ground so would have to come back. That is if it can be achieved economically.

Offline ClaytonBirchenough

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Re: Reusable Second Stage Configuration
« Reply #4 on: 06/21/2013 01:28 am »
A possible second stage reusable configuration I imagine (just imagining, not saying this is anywhere near what Spacex will do) is a scaled up Dragon second stage. The Dragon would sit upside down compared to a normal launch, and would we 5.2 m in diameter at the top. This is the same configuration as the Mars One Dragon, which is said to have 25 m3 of internal volume. Not sure if 25 m3 is enough to hold the engine and propellant, but keeping the same shape as a normal Dragon but scaled-up would ease development. Just thinking out loud, will be interesting to see if and how Spacex's pulls off second stage reusability.
Clayton Birchenough

Offline Garrett

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Re: Reusable Second Stage Configuration
« Reply #5 on: 06/21/2013 04:15 pm »
I had long struggled to think of a really good configuration for the fully reusable second stage. Unstable nose first or hard to shield engine end first
What's wrong with SpaceX's configuration (see attached photo)? Who said it was unstable?

Quote
The fairing could fly around and swallow the second stage, engine end first, before re-entry.
That was a joke, right?
- "Nothing shocks me. I'm a scientist." - Indiana Jones

Offline Jim

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Re: Reusable Second Stage Configuration
« Reply #6 on: 06/21/2013 05:50 pm »

A couple of differences I wouldn't be surprised by in a real reusable upper stage are a fatter front end with a taper toward the rear, and landing thrusters near the top. 

Based on what? 

Offline wolfpack

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Re: Reusable Second Stage Configuration
« Reply #7 on: 06/21/2013 05:53 pm »
Better to invest money in a re-startable, cryo second stage than a reusable one. Not all second stages are coming back anyway.

Offline Jim

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Re: Reusable Second Stage Configuration
« Reply #8 on: 06/21/2013 06:13 pm »

Not much.  I liked that suggestion in the thread that it might be more Dragon-like.  If I was bringing new information rather than speculating, I'd have referenced my source.

Have you ever seen a completely cylindrical reentry vehicle?

I haven't, and that is pretty close to being the extent of my reasoning process.

Have you seen a tapered pressurized tank that supports a load on both ends?

Offline Oli

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Re: Reusable Second Stage Configuration
« Reply #9 on: 06/21/2013 06:17 pm »
Quote from: Garrett
What's wrong with SpaceX's configuration (see attached photo)?

Well, as has been mentioned, the walls are not protected and it doesn't generate any lift, hence no cross-range capability (correct me if I'm wrong).

Quote from: wolfpack
Better to invest money in a re-startable, cryo second stage than a reusable one. Not all second stages are coming back anyway.

Yep, the reusable first stage already halfs the payload, approximately (Mach 6 instead of Mach 10 separation). With a reusable second stage I doubt there would be anything left.





Offline R7

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Re: Reusable Second Stage Configuration
« Reply #10 on: 06/21/2013 06:20 pm »
Have you seen a tapered pressurized tank that supports a load on both ends?

?
AD·ASTRA·ASTRORVM·GRATIA

Offline Jim

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Re: Reusable Second Stage Configuration
« Reply #11 on: 06/21/2013 06:27 pm »
Have you seen a tapered pressurized tank that supports a load on both ends?

?

Not the whole stage including common bulkhead in the middle of it.

Offline Jim

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Re: Reusable Second Stage Configuration
« Reply #12 on: 06/21/2013 06:28 pm »
Have you seen a tapered pressurized tank that supports a load on both ends?
If you want to explain why you think it would be a problem, I'm interested in hearing it.

The reentry issue is minor compared to the structural one

Offline bubbagret

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Re: Reusable Second Stage Configuration
« Reply #13 on: 06/21/2013 06:29 pm »
Have you seen a tapered pressurized tank that supports a load on both ends?

?

Semyorka, Molnia, Polyot, Vostok, Soyuz.

Also see http://www.arianespace.com/launch-services-soyuz/soyuz-introduction.asp

"Following liftoff, the first-stage’s boosters burn for approximately 118 seconds and are then jettisoned. Thrust is transferred through a ball joint located at the top of the boosters’ cone-shaped structure, which is attached to the central core by two rear struts."

Offline R7

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Re: Reusable Second Stage Configuration
« Reply #14 on: 06/21/2013 06:31 pm »
To minimize g forces on re-entry you don't want your heat shield diameter to be any larger than necessary.

AIUI it doesn't work quite like that. Reentry g-forces depends more on entry angle and whether you have any lift or not. Ballistic coefficient affects more how high you experience maximum deceleration.

Anyway, it's unmanned, no need to get fancy. Quick ballistic reentry, shorter heat pulse, easier for the heat shield and structure.
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Offline Jim

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Re: Reusable Second Stage Configuration
« Reply #15 on: 06/21/2013 06:32 pm »

Semyorka, Molnia, Polyot, Vostok, Soyuz.


Those don't rely on internal pressure for support.

Offline bubbagret

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Re: Reusable Second Stage Configuration
« Reply #16 on: 06/21/2013 06:35 pm »

Semyorka, Molnia, Polyot, Vostok, Soyuz.


Those don't rely on internal pressure for support.

So we are only equating Atlas 1, and Centaur type structures to F9-S2? I didn't realize that.

Offline tafbass

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Re: Reusable Second Stage Configuration
« Reply #17 on: 06/21/2013 06:37 pm »
I think that there are two things that need to be accomplished at the same time to provide a practical solution. First managing the pressure in the tank during re-entry so there is on over pressurization on re-entry the load are transferred aft just like in a launch. More like the old atlas. Next a small ballistic aerobraking balut much like a child's waist floater at the base of the second stage. This would add parasitic drag and help stability in a nose down attitude with the CG being located aft.

Offline Jim

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Re: Reusable Second Stage Configuration
« Reply #18 on: 06/21/2013 06:42 pm »

So we are only equating Atlas 1, and Centaur type structures to F9-S2? I didn't realize that.

They are structurally stable for ground ops but require pressurization for flight loads.  That is how all Spacex launch vehicles are designed.  Whereas the Soyuz is built like a tank and can support people walking on it.
« Last Edit: 06/21/2013 06:44 pm by Jim »

Offline R7

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Re: Reusable Second Stage Configuration
« Reply #19 on: 06/21/2013 06:46 pm »
Whereas the Soyuz is built like a tank and can support people walking on it.

But is all booster engine thrust transferred to the ball joint in the tip via skin or doesn't it rely on internal pressures in the tanks too to do the job?

(Gotta ask, don't know for sure, shame on me)
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