Author Topic: F9 Second Stage Reusability  (Read 190442 times)

Offline ArbitraryConstant

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Re: F9 Second Stage Reusability
« Reply #400 on: 04/17/2017 05:19 PM »
But it does show it landing tail down with no other obvious engines firing apart from the main one.
Disagree, video shows a number of thruster plumes around the edge of the stage quite clearly.
« Last Edit: 04/17/2017 05:19 PM by ArbitraryConstant »

Offline john smith 19

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Re: F9 Second Stage Reusability
« Reply #401 on: 04/17/2017 10:24 PM »
But it does show it landing tail down with no other obvious engines firing apart from the main one.
Disagree, video shows a number of thruster plumes around the edge of the stage quite clearly.
You're behind the curve. As I already noted.
I have and you're right there does appear to be some non main engine firing from inside the main skirt.

However what's still missing is how the US goes from being engine first to being nose first (and how it's kept nose first) during reentry and how it goes from nose first to tail sitting to fire whatever engines are being used for landing.
My instinct is SX will use what they know and what works.  Grid fins, being mostly drag devices should keep the nose down during most of the deceleration following a main engine burn. If they can respond fast enough that would suggest they could bring about a controlled "pendulum swing" of the stage to the vertical in time to be ready for landing.

The alternative would be a full 180deg flip and then let the tail end "fall" to vertical.

The joker in the pack is this major shift in attitude has to occur deep in the atmosphere where the aerodynamic loads are on the side of the stage.
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Offline ArbitraryConstant

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Re: F9 Second Stage Reusability
« Reply #402 on: 04/18/2017 01:58 AM »
My instinct is SX will use what they know and what works.  Grid fins, being mostly drag devices should keep the nose down during most of the deceleration following a main engine burn.
I'm skeptical any reasonable grid fin would survive full reentry heating, it would need to be some other material like PICA-X flaps. Also seems like a lifting entry is more desirable, so center of gravity would be somewhat off center. In this scenario, the transition to vertical flight can be accomplished by increasing angle of attack until stall.

Offline meekGee

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Re: F9 Second Stage Reusability
« Reply #403 on: 04/18/2017 04:41 AM »
My instinct is SX will use what they know and what works.  Grid fins, being mostly drag devices should keep the nose down during most of the deceleration following a main engine burn.
I'm skeptical any reasonable grid fin would survive full reentry heating, it would need to be some other material like PICA-X flaps. Also seems like a lifting entry is more desirable, so center of gravity would be somewhat off center. In this scenario, the transition to vertical flight can be accomplished by increasing angle of attack until stall.

Agreed - what they know works in a different regime.  Grid fins may still be used, but not to provide guidance/stabilization during reentry.

The cleanest way to achieve active guidance during reentry is c.g. shifting.  But AFAIK, it's used to tweak the angle of naturally stable bodies, not to actively keep them from flipping around and tumbling.
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Offline guckyfan

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Re: F9 Second Stage Reusability
« Reply #404 on: 04/18/2017 08:29 AM »
The cleanest way to achieve active guidance during reentry is c.g. shifting. 

Yes, but when this is not feasible shifting center of drag is good as well. IMO PicaX covered drag and steering flaps at the bottom will do that nicely. Similar to the 2 flaps on ITS. They may need more than two lacking the wing like extensions on ITS.

Offline meekGee

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Re: F9 Second Stage Reusability
« Reply #405 on: 04/18/2017 01:17 PM »
The cleanest way to achieve active guidance during reentry is c.g. shifting. 

Yes, but when this is not feasible shifting center of drag is good as well. IMO PicaX covered drag and steering flaps at the bottom will do that nicely. Similar to the 2 flaps on ITS. They may need more than two lacking the wing like extensions on ITS.
True, but the flaps are more effective in belly first flight, since they see non separated  flow.  If coming in head first, flaps in the back may be less effective.
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Offline john smith 19

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Re: F9 Second Stage Reusability
« Reply #406 on: 04/18/2017 01:45 PM »
I'm skeptical any reasonable grid fin would survive full reentry heating, it would need to be some other material like PICA-X flaps. Also seems like a lifting entry is more desirable, so center of gravity would be somewhat off center. In this scenario, the transition to vertical flight can be accomplished by increasing angle of attack until stall.
You have to understand that all capsule entries since Mercury have been lifting body entries. Once you have offset Cg then you can vary the relevant vectors and get lift. I think the Russians opted for unconditional stability, that's why they stayed spherical.

Musk has said the new grid fins they are planning to use will increase stage L/D ratio to 1. Apollo was about 0.5 at hypersonic speed. What I'm not clear on is how got a cross range this will the stage and wheather it's good enough to do a RTLS after the first orbit or if it will remain the case that the simplest move is to wait about a day til the launch site comes back under its ground track.
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Offline RedLineTrain

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Re: F9 Second Stage Reusability
« Reply #407 on: 04/18/2017 07:59 PM »
I'm skeptical any reasonable grid fin would survive full reentry heating, it would need to be some other material like PICA-X flaps.

Titanium is fairly durable with heating.  For example, see X-33 as an example of a titanium SSTO vehicle.
« Last Edit: 04/18/2017 08:07 PM by RedLineTrain »

Offline sevenperforce

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Re: F9 Second Stage Reusability
« Reply #408 on: 04/18/2017 08:02 PM »
Is there a way of placing some sort of ITS-wing-like extensions on a redesigned upper stage to allow for a biconic lifting-body re-entry, without compromising aerodynamic stability during launch?

Not for S2 reuse on Falcon Heavy, but something further down the road.

The hardware difference between an expendable first stage and a reusable first stage isn't much: bolt on the legs and tap the helium pressurant system, bolt on the grid fins and add their control system, add some TPS and you're good to go. Could we see something similar for a reusable second stage? Like, bolt on the wing extensions, wrap TPS around one side of the stage, and install SuperDracos in the wing extensions?

Offline Gliderflyer

Re: F9 Second Stage Reusability
« Reply #409 on: 04/19/2017 01:12 AM »
I'm skeptical any reasonable grid fin would survive full reentry heating, it would need to be some other material like PICA-X flaps.

Titanium is fairly durable with heating.  For example, see X-33 as an example of a titanium SSTO vehicle.
I thought that the X-33 was inconel, but in any case, that isn't the problem. Aerodynamic heating is much higher on sharp edges than it is on blunt ones, and grid fins consist entirely of sharp edges. The X-33 was flat and "fluffy" so it could get away with using a relatively normal material for the TPS. There might be some magic material that would allow a grid fin to survive orbital reentry heating, but it would be MUCH easier to use flat plates/body flaps.
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Offline Robotbeat

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Re: F9 Second Stage Reusability
« Reply #410 on: 04/19/2017 01:20 AM »
Both wrong twice. X-33 wasn't SSTO, and its main structure was carbon fiber.
« Last Edit: 04/19/2017 01:21 AM by Robotbeat »
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Offline HMXHMX

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Re: F9 Second Stage Reusability
« Reply #411 on: 04/19/2017 01:28 AM »
Both wrong twice. X-33 wasn't SSTO, and its main structure was carbon fiber.

I think Gliderflyer might be referring to the Inco "tiles" that covered the carbon fiber frame on the windward side.

http://www.styleofspeed.com/images/space/x-33/tps_c.jpg

Offline Gliderflyer

Re: F9 Second Stage Reusability
« Reply #412 on: 04/19/2017 02:35 AM »
Both wrong twice. X-33 wasn't SSTO, and its main structure was carbon fiber.

I think Gliderflyer might be referring to the Inco "tiles" that covered the carbon fiber frame on the windward side.

http://www.styleofspeed.com/images/space/x-33/tps_c.jpg
Yep, I was referring to the tiles. I forgot that the X-33 was only the mach 13 version (I have a bad habit of using X-33/Venture Star interchangeably), but I assume that Venture Star would have used the same TPS.
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Offline ChrisWilson68

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Re: F9 Second Stage Reusability
« Reply #413 on: 04/19/2017 07:58 AM »
Yeah the grid fins have sharp edges, but they're going to be behind the main shockwave.  The blunt main heat shield will take the bulk of the heating.  Actually, more accurately, the atmosphere compressed by the blunt main heat shield will take the bulk of the heating.  The blunt main heat shield will take less of it, but still a lot more than any grid fins back on the body will.

Look at how the main shock wave is far to the side as you go back for a re-entry capsule.  For S2 the sides are straight, not angled back, but they are still far from the shock wave.  The grid fins wouldn't reach out far enough to get the kind of heating the main heat shield will get.

http://www.popsci.com/sites/popsci.com/files/styles/large_1x_/public/blunt-body-shockwave.jpg?itok=dDTHT4aH

Offline wannamoonbase

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Re: F9 Second Stage Reusability
« Reply #414 on: 04/19/2017 08:23 AM »
Another idea would be a Rogallo wing, like that employed on the Gemini TT-1 test vehicle:



- I am more of the belief that they'll try with a simple parachute and grab it with a helicopter. Seems to me that it would be the lightest and most practical solution.

I have to agree that a parachute in air recovery would be the lightest weight option. 

It's a reasonable starting point.  Get some S2's back, learn, iterate.
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Offline sevenperforce

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Re: F9 Second Stage Reusability
« Reply #415 on: 04/19/2017 01:30 PM »
I have to agree that a parachute in air recovery would be the lightest weight option. 

It's a reasonable starting point.  Get some S2's back, learn, iterate.
Where would they put the TPS, how would they prevent ballistic re-entry, and where would they put the chute?

Offline RedLineTrain

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Re: F9 Second Stage Reusability
« Reply #416 on: 04/19/2017 01:57 PM »
Both wrong twice. X-33 wasn't SSTO, and its main structure was carbon fiber.

I stand corrected.  At least early on, Venturestar's skin was planned to be titanium.  I guess that must have changed.

Offline adrianwyard

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Re: F9 Second Stage Reusability
« Reply #417 on: 04/19/2017 02:12 PM »
Yeah the grid fins have sharp edges, but they're going to be behind the main shockwave.  The blunt main heat shield will take the bulk of the heating.  Actually, more accurately, the atmosphere compressed by the blunt main heat shield will take the bulk of the heating.  The blunt main heat shield will take less of it, but still a lot more than any grid fins back on the body will.

Look at how the main shock wave is far to the side as you go back for a re-entry capsule.  For S2 the sides are straight, not angled back, but they are still far from the shock wave.  The grid fins wouldn't reach out far enough to get the kind of heating the main heat shield will get.

http://www.popsci.com/sites/popsci.com/files/styles/large_1x_/public/blunt-body-shockwave.jpg?itok=dDTHT4aH

If the grid-fins are deep in the wake they won't help solve the pitch/yaw control problems S2 appears to have; you need to keep it upright with the heat shield facing into the airstream. So... spin stabilize S2 for entry, and only use the grid fins for terminal guidance a la S1.

In fact spinning up the S2 for entry might be something you could do with a stock stage. I guess it depends where the thrusters are and if it naturally spins in the desired axis.

Offline Rocket Science

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Re: F9 Second Stage Reusability
« Reply #418 on: 04/19/2017 02:19 PM »
Don't get all myopic by strictly focusing on CoG... It is the relationship between it and the CoP for lifting entry. The CoP will shift while traveling from hypersonic, supersonic and subsonic flight regimes. Not discussing CoP and strictly focusing on CoG is not adequately addressing the challenge of lifting entry...
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Offline sevenperforce

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Re: F9 Second Stage Reusability
« Reply #419 on: 04/19/2017 02:20 PM »
Yeah the grid fins have sharp edges, but they're going to be behind the main shockwave.  The blunt main heat shield will take the bulk of the heating.  Actually, more accurately, the atmosphere compressed by the blunt main heat shield will take the bulk of the heating.  The blunt main heat shield will take less of it, but still a lot more than any grid fins back on the body will.

Look at how the main shock wave is far to the side as you go back for a re-entry capsule.  For S2 the sides are straight, not angled back, but they are still far from the shock wave.  The grid fins wouldn't reach out far enough to get the kind of heating the main heat shield will get.

http://www.popsci.com/sites/popsci.com/files/styles/large_1x_/public/blunt-body-shockwave.jpg?itok=dDTHT4aH

If the grid-fins are deep in the wake they won't help solve the pitch/yaw control problems S2 appears to have; you need to keep it upright with the heat shield facing into the airstream. So... spin stabilize S2 for entry, and only use the grid fins for terminal guidance a la S1.

In fact spinning up the S2 for entry might be something you could do with a stock stage. I guess it depends where the thrusters are and if it naturally spins in the desired axis.
Oh, I didn't think of spin-stabilization. That's a fantastic idea.

Per the Falcon 9 user's guide: "In addition, the second stage contains a cold nitrogen gas (GN2) attitude control system (ACS) for pointing and roll control." The included table notes that while the first stage uses engine gimbals for pitch, yaw, and roll control, the second stage uses the cold gas thrusters for all three, with pitch and yaw also aided by engine gimbal during burns.

I also noted that in the attached SpaceX CGI rendering, there are a series of flaps around the engine that aren't present in the current flown model...TPS guidance flaps?

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