Author Topic: F9 Second Stage Reusability  (Read 212629 times)

Online Norm38

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Re: F9 Second Stage Reusability
« Reply #680 on: 04/23/2018 04:41 PM »
Saw this on YouTube today, thought I'd post it.  Didn't hear anything new for the regulars, but may be good for the non-engineers.


Online Comga

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Re: F9 Second Stage Reusability
« Reply #681 on: 04/29/2018 02:03 PM »
Saw this on YouTube today, thought I'd post it.  Didn't hear anything new for the regulars, but may be good for the non-engineers.

Nah
I would bet against each of this guy’s guesses and interpretations.
It’s not “obvious”. It doesn’t “have to be”.
SpaceX is not going to rig up some complex structure like HIAD or whatever
They can’t surround the second stage in airbags. It’s too big and they couldn’t be well distributed
All of his borrowed video snippets are irrelevant.

edit: typos
« Last Edit: 04/29/2018 02:05 PM by Comga »
What kind of wastrels would dump a perfectly good booster in the ocean after just one use?

Offline deruch

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Re: F9 Second Stage Reusability
« Reply #682 on: 04/29/2018 03:51 PM »
Saw this on YouTube today, thought I'd post it.  Didn't hear anything new for the regulars, but may be good for the non-engineers.

Nah
I would bet against each of this guy’s guesses and interpretations.
It’s not “obvious”. It doesn’t “have to be”.
SpaceX is not going to rig up some complex structure like HIAD or whatever
They can’t surround the second stage in airbags. It’s too big and they couldn’t be well distributed
All of his borrowed video snippets are irrelevant.

edit: typos

If they were going to only try to recover the engines/thrust structure ala an orbital SMART, they might go with something like a HIAD.  For a full 2nd stage, they'll try other things.
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Offline wannamoonbase

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Re: F9 Second Stage Reusability
« Reply #683 on: 04/30/2018 01:37 PM »
I think like many SpaceX development projects they will work in steps and take incremental steps from the least amount they think is possible and work up to finding out what is the minimal needed.

I can see them trying as a first step to just re-enter and not fly to a controlled path or recover.

After all that seems like the largest technical challenge.

As for the bouncy house, that's how fairing recover was described and EM said that Mr. Steven could catch a returning Dragon.  So maybe the SpaceX navy would be catching Upper stages as well.

Should be interesting to watch.
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Offline john smith 19

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Re: F9 Second Stage Reusability
« Reply #684 on: 04/30/2018 02:29 PM »

Nah
I would bet against each of this guy’s guesses and interpretations.
It’s not “obvious”. It doesn’t “have to be”.
SpaceX is not going to rig up some complex structure like HIAD or whatever
They can’t surround the second stage in airbags. It’s too big and they couldn’t be well distributed
All of his borrowed video snippets are irrelevant.
True.
Just  the KE alone (27 mega m/s Vs 5 mega m/s) makes it 29x harder in energy dissipated before we get to the potential energy of the increased altitude.

I comes down to this.
1) You want the engines in the rear.
2) A rear heavy object wants to flip heavy end forward.
3) Too much side load and the (very) lightweight tank walls collapse.

How you keep the loads along the stage, and the engines pointing backward, are the crux of the problem.


Now a really big skirt (with minimum TPS) just above the engine bay can
a) Radically increase drag in the high (near vacuum) atmosphere so the stage is moving much more slowly before it descends to the thicker low atmosphere. big deceleration. Small(ish) heating.
b) Keep the light end (with a thick, simple PICX heatshield) pointing into the airstream.

As long as it last long enough to get into the air density where the grid fins can work then your home dry.

But the exact details are very tricky and clearly SX have learned some new science since 2014 when Musk last visited this issue.
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Offline wannamoonbase

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Re: F9 Second Stage Reusability
« Reply #685 on: 04/30/2018 03:32 PM »
But the exact details are very tricky and clearly SX have learned some new science since 2014 when Musk last visited this issue.

Maybe they just have enough performance margin for some LEO missions to make an attempt.

Block 5 is over kill for many possible missions, may as well make use of that when possible to experiment.

Plus if they manage to make it work they could lower the costs of such missions even further.  That would be an interesting market option.
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Online envy887

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Re: F9 Second Stage Reusability
« Reply #686 on: 04/30/2018 03:40 PM »

Nah
I would bet against each of this guy’s guesses and interpretations.
It’s not “obvious”. It doesn’t “have to be”.
SpaceX is not going to rig up some complex structure like HIAD or whatever
They can’t surround the second stage in airbags. It’s too big and they couldn’t be well distributed
All of his borrowed video snippets are irrelevant.
True.
Just  the KE alone (27 mega m/s Vs 5 mega m/s) makes it 29x harder in energy dissipated before we get to the potential energy of the increased altitude.

I comes down to this.
1) You want the engines in the rear.
2) A rear heavy object wants to flip heavy end forward.
3) Too much side load and the (very) lightweight tank walls collapse.

How you keep the loads along the stage, and the engines pointing backward, are the crux of the problem.


Now a really big skirt (with minimum TPS) just above the engine bay can
a) Radically increase drag in the high (near vacuum) atmosphere so the stage is moving much more slowly before it descends to the thicker low atmosphere. big deceleration. Small(ish) heating.
b) Keep the light end (with a thick, simple PICX heatshield) pointing into the airstream.

As long as it last long enough to get into the air density where the grid fins can work then your home dry.

But the exact details are very tricky and clearly SX have learned some new science since 2014 when Musk last visited this issue.

An inflatable radically changes the COP/COM equation, the engine location becomes less critical.

With a 100x increase in ballistic coefficient, you don't need PICA-X. Stagnation temperatures drop to ~800 C or lower and easily into the working range of COTS ceramics and metals or a coating of SPAM.

Offline speedevil

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Re: F9 Second Stage Reusability
« Reply #687 on: 04/30/2018 04:08 PM »
With a 100x increase in ballistic coefficient, you don't need PICA-X. Stagnation temperatures drop to ~800 C or lower and easily into the working range of COTS ceramics and metals or a coating of SPAM.
Stagnation temperatures drop to 800C and heat flux at that temperature goes way down, meaning some things might not need protection at all for the several tens of seconds of peak heating.

Excited to see what it actually is.

Offline kraisee

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Re: F9 Second Stage Reusability
« Reply #688 on: 05/02/2018 01:58 AM »
With a 100x increase in ballistic coefficient, you don't need PICA-X. Stagnation temperatures drop to ~800 C or lower and easily into the working range of COTS ceramics and metals or a coating of SPAM.

With some fried PINEAPPLE, that'll make for a delicious snack after they land. :)

Oh...

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Offline john smith 19

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Re: F9 Second Stage Reusability
« Reply #689 on: 05/02/2018 07:32 AM »
An inflatable radically changes the COP/COM equation, the engine location becomes less critical.

With a 100x increase in ballistic coefficient, you don't need PICA-X. Stagnation temperatures drop to ~800 C or lower and easily into the working range of COTS ceramics and metals or a coating of SPAM.
Certainly shifts the CoP. Not so sure about the Centre of Mass. AFAIK the consensus remains the engine bay will be the heaviest section of the stage.

To lower the ballistic coefficient 100x needs a skirt about 16.5m (54 feet)  in width.

That's a pretty substantial unfurling task. Not impossible, but tough. If it can be made like those emergency slides aircraft use you could get quite a lot of area in a fairly small space, using inflatable tubes to stiffen it.
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Online matthewkantar

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Re: F9 Second Stage Reusability
« Reply #690 on: 05/02/2018 12:22 PM »
Quote from Elon Musk in a comment below the recent Instagram photo of the fairing descending under a parachute: "No, the upper stage engine is designed for vacuum operation only. We will either bring it in hot and fast Dragon style with a heat shield on front or slower with a giant party balloon."

I guess they are still in the concept phase of design on this. 

Offline wannamoonbase

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Re: F9 Second Stage Reusability
« Reply #691 on: 05/02/2018 12:33 PM »
Quote from Elon Musk in a comment below the recent Instagram photo of the fairing descending under a parachute: "No, the upper stage engine is designed for vacuum operation only. We will either bring it in hot and fast Dragon style with a heat shield on front or slower with a giant party balloon."

I guess they are still in the concept phase of design on this. 

They have existing heat shield capabilities, I believe they’ll start there. 

Edit: Spelling.
« Last Edit: 05/02/2018 05:35 PM by wannamoonbase »
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Online envy887

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Re: F9 Second Stage Reusability
« Reply #692 on: 05/02/2018 01:27 PM »
An inflatable radically changes the COP/COM equation, the engine location becomes less critical.

With a 100x increase in ballistic coefficient, you don't need PICA-X. Stagnation temperatures drop to ~800 C or lower and easily into the working range of COTS ceramics and metals or a coating of SPAM.
Certainly shifts the CoP. Not so sure about the Centre of Mass. AFAIK the consensus remains the engine bay will be the heaviest section of the stage.

To lower the ballistic coefficient 100x needs a skirt about 16.5m (54 feet)  in width.

That's a pretty substantial unfurling task. Not impossible, but tough. If it can be made like those emergency slides aircraft use you could get quite a lot of area in a fairly small space, using inflatable tubes to stiffen it.

A toroidal ballute around the engine would probably be easier than a skirt, but a simple spherical towed ballute hanging off the bottom of the stage would probably be easiest.
« Last Edit: 05/02/2018 01:27 PM by envy887 »

Offline john smith 19

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Re: F9 Second Stage Reusability
« Reply #693 on: 05/02/2018 04:21 PM »
An inflatable radically changes the COP/COM equation, the engine location becomes less critical.

With a 100x increase in ballistic coefficient, you don't need PICA-X. Stagnation temperatures drop to ~800 C or lower and easily into the working range of COTS ceramics and metals or a coating of SPAM.
Certainly shifts the CoP. Not so sure about the Centre of Mass. AFAIK the consensus remains the engine bay will be the heaviest section of the stage.

To lower the ballistic coefficient 100x needs a skirt about 16.5m (54 feet)  in width.

That's a pretty substantial unfurling task. Not impossible, but tough. If it can be made like those emergency slides aircraft use you could get quite a lot of area in a fairly small space, using inflatable tubes to stiffen it.

A toroidal ballute around the engine would probably be easier than a skirt, but a simple spherical towed ballute hanging off the bottom of the stage would probably be easiest.
Like a sort of space "sea anchor" ?

One problem with that would be the mounting. I'd think you'd want to put it as close to the center as possible but that's occupied by the engine. So off center you've got a force tugging the off its line, or you need say a 3 or 4 point attachment arrangement around the nozzle to equalize loads. Again, tricky to deploy reliably. A smaller ballute, one per mounting point, might be easier, but now you need all of them to inflate or you get the the uneven loading.

Just remembered. 800c is probably in the range of the Dunlop developed woven metal "airmat" technology. Unfortunately I don't know of any plastics that go above 400c (IIRC 300-350c is pretty challenging for regular use).

OTOH probably well within the range of a layer of flexible PICAX, which SX should have access to.
« Last Edit: 05/02/2018 04:24 PM by john smith 19 »
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Online envy887

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Re: F9 Second Stage Reusability
« Reply #694 on: 05/02/2018 05:35 PM »
An inflatable radically changes the COP/COM equation, the engine location becomes less critical.

With a 100x increase in ballistic coefficient, you don't need PICA-X. Stagnation temperatures drop to ~800 C or lower and easily into the working range of COTS ceramics and metals or a coating of SPAM.
Certainly shifts the CoP. Not so sure about the Centre of Mass. AFAIK the consensus remains the engine bay will be the heaviest section of the stage.

To lower the ballistic coefficient 100x needs a skirt about 16.5m (54 feet)  in width.

That's a pretty substantial unfurling task. Not impossible, but tough. If it can be made like those emergency slides aircraft use you could get quite a lot of area in a fairly small space, using inflatable tubes to stiffen it.

A toroidal ballute around the engine would probably be easier than a skirt, but a simple spherical towed ballute hanging off the bottom of the stage would probably be easiest.
Like a sort of space "sea anchor" ?

One problem with that would be the mounting. I'd think you'd want to put it as close to the center as possible but that's occupied by the engine. So off center you've got a force tugging the off its line, or you need say a 3 or 4 point attachment arrangement around the nozzle to equalize loads. Again, tricky to deploy reliably. A smaller ballute, one per mounting point, might be easier, but now you need all of them to inflate or you get the the uneven loading.

Just remembered. 800c is probably in the range of the Dunlop developed woven metal "airmat" technology. Unfortunately I don't know of any plastics that go above 400c (IIRC 300-350c is pretty challenging for regular use).

OTOH probably well within the range of a layer of flexible PICAX, which SX should have access to.

That's the idea. Why wouldn't an off-center mounting work? There's no reason the stage has to be pointed directly into the airflow. A slightly off-center mount would cause the stage to tip away from the tow rope so that the nozzle bell would clear it.

A 400 C material might work if the heat flux is low enough that you can dissipate it radiatively. Many highly stressed materials operate in environments beyond their melting temperatures (jet turbine blades), and sometimes beyond their boiling temperatures (regeneratively cooled rocket engines).

If you need more temperature resistant materials, a thin layer of woven ceramic might work. Like 3M Nextel with a melting point of 1800 C. It's 980 g/m^2 so 1,300 kg of it would cover the hot half of a 30 m sphere.

Online meekGee

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Re: F9 Second Stage Reusability
« Reply #695 on: 05/03/2018 06:38 PM »
An inflatable radically changes the COP/COM equation, the engine location becomes less critical.

With a 100x increase in ballistic coefficient, you don't need PICA-X. Stagnation temperatures drop to ~800 C or lower and easily into the working range of COTS ceramics and metals or a coating of SPAM.
Certainly shifts the CoP. Not so sure about the Centre of Mass. AFAIK the consensus remains the engine bay will be the heaviest section of the stage.

To lower the ballistic coefficient 100x needs a skirt about 16.5m (54 feet)  in width.

That's a pretty substantial unfurling task. Not impossible, but tough. If it can be made like those emergency slides aircraft use you could get quite a lot of area in a fairly small space, using inflatable tubes to stiffen it.

A toroidal ballute around the engine would probably be easier than a skirt, but a simple spherical towed ballute hanging off the bottom of the stage would probably be easiest.
I'm also reading it as a towed balloon, but the difficulty there are induced gyrating motions...  Maybe they've figured out a way to actively damp them.

Maybe the baloon is not towed freely but rather attached to the back of the engine bay by really short tethers.  Like 6 of them.  That would be more stable to begin with.

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Re: F9 Second Stage Reusability
« Reply #696 on: 05/03/2018 07:01 PM »
Does anybody have any calcs about what the g-force is for the trailing balloon idea. I assume because it has a higher ballistic coefficient it decelerates faster. This would influence the strength of the ropes used to attach the balloon to the S2.

EDIT:
to clarify.
I was thinking of g's for S2 alone versus S2 with giant trailing party balloon.
So the answer below seems correct that it is only the height that changes and not the g-force.
« Last Edit: 05/03/2018 08:41 PM by rsdavis9 »
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Offline speedevil

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Re: F9 Second Stage Reusability
« Reply #697 on: 05/03/2018 07:35 PM »
Does anybody have any calcs about what the g-force is for the trailing balloon idea. I assume because it has a higher ballistic coefficient it decelerates faster.
To a first order, it does not.
It decelerates at the same rate, just where the atmosphere is 100* less dense.
The deceleration rate is in fact slightly different because the atmosphere scale height  at the entry interface (the amount the atmosphere is changing due to height) varies due to a temperature discontinuity in the atmosphere).

Offline aero

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Re: F9 Second Stage Reusability
« Reply #698 on: 05/03/2018 08:01 PM »
Does anybody have any calcs about what the g-force is for the trailing balloon idea. I assume because it has a higher ballistic coefficient it decelerates faster.
To a first order, it does not.
It decelerates at the same rate, just where the atmosphere is 100* less dense.
The deceleration rate is in fact slightly different because the atmosphere scale height  at the entry interface (the amount the atmosphere is changing due to height) varies due to a temperature discontinuity in the atmosphere).

I think that rsdavis9 misspoke. Yes, they decelerate at the same rate, they are tied together after all. But the balloon experiences a much greater drag force than the first stage, that is its whole purpose. The drag force will hold tension on the lines attaching the two. How great can this drag force be expected to be?

If the balloon increases the deceleration by 0.1 g above that of stage 1 alone, then the drag force would equal about 2 tons of force. So the question boils down to this, "What would be the optimum and maximum increase of deceleration of the assembly be between balloon deployment and balloon release?

That is, increase over that of stage 1 alone.
« Last Edit: 05/03/2018 08:02 PM by aero »
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Online envy887

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Re: F9 Second Stage Reusability
« Reply #699 on: 05/03/2018 08:11 PM »
Does anybody have any calcs about what the g-force is for the trailing balloon idea. I assume because it has a higher ballistic coefficient it decelerates faster.
To a first order, it does not.
It decelerates at the same rate, just where the atmosphere is 100* less dense.
The deceleration rate is in fact slightly different because the atmosphere scale height  at the entry interface (the amount the atmosphere is changing due to height) varies due to a temperature discontinuity in the atmosphere).

I think that rsdavis9 misspoke. Yes, they decelerate at the same rate, they are tied together after all. But the balloon experiences a much greater drag force than the first stage, that is its whole purpose. The drag force will hold tension on the lines attaching the two. How great can this drag force be expected to be?

If the balloon increases the deceleration by 0.1 g above that of stage 1 alone, then the drag force would equal about 2 tons of force. So the question boils down to this, "What would be the optimum and maximum increase of deceleration of the assembly be between balloon deployment and balloon release?

That is, increase over that of stage 1 alone.

To the first order, the drag force on the balloon is 100x greater than that on the stage. It's basically hanging the mass of the stage off the balloon, at whatever g-load deceleration rate the entry requires.

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