Author Topic: F9 Second Stage Reusability  (Read 140090 times)

Offline dglow

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Re: F9 Second Stage Reusability
« Reply #60 on: 04/01/2017 04:12 PM »
Or an aft mounted toroidal inflatable skirt using similar technology to HIAD, and a forward mounted Pica-X heatshield.

Entering nose first, shuttlecock style?

Isn't that what they showed in their 2011 reusability video? TPS covering the top of the stage and partially down one of the sides.

That video also showed an extending/retracting engine bell to create clearance for the legs, IIRC.

Offline Kaputnik

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Re: F9 Second Stage Reusability
« Reply #61 on: 04/01/2017 04:22 PM »
. Hope someone answers the ion thruster/photovoltaic panel deorbit idea that someone upthread suggested.

Erm, I did...
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Online douglas100

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Re: F9 Second Stage Reusability
« Reply #62 on: 04/01/2017 04:29 PM »

Isn't that what they showed in their 2011 reusability video? TPS covering the top of the stage and partially down one of the sides...

Yes, but minus the toroidal aft skirt. In previous discussion folks have questioned the stability of entering nose first when most of the mass is in the rear. The skirt would enhance stability, shuttlecock style as already mentioned. An alternative might be deployable flaps covered in TPS.

Quote
That video also showed an extending/retracting engine bell to create clearance for the legs, IIRC.

I like the idea of air capture as mentioned by meekgee and others. It avoids flipping the stage, making any modifications to the engine and eliminates the weight of the legs. It also adds operational flexibility and safety. The stage can be recovered offshore and be brought directly back to the launch point. The recovery equipment on the stage would then only be a heatshield and a steerable parachute. The capture helicopter would do the rest.
Douglas Clark

Offline Kaputnik

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Re: F9 Second Stage Reusability
« Reply #63 on: 04/01/2017 04:31 PM »
Or an aft mounted toroidal inflatable skirt using similar technology to HIAD, and a forward mounted Pica-X heatshield.

Entering nose first, shuttlecock style?

Isn't that what they showed in their 2011 reusability video? TPS covering the top of the stage and partially down one of the sides.

That video also showed an extending/retracting engine bell to create clearance for the legs, IIRC.

The 2011 video didn't show an inflatable skirt, it was more of a sideways entry. For all we know that is still exactly what they plan to do. The stage might already be strong enough in that axis, unlike the much finer first stage.
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Offline GORDAP

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Re: F9 Second Stage Reusability
« Reply #64 on: 04/01/2017 04:57 PM »
I think SpaceX has a fine line to skate here:  They don't want to modify the US so much that it invalidates this first flight as a test flight of the FH in any way.

On the other hand, they probably have some ideas or plans on a future design of a reusable US and this flight is a good opportunity to use as a test bed for those ideas.  That is, they don't in any way plan to evolve the present Merlin US stage to be reusable - Musk has pretty much indicated the ISP is too low for this - but they can use this US as a space based 'grasshopper'. 

So what would be some minimal changes that could answer some basic questions?  I'm thinking maybe they could just add TPS to the top and TPS covered titanium grid fins to the 'base'.  Then have the stage descend top forward, with the fins used to add a bit of drag and precision steering.  Would grid fins back near the engine help with the CG being on the wrong end for stable flight? 

Anyway, forget soft landing; just aim for a spot in the ocean and see how close you get.  Maybe have a drone ship nearby to record video, etc.  This would at least answer if they can survive reentry with this mode and if they can come close to the intended spot.

Offline ChrisWilson68

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Re: F9 Second Stage Reusability
« Reply #65 on: 04/01/2017 04:59 PM »
Two words: bouncy castle! :-)

Offline Kaputnik

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Re: F9 Second Stage Reusability
« Reply #66 on: 04/01/2017 05:03 PM »
They couod just keep a heap of residual propellant in the stage and enter nose first- this would significantly affect the CG and increase stability.
Or, thinking this through, add ballast (shudder) to the top of the stage...
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Offline saliva_sweet

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Re: F9 Second Stage Reusability
« Reply #67 on: 04/01/2017 05:13 PM »
They couod just keep a heap of residual propellant in the stage and enter nose first- this would significantly affect the CG and increase stability.
Or, thinking this through, add ballast (shudder) to the top of the stage...

They have plenty of payload capacity to add as much ballast as they could ever need. What does a single merlin weigh anyway? ~1t I'd wager.

Offline Lar

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Re: F9 Second Stage Reusability
« Reply #68 on: 04/01/2017 05:17 PM »
Hi all,

Now I just cant imagine a tube flying aerodynamically through the atmosphere without grid fins, theres no way S2 wont have grid fins will it? Can grid fins survive reentry without burning up, imagine grid fins covered in pica-x.

maybe instead of grid they could do simple flat fins like blue origin

I think that's a bad idea. Grid fins are far more efficient in terms of control authority/lift per unit mass. That's why people use them.  Blue may reconsider their thinking on flat fins, we'll see..  Flat fins may heat less though... except at the leading edge. Grid fins are ALL leading edge :)
"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 ChrisWilson68

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Re: F9 Second Stage Reusability
« Reply #69 on: 04/01/2017 05:19 PM »
Hi all,

Now I just cant imagine a tube flying aerodynamically through the atmosphere without grid fins, theres no way S2 wont have grid fins will it? Can grid fins survive reentry without burning up, imagine grid fins covered in pica-x.

maybe instead of grid they could do simple flat fins like blue origin

I think that's a bad idea. Grid fins are far more efficient in terms of control authority/lift per unit mass. That's why people use them.  Blue may reconsider their thinking on flat fins, we'll see..  Flat fins may heat less though... except at the leading edge. Grid fins are ALL leading edge :)

Also, grid fins are much better for high mach numbers.  Blue Origin hasn't dealt with those kinds of speeds yet.  When they do, I expect them to go with grid fins.  That's what grid fins were invented for.

Offline Space Ghost 1962

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Re: F9 Second Stage Reusability
« Reply #70 on: 04/01/2017 05:53 PM »
You can waste performance on the Falcon Heavy demo mission.
True. And he's hinted at something ... surprising.

Here's how you might use that (but for a lot of reasons its unlikely).

You could either loft an entire, fueled stage in the fairing ... or tanks to refill the second stage, including excess for boil-off.

In effect you'd want to end up on orbit with a fully loaded US that has a Dragon heat shield on top.

The autonomous program would check out the refuel, and orient/phase the stage for a grazing entry precisely.

The US RCS would control buffeting as EI occured, getting the stage as low as possible given CG, with perhaps a cooled/vented heat shield. Then, before aerodynamic pressure built up too high, the stage would flip and RCS would maintain orientation. You'd go as low as you could heating the engine bell, then ignite and burn for an extended entry burn, likely wiggling the gimbal to increase drag, and likely burning to exhaustion. Hopefully making it to transonic.

Passive CG would keep you from tumbling. Assuming you survive to free fall velocities, you pop the chutes around 5K feet, and perhaps there's something floating on the surface for a little while to be admired.

You

Offline mr. mark

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Re: F9 Second Stage Reusability
« Reply #71 on: 04/01/2017 06:11 PM »
What no one seems to be talking about is that the FH launch is 5 months out. Clearly SpaceX already has a second stage prototype ready to go. You just don't slap things on to an existing stage. This brings up an important point. Is SpaceX conducting super secret testing such as a black ops program for future designs away from the public eye.

Offline alang

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Re: F9 Second Stage Reusability
« Reply #72 on: 04/01/2017 06:15 PM »
You can waste performance on the Falcon Heavy demo mission.
True. And he's hinted at something ... surprising.

Here's how you might use that (but for a lot of reasons its unlikely).

You could either loft an entire, fueled stage in the fairing ... or tanks to refill the second stage, including excess for boil-off.

In effect you'd want to end up on orbit with a fully loaded US that has a Dragon heat shield on top.

The autonomous program would check out the refuel, and orient/phase the stage for a grazing entry precisely.

The US RCS would control buffeting as EI occured, getting the stage as low as possible given CG, with perhaps a cooled/vented heat shield. Then, before aerodynamic pressure built up too high, the stage would flip and RCS would maintain orientation. You'd go as low as you could heating the engine bell, then ignite and burn for an extended entry burn, likely wiggling the gimbal to increase drag, and likely burning to exhaustion. Hopefully making it to transonic.

Passive CG would keep you from tumbling. Assuming you survive to free fall velocities, you pop the chutes around 5K feet, and perhaps there's something floating on the surface for a little while to be admired.

You

This is getting into party thread territory, so maybe it would need another thread: I wonder what the aerodynamics of a dragon mounted upside down on the payload adapter would be...:-)

Online cppetrie

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Re: F9 Second Stage Reusability
« Reply #73 on: 04/01/2017 06:18 PM »
Is SpaceX conducting super secret testing such as a black ops program for future designs away from the public eye.
I can't imagine the answer to this question is anything other than absolutely yes. As a bleeding edge aerospace company how could they not have numerous R&D projects that aren't public. There is some stuff that shows up in L2 that isn't "public" but there is probably plenty more than isn't at all known outside of the small group working on it.

Offline saliva_sweet

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Re: F9 Second Stage Reusability
« Reply #74 on: 04/01/2017 06:28 PM »
Clearly SpaceX already has a second stage prototype ready to go. You just don't slap things on to an existing stage. This brings up an important point. Is SpaceX conducting super secret testing such as a black ops program for future designs away from the public eye.

I seriously doubt that. He wouldn't have called it "hail mary" then. Spacex doesn't have the time or resources to seriously devote to a one off hail mary attempt right now. I think it's more likely Elon is considering giving a couple of interns access to a used dragon and a healthy budget of zero, telling them to make something happen. Thinking it might be worth to fly that in lieu of a couple of extra tons of cheese (cheese is expensive you know).
« Last Edit: 04/01/2017 06:29 PM by saliva_sweet »

Offline mr. mark

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Re: F9 Second Stage Reusability
« Reply #75 on: 04/01/2017 07:09 PM »
Highly doubtful of that. New designs take years to develop. You just don't tell interns to throw something together. I wouldn't doubt they have the prototype sitting somewhere in Hawthorne ready and waiting.

Offline saliva_sweet

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Re: F9 Second Stage Reusability
« Reply #76 on: 04/01/2017 07:33 PM »
New designs take years to develop.

Spacex doesn't have years to develop a hail mary reusable second stage as a cheese replacement for the falcon heavy demo :'(

Online Kosmos2001

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Re: F9 Second Stage Reusability
« Reply #77 on: 04/01/2017 07:46 PM »
My maths and physics is very rusty but...

According to Wikipedia[1], the Merlin 1D can throttle to 360 kN (81,000 lbf).
According to Space Launch Report[2], the F9 second stage dry mass is ~4.5t

We know F=m.a[3] where F=force in N, m=mass in Kg and a=acceleration in m/s/s
We also know g=acceleration-due-to-gravity=9.8m/s/s

so at 1g F=4500.9.8/1000=44kN
and 360/44=8.16
so it seems that a Merlin 1Dvac could land a second stage with an ~8g hoover slam.

Two question for the experts on NSF:
1) Is this calculation correct?
2) Could the stage survive 8g?

[1] https://en.wikipedia.org/wiki/Merlin_(rocket_engine_family)#Merlin_1D_Vacuum
[2] http://www.spacelaunchreport.com/falcon9ft.html
[3] https://en.wikipedia.org/wiki/Newton_(unit)#Definition

IDK, but probably using the MDVac close to SL would generate problems associated with flow separation.

Offline northenarc

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Re: F9 Second Stage Reusability
« Reply #78 on: 04/01/2017 07:49 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.   

Offline mme

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Re: F9 Second Stage Reusability
« Reply #79 on: 04/01/2017 08:18 PM »
Because of the time constraints, I proposed that they could try a system on the Falcon Heavy Demo mission with 4 legs in the payload fairing. Splay them out and perform re-entry from LEO. Spreading the heat over the larger area would be interesting and might inform the development of a fairing/heat-shield combo to protect the second stage.
It might be something like a 3 part fairing with two parts recoverable and the 3rd part sliding into position downward to protect the stage.
How it would land? Deployable paraglider 'chute like the fairing onto a "bouncy castle" (air-bag like) barge.
 
That gives me an idea (sorry if it's been suggested.) Behold, my first NFS Rube Goldberg device.  It must be contagious:

Rocket LEGO Landing Element.  A landing module that connects to the PAF and is used as a PAF for the "real" payload.  It includes integrated SDs and landing legs and it's mass shifts the COM forward so it reenters "head first." No idea how to add a heat shield or control the trajectory w/o some sort of fins.

Similar idea could use a controllable parafoil instead of propulsive landing (or maybe with propulsive assist.)

Basically I like the idea of the PAF as a PAF so that there is only one S2 and you plug on the "reuse option."  Some assembly and lots of hand waving required.
Space is not Highlander.  There can, and will, be more than one.

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