Author Topic: F9 Second Stage Reusability  (Read 138447 times)

Offline john smith 19

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Re: F9 Second Stage Reusability
« Reply #360 on: 04/14/2017 03:55 PM »
Would Kestrels would make good landing engines?  They don't use turbopumps, right fuel, thrust seems in the right range for landing, and they're already a proven technology for SpaceX.  Instead of using dead weight for ballast in the nose, that space could be a tank, fuel could be pumped between ballast and main tanks to shift center of gravity so your landing fuel is also your ballast. 

While dissimilar in fuel and scale; mixing different size engines for vacuum and landing is a concept they're planning for ITS.
Before looking for trouble it's better to look at the simplest possible solution.

The simplest solution is to land like the booster stage with the Merlin engine firing through its full vacuum nozzle.

So the questions are
1) Can the nozzle end be made survive the air flow, given that with the stage slightly nose down part of it will be in the air stream (for example could slowly spinning the stage on the long axis be enough to spread the heat)?

2)Can the Vac nozzle be run without destructive flow separation at Sea Level pressure. Note it's asymmetrical flow sep that's the problem. A smooth peeling off the wall all the way around is survivable. The Rocketdyne team that worked on the original J-2X programme were able to substantially increase the nozzle size at which flow sep started by relatively simple nozzle reshaping.

3)Can the Merlin throttle down enough to land IE below landing mass? The improved maximum thrust actually makes this problem worse as you need a smaller fraction of a larger number to equal the landing stage weight. However even the SSME was tested and capable of throttling down to about 17% of full power. Note the Pintle injector SX use is capable of "deep" throttling (which IIRC is below 15% of full thrust). Splitting the propellant between 2 sets of injectors (say 0-10% thrust and the other cutting in to give 11-100%) is another option.

If the answer to all these question is yes then the Merlin main engine is all you need, but I think it's going to take serious data collection and simulation to prove it (and if not why not) and only SX know for sure (or rather with a high degree of probability).
« Last Edit: 04/14/2017 03:57 PM by john smith 19 »
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Offline babakm

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Re: F9 Second Stage Reusability
« Reply #361 on: 04/14/2017 04:57 PM »
Wondering:  Considering the remaining LOX visible after SECO in the webcast and noted by many in the Echostar discussion thread, did they do a dry run of recovery (or at least the reentry burn)?

https://forum.nasaspaceflight.com/index.php?topic=40374.msg1654865#msg1654865
« Last Edit: 04/14/2017 05:02 PM by babakm »

Offline rakaydos

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Re: F9 Second Stage Reusability
« Reply #362 on: 04/14/2017 06:03 PM »
Would Kestrels would make good landing engines?  They don't use turbopumps, right fuel, thrust seems in the right range for landing, and they're already a proven technology for SpaceX.  Instead of using dead weight for ballast in the nose, that space could be a tank, fuel could be pumped between ballast and main tanks to shift center of gravity so your landing fuel is also your ballast. 

While dissimilar in fuel and scale; mixing different size engines for vacuum and landing is a concept they're planning for ITS.
Before looking for trouble it's better to look at the simplest possible solution.

The simplest solution is to land like the booster stage with the Merlin engine firing through its full vacuum nozzle.

So the questions are
1) Can the nozzle end be made survive the air flow, given that with the stage slightly nose down part of it will be in the air stream (for example could slowly spinning the stage on the long axis be enough to spread the heat)?

2)Can the Vac nozzle be run without destructive flow separation at Sea Level pressure. Note it's asymmetrical flow sep that's the problem. A smooth peeling off the wall all the way around is survivable. The Rocketdyne team that worked on the original J-2X programme were able to substantially increase the nozzle size at which flow sep started by relatively simple nozzle reshaping.

3)Can the Merlin throttle down enough to land IE below landing mass? The improved maximum thrust actually makes this problem worse as you need a smaller fraction of a larger number to equal the landing stage weight. However even the SSME was tested and capable of throttling down to about 17% of full power. Note the Pintle injector SX use is capable of "deep" throttling (which IIRC is below 15% of full thrust). Splitting the propellant between 2 sets of injectors (say 0-10% thrust and the other cutting in to give 11-100%) is another option.

If the answer to all these question is yes then the Merlin main engine is all you need, but I think it's going to take serious data collection and simulation to prove it (and if not why not) and only SX know for sure (or rather with a high degree of probability).
While that can be looked into, it remains a longshot, and even SpaceX's original video does not suggest that they thought it was possible.

I've heard that a minimum throttle hoverslam would be at something like 8g for a dry upperstage.

Offline philw1776

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Re: F9 Second Stage Reusability
« Reply #363 on: 04/14/2017 06:40 PM »
Found this:

Here's what I have....

https://forum.nasaspaceflight.com/index.php?action=dlattach;topic=36039.0;attach=621091;sess=43050

Quote
PICA-X heat shield material boasts a density of only 0.27 g/ cm3, near the density of solid cork (0.24 g/cm3) [22]. The material needs only be a few centimeters thick (typically around 6 cm)


So roughly, a 5m wide 6cm thick PICA-X heat shield masses under 350Kg.  A good start.  Say 500Kg total to cover other hot spots.
If the recovery system masses 1.5 to 2 tons the 2 ton payload loss may be acceptable for a number of launches.
But it does look like S2 recovery will not be anywhere close to all missions because of any substantive payload mass subtraction.  More S1s recovered % wise.
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Re: F9 Second Stage Reusability
« Reply #364 on: 04/14/2017 06:55 PM »
But it does look like S2 recovery will not be anywhere close to all missions because of any substantive payload mass subtraction.  More S1s recovered % wise.

They could shift those missions to FH 3 core RTLS. It depends on how much cores will cost per flight with reuse.

Offline CharlieWildman

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Re: F9 Second Stage Reusability
« Reply #365 on: 04/15/2017 12:55 AM »
Thanks every one for finding the PicaX density.  I went with .27g/cm^3.

New drawings show the drag flaps and the start of a modular nose structure and heat shield.  Nose heat shield with 3 inch thick PicaX comes in at 637 lbs. Guesstimated aluminum structure is 1,248 lbs

Drag flaps are modeled with 2 inch aluminum honeycomb topped with .093 inch thick carbon skins.  2 inches of PicaX cover one surface and three edges. Total weight for each flap is 61.6 lb.  Flaps are 4 x 4 feet. 

Drag flap size is based on the old SpaceX drawing.  They look small to me given the CG and stretched 2nd stage.  Might make sense to move them farther aft and increase their area. And  as RocketScience mentions, there is a lot of wight that will have to go in to the rear end to get the flaps moving.
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Re: F9 Second Stage Reusability
« Reply #366 on: 04/15/2017 01:05 AM »
A question, if you don't mind.  What do you get for center of gravity when you reverse this situation?  If you put the control flaps at the top and the heat shielding around the base and the engine but not the engine bell.  Allow the engine bell to keep most of the heat for heat from everything.

Offline CharlieWildman

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Re: F9 Second Stage Reusability
« Reply #367 on: 04/15/2017 01:26 AM »
A question, if you don't mind.  What do you get for center of gravity when you reverse this situation?  If you put the control flaps at the top and the heat shielding around the base and the engine but not the engine bell.  Allow the engine bell to keep most of the heat for heat from everything.

That is a very interesting question.  Something I have been thinking about too.  Don't have time tonight, but in the next day or two I'll rearrange the model with the flaps at the nose and heat shield at the rear along with a retracting engine bell.   
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Re: F9 Second Stage Reusability
« Reply #368 on: 04/15/2017 01:33 AM »
No retracting engine bell, let it set up a shockwave as far from the stage as possible. The material is so thin and so susceptible to damage at the end that most likely no matter what you do you will not be able to reuse it. Once on the ground, it could be easily unbolted and a replacement bolted in place. The material from the old engine bell would, of course, be recycled. Most likely entering engine bell 1st the thinner material would just be folded over but still would set up a shockwave that would help protect the rest of the stage.

Offline pmonta

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Re: F9 Second Stage Reusability
« Reply #369 on: 04/15/2017 04:49 AM »
To go way out on a limb with a probably ridiculous idea:  ideally the heat shield would be in front of the second-stage engine during reentry.  That's kind of hard to manage during ascent for obvious reasons.  So carry the heat shield uphill at the top of the second stage (opposite the engine), then, once in orbit, detach the shield, maneuver it, and reattach at the engine end.

There's plenty of time---the stage is in orbit---and humans can help with teleoperation of the robotics if needed.  Perhaps there would be a small kit of struts and fasteners along with the shield, and anchor points around the S2 engine base.

Is it reasonable to deorbit with RCS?

Offline Rocket Science

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Re: F9 Second Stage Reusability
« Reply #370 on: 04/15/2017 08:33 AM »
Thanks every one for finding the PicaX density.  I went with .27g/cm^3.

New drawings show the drag flaps and the start of a modular nose structure and heat shield.  Nose heat shield with 3 inch thick PicaX comes in at 637 lbs. Guesstimated aluminum structure is 1,248 lbs

Drag flaps are modeled with 2 inch aluminum honeycomb topped with .093 inch thick carbon skins.  2 inches of PicaX cover one surface and three edges. Total weight for each flap is 61.6 lb.  Flaps are 4 x 4 feet. 

Drag flap size is based on the old SpaceX drawing.  They look small to me given the CG and stretched 2nd stage.  Might make sense to move them farther aft and increase their area. And  as RocketScience mentions, there is a lot of wight that will have to go in to the rear end to get the flaps moving.
Nice work Charlie, keep at it! :)
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Offline john smith 19

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Re: F9 Second Stage Reusability
« Reply #371 on: 04/15/2017 09:31 AM »
While that can be looked into, it remains a longshot, and even SpaceX's original video does not suggest that they thought it was possible.

I've heard that a minimum throttle hoverslam would be at something like 8g for a dry upperstage.
Watch the video again from 1:18 to 1:59

It shows the US flipping, then a de-orbit burn. It does not show it flipping again to do a nose first entry (although that's what it does show). Nor does it show how the US does the 90deg shift from nose front to nose top (or the 180 deg flip post re-entry). Both of which occur when the stage is deep in the atmosphere and GN2 or even Dracos would probably not have the authority to overcome the drag forces without ripping the stage apart.

But it does show it landing tail down with no other obvious engines firing apart from the main one.

That suggests either it's the main engine doing the work or any separate landing engines are inside the tail skirt.

Mechanically and structurally the simplest answer is to use what has to be on the stage anyway IE the main engine, provided it can survive the environment and throttle down enough.
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Offline Kaputnik

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Re: F9 Second Stage Reusability
« Reply #372 on: 04/15/2017 09:34 AM »
No retracting engine bell, let it set up a shockwave as far from the stage as possible. The material is so thin and so susceptible to damage at the end that most likely no matter what you do you will not be able to reuse it. Once on the ground, it could be easily unbolted and a replacement bolted in place. The material from the old engine bell would, of course, be recycled. Most likely entering engine bell 1st the thinner material would just be folded over but still would set up a shockwave that would help protect the rest of the stage.

Surely the nozzle extension would deform in a somewhat random way- sounds a bit tricky to model- would the stage have sufficient control authority to deal with this? Worst case might be the nozzle folds over to one side, producing an asymmetric aero load.
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Offline rakaydos

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Re: F9 Second Stage Reusability
« Reply #373 on: 04/15/2017 04:11 PM »
While that can be looked into, it remains a longshot, and even SpaceX's original video does not suggest that they thought it was possible.

I've heard that a minimum throttle hoverslam would be at something like 8g for a dry upperstage.
Watch the video again from 1:18 to 1:59

It shows the US flipping, then a de-orbit burn. It does not show it flipping again to do a nose first entry (although that's what it does show). Nor does it show how the US does the 90deg shift from nose front to nose top (or the 180 deg flip post re-entry). Both of which occur when the stage is deep in the atmosphere and GN2 or even Dracos would probably not have the authority to overcome the drag forces without ripping the stage apart.

But it does show it landing tail down with no other obvious engines firing apart from the main one.

That suggests either it's the main engine doing the work or any separate landing engines are inside the tail skirt.

Mechanically and structurally the simplest answer is to use what has to be on the stage anyway IE the main engine, provided it can survive the environment and throttle down enough.
Watch it again. First during the flip, see where the thrusters next to the engine bell are that fire during the flip. Then the landing, where you can see 4 of those thrusters firing, not the main bell.

Offline GWH

Re: F9 Second Stage Reusability
« Reply #374 on: 04/15/2017 04:34 PM »
You guys are arguing about a really old video that I am pretty sure Musk himself had said was done with a lot of artistic license. I wouldn't get hung up on anything.

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Re: F9 Second Stage Reusability
« Reply #375 on: 04/15/2017 05:31 PM »
No retracting engine bell, let it set up a shockwave as far from the stage as possible. The material is so thin and so susceptible to damage at the end that most likely no matter what you do you will not be able to reuse it. Once on the ground, it could be easily unbolted and a replacement bolted in place. The material from the old engine bell would, of course, be recycled. Most likely entering engine bell 1st the thinner material would just be folded over but still would set up a shockwave that would help protect the rest of the stage.

Surely the nozzle extension would deform in a somewhat random way- sounds a bit tricky to model- would the stage have sufficient control authority to deal with this? Worst case might be the nozzle folds over to one side, producing an asymmetric aero load.

Yes probably, but so are some arrowheads, that’s what feathers are for.  Besides, it could possibly be an advantage.

When you are trying to get something to do a thing with a minimum of effort is usually best to figure out what it wants to do on its own and see if you can work with that.  If the center of gravity is more toward the engine because of the engine's weight and this means that it wants to reenter engine 1st why not see if you can work with that.  If you have to add some TPS why not add it to make the stage do more of what is going to do anyway.  If you’re adding TPS to protect the most valuable component on the stage, the engine, this would just move the center of gravity more toward the engine making it more stable.  As long as it’s not too unstable the only additional control authority needed is to set up a role that will cancel out the random direction.  You could even use this, to some extent, to help aim the stage downrange.

If this is not enough stability you could add, as others have suggested, flaps at the opposite end.  If this is still not enough stability you could add a HIAD like device.  Adding a HIAD has the added advantage of producing more drag sooner/higher and will thus reduce the heat load later on.  Although it is still probably possible to steer with a HIAD it may be better to use movable flaps for this instead.

For a 1st attempt just trying to get the stage to survive reentry and get telemetry from it would probably be more than adequate.  After you get past this step you can add the additional bells and whistles (flaps etc.) eventually with the stage coming down under a large parachute.

Offline CharlieWildman

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Re: F9 Second Stage Reusability
« Reply #376 on: 04/15/2017 06:47 PM »
Here is a comparison showing the rough CG location for nose first and engine first re-entry.
I added a painfully guesstimated carbon structure (interstage like) to the engine first model to connect the heat shield aluminum structure to the tank.  It weighs in at 153 Kg for the bell jettisoning model and 360 kg for the retracting bell model.

Adding Superdracos, prop tanks and plumbing will help the CG situation for both nose first (assuming Dracos in the nose) and engine first methods.  Any residual prop in the main tanks, assuming both RP1 and LOX, will also help the CG for both methods.

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Re: F9 Second Stage Reusability
« Reply #377 on: 04/15/2017 11:59 PM »
how about a small wing, optimized for hypersonic and Mach 2+ flight ?
Optimally it would provide a lot of lift above 50Km altitude, which could be used both to give the stage a lot of cross range to facilitate getting to the LZ/ASDS, substantially reducing re-entry speeds, perhaps even fixing CG problems (place the wings at CG).
But landing would still be done straight up.
The shuttle orbiter massed 100 tons. A 2nd stage with re-entry gear perhaps would mass 5-6 tons.
Shuttle wings provided enough lift for a ~200 knot landing. These wings would be of little use at terminal speed below 5km altitude.
Possibly the wings could even help on the way up, as long as L:D > 3.

The re-entry scenario would be to put the stage on an elliptical orbit such that the perigee skims the upper atmosphere just enough to produce a little lift (and a little drag) at perigee. At first the stage would want to go up both by its initial orbit and the lift it produces, but drag quickly slows the stage enough such that its orbit no longer pushes it up. Then it becomes a descent at lowest drag levels = lowest heating.

I'm thinking something like the X15 wing with a lot of TPS on the bottom/leading edge.

The dream scenario is to give the stage 1000Km of cross range, including a short minimum thrust engine burn after drag heating is low enough (but the air is still very thin). Enough to lift the stage by as much as 10 Km and increase speed quite a bit.

Probably a stupid idea. Shoot it down already  ;D
« Last Edit: 04/16/2017 12:01 AM by macpacheco »
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Offline Rocket Science

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Re: F9 Second Stage Reusability
« Reply #378 on: 04/16/2017 01:24 AM »
how about a small wing, optimized for hypersonic and Mach 2+ flight ?
Optimally it would provide a lot of lift above 50Km altitude, which could be used both to give the stage a lot of cross range to facilitate getting to the LZ/ASDS, substantially reducing re-entry speeds, perhaps even fixing CG problems (place the wings at CG).
But landing would still be done straight up.
The shuttle orbiter massed 100 tons. A 2nd stage with re-entry gear perhaps would mass 5-6 tons.
Shuttle wings provided enough lift for a ~200 knot landing. These wings would be of little use at terminal speed below 5km altitude.
Possibly the wings could even help on the way up, as long as L:D > 3.

The re-entry scenario would be to put the stage on an elliptical orbit such that the perigee skims the upper atmosphere just enough to produce a little lift (and a little drag) at perigee. At first the stage would want to go up both by its initial orbit and the lift it produces, but drag quickly slows the stage enough such that its orbit no longer pushes it up. Then it becomes a descent at lowest drag levels = lowest heating.

I'm thinking something like the X15 wing with a lot of TPS on the bottom/leading edge.

The dream scenario is to give the stage 1000Km of cross range, including a short minimum thrust engine burn after drag heating is low enough (but the air is still very thin). Enough to lift the stage by as much as 10 Km and increase speed quite a bit.

Probably a stupid idea. Shoot it down already  ;D
Not really, I posted a concept using an "X-37 like" S2 several pages back to land horizontally on a runway: post#81
https://forum.nasaspaceflight.com/index.php?topic=42637.80
« Last Edit: 04/16/2017 12:35 PM by Rocket Science »
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Re: F9 Second Stage Reusability
« Reply #379 on: 04/16/2017 01:52 AM »
Not really, I posted a concept using an "X-37 like" S2 several pages back to land horizontally on a runway: post#81
https://forum.nasaspaceflight.com/index.php?topic=42637.80
But is SpaceX interested in such an idea ?
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