Author Topic: F9 Second Stage Reusability  (Read 212969 times)

Offline darkenfast

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Re: F9 Second Stage Reusability
« Reply #380 on: 04/16/2017 03:31 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.

May I add something from the safety of my uneddicated armchair?  Change to three flaps, to save weight, maybe even two (with roll control by other means).  Add a small extension (2'?), to the front of the second stage cylinder, capped by the heat shield.  Into that goes a Draco-derived (NOT Super Draco) set of tanks and thrusters to keep control during re-entry and a similar-to-the-fairing (but larger) steerable parachute to recover on the same bag that the fairings will (hopefully!) land on, perhaps the next day.  Cable risers would run down the side to orient the stage for landing.  I agree with others that the nozzle extension will not survive and will be jettisoned.   This adds more useful weight to the front end and keeps overall weight increase to about the minimum that I can see.

Which, of course, means that it's probably all wrong!

Offline RDMM2081

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Re: F9 Second Stage Reusability
« Reply #381 on: 04/16/2017 05:07 AM »
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.

Please, forgive the late-night speculation.   But I have followed this thread and read all the (ideas) to date, none have struck me yet as a "spaceX" solution.  Suddenly this seems close! (If you flip it back around!) Add some amount of PicaX as necessary to the side wall, jettison the vac-nozzle(can only the extended vac-nozzle be jettisoned, leaving a SL nozzle intact?), extend a (grid-fin-ish) flap (with heavy heat xfer properties) TOWARDS the nozzle/CG, ride that in, and THEN, hmm, I dunno, pick a landing method? Mvac seems unlikely (imho, throttling issues), and so does the addition of superdracos (crazy mass penalties!) which leaves "float-testing" (salt-water bath, minus 100 style points) or some type of mid-air recovery (also not perfect, but the mass penalties otherwise are, um, "silly" (imho))

I dunno, probably shouldn't bother typing up stuff like this after the amount of tequila which was actually involved.....
« Last Edit: 04/16/2017 05:41 AM by RDMM2081 »

Offline RDMM2081

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Re: F9 Second Stage Reusability
« Reply #382 on: 04/16/2017 05:32 AM »
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.

I feel like I need to type more words, sorry mods if you need to delete or combine. But take the top picture, except imagine nozzle first re-entry, extend the flap(entry-wise?) and reinforce the pica type properties, now you shield the sensitive engine nozzle (whether you think it's usable for entry/landing at all) (at least you protect it somewhat for re-use) and I think you spread the heating loads across the stage somewhat. 

Seems like this flap mod could add less mass(maintain current CG anyway), add a "lifting-body" aspect, (IANARS), and obviously won't solve every problem and create world peace, but at least now the "last mile" recovery bit is hopefully on the table (think back to "soft water landing" on Cassiopeia, did not solve the problem, but it cut the solution-set by ~80% (or whatever, pick a number))

Online guckyfan

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Re: F9 Second Stage Reusability
« Reply #383 on: 04/16/2017 06:44 AM »
Landing prop tanks at the top make sense to me, that's most of the weight. But legs and engines at the top don't add up. You need an aerodynamic side for reentry, not more equipment that needs protection. Any remaining stability issues need to be adressed with flaps. the early animation does it that way, ITS does it that way, so will a reusable upper stage IMO.

Offline Rocket Science

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Re: F9 Second Stage Reusability
« Reply #384 on: 04/16/2017 04:08 PM »
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 ?
That, I can not answer for you... What I can answer is that Elon "is" interested in being successful... What form that architecture will look like will be driven purely by physics and economics, the data will speak for itself. Any approach that may work shouldn't be ruled out at this point. He may ideological, but not foolish IMHO
« Last Edit: 04/16/2017 04:13 PM by Rocket Science »
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Offline Lar

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Re: F9 Second Stage Reusability
« Reply #385 on: 04/16/2017 04:52 PM »
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 ?
That, I can not answer for you... What I can answer is that Elon "is" interested in being successful... What form that architecture will look like will be driven purely by physics and economics, the data will speak for itself. Any approach that may work shouldn't be ruled out at this point. He may ideological, but not foolish IMHO

He has in the past disdained wings as not worth the mass. But grid fins are a kind of wing (if you stretch the definition) and if someone put together a proposal that showed that wings of a certain size were the least mass penalty (integrating across the increased drag on the way up, the probability of success, and all the other variables in the trade space) way to recover S2, I think he'd give it very serious and detailed consideration.

So yeah. what Rocket Science said.
« Last Edit: 04/16/2017 04:53 PM by Lar »
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Offline CharlieWildman

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Re: F9 Second Stage Reusability
« Reply #386 on: 04/16/2017 06:02 PM »
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 ?
That, I can not answer for you... What I can answer is that Elon "is" interested in being successful... What form that architecture will look like will be driven purely by physics and economics, the data will speak for itself. Any approach that may work shouldn't be ruled out at this point. He may ideological, but not foolish IMHO

He has in the past disdained wings as not worth the mass. But grid fins are a kind of wing (if you stretch the definition) and if someone put together a proposal that showed that wings of a certain size were the least mass penalty (integrating across the increased drag on the way up, the probability of success, and all the other variables in the trade space) way to recover S2, I think he'd give it very serious and detailed consideration.

So yeah. what Rocket Science said.

Sigh. If I only had an engineering team and a wind tunnel...
I was actually playing with the idea of CADing up a 2nd stage with X-37 ish wings and tail and doing some DIY aerodynamic testing.  There would be so much structural guess work though...  I think any findings would be useless for determining the mass hit.  So shelving that idea for now. 
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Online Lemurion

Re: F9 Second Stage Reusability
« Reply #387 on: 04/16/2017 07:21 PM »
No matter what I see, I keep coming back to a tail-first entry. I can't call myself an expert, but the one thing I'm sure SpaceX won't do is add mass primarily to move the CG. If gravity is pulling the upper stage into a tail-first orientation, I think SpaceX will work with it rather than against it.

Offline mme

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Re: F9 Second Stage Reusability
« Reply #388 on: 04/16/2017 07:54 PM »
No matter what I see, I keep coming back to a tail-first entry. I can't call myself an expert, but the one thing I'm sure SpaceX won't do is add mass primarily to move the CG. If gravity is pulling the upper stage into a tail-first orientation, I think SpaceX will work with it rather than against it.
Ah, but there is a difference between adding mass "just to move the CG" and the potential situation where the mass of a heatshield, SDs, fuel, landing legs etc. is enough to move CG.  In other words, recovery will require more mass.  You might as well put it to use.

In the end I think it will come down to a trade off between engineering complexity (and therefore probable reliability), total mass penalty and what is physically possible starting with the current S2 design.

I have no confidence in what approach they will take.
« Last Edit: 04/16/2017 07:54 PM by mme »
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Offline macpacheco

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Re: F9 Second Stage Reusability
« Reply #389 on: 04/17/2017 03:09 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 ?
That, I can not answer for you... What I can answer is that Elon "is" interested in being successful... What form that architecture will look like will be driven purely by physics and economics, the data will speak for itself. Any approach that may work shouldn't be ruled out at this point. He may ideological, but not foolish IMHO

He has in the past disdained wings as not worth the mass. But grid fins are a kind of wing (if you stretch the definition) and if someone put together a proposal that showed that wings of a certain size were the least mass penalty (integrating across the increased drag on the way up, the probability of success, and all the other variables in the trade space) way to recover S2, I think he'd give it very serious and detailed consideration.

So yeah. what Rocket Science said.

I wouldn't interpret Musk's statements as disdain, but rather the bigger thinking that wings are 100% useless on the moon and of little use on the thin Martian atmosphere. He wanted a generic solution. I have a lot of respect for that.
But F9/FH upper stages aren't intended to ever land on the Moon or Mars. There's already very long runways at the cape and at vandy.
So I wouldn't be surprised if EM decides to go that route. I'm sure what we're thinking has already been discussed internally.
It wouldn't be the first time he changed his mind.
But I sense he already has a better solution and a more generic one.
What I want is they find any solution that works well and advance the goal post several miles. Whatever the solution is.
« Last Edit: 04/17/2017 03:10 AM by macpacheco »
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Offline adrianwyard

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Re: F9 Second Stage Reusability
« Reply #390 on: 04/17/2017 05:18 AM »
Re disdain for wings: We do at least know that making use of aerodynamic lift is in favor at SpaceX: They experimented with it on the last Falcon launch. And the ITS designs are clearly 'lifting bodies' with entry attitudes closer to the shuttle orbiter than the tail-first boosters.

If SpaceX were designing the Falcon stage 2 now I'd easily believe it would look and behave like a massively scaled down ITS, and part of the justification for that design would be proving/maturing plans for ITS.

But what's actually coming in the near term S2 recovery attempts? I've no idea.

Online Oersted

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Re: F9 Second Stage Reusability
« Reply #391 on: 04/17/2017 12:00 PM »
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.

Offline robert_d

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Re: F9 Second Stage Reusability
« Reply #392 on: 04/17/2017 01:48 PM »
I wonder if while they were looking at options for recovering the payload fairing, they discovered that the strength and flexibility of new materials made a parasail/parafoil option viable instead of the legs. And, if they had calculated the cost for a "bouncy castle" recovery ship as too high, spreading it over many fairings/several 2nd stages might just possibly be economic. Fairing recovered on East coast and 2nd stage on West, and possibly visa versa for west coast launches. Could also be that PICA-X has worked well enough on dragon that weight estimates for 2nd stage have come down.

The 'secret sauce' might be some dual use capability. Since second stages must already be strong enough to support Falcon Heavy payloads, Maybe they can also support super-dracos at least on the F9. Maybe these tanks could also change the outer mold line to increase lift.

Offline Rocket Science

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Re: F9 Second Stage Reusability
« Reply #393 on: 04/17/2017 03:12 PM »
Re disdain for wings: We do at least know that making use of aerodynamic lift is in favor at SpaceX: They experimented with it on the last Falcon launch. And the ITS designs are clearly 'lifting bodies' with entry attitudes closer to the shuttle orbiter than the tail-first boosters.

If SpaceX were designing the Falcon stage 2 now I'd easily believe it would look and behave like a massively scaled down ITS, and part of the justification for that design would be proving/maturing plans for ITS.

But what's actually coming in the near term S2 recovery attempts? I've no idea.
Agreed, who knows? Keeps things interesting IMHO! :) Just a note about a lifting body plan-form, you need those stubby wings on the hybrid-like X-37 to  give it more cross-range and keep the landing speeds reasonable...
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Offline john smith 19

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Re: F9 Second Stage Reusability
« Reply #394 on: 04/17/2017 03:42 PM »
Sigh. If I only had an engineering team and a wind tunnel...
I was actually playing with the idea of CADing up a 2nd stage with X-37 ish wings and tail and doing some DIY aerodynamic testing.  There would be so much structural guess work though...  I think any findings would be useless for determining the mass hit.  So shelving that idea for now.
In CAD, anything is possible.  :(

But your engineering instincts are right to be weary of wings.

The key problem is not making them light enough to carry the load, or how to extend them if they are retractable.

It's that those loads are now pressing on the side of a structure that's been designed (from day one) to be very strong lengthwise. It's the old soda can analogy. You can stack 10 full loaded soda cans on one empty provided it's longwise. Put the empty on its side and it's crushed.

It's true most large aircraft are cylinders-with-wings-stuck-on-them but those cylinders don't have to sit on their tails prior to launch.

I don't think there have been more than 10  "tail sitter" aircraft designs in the whole history of flight (including Shuttle, Buran and the X37b). AFAIK outside Shuttle and Buran all were all 1 (or no) person vehicles for military test programmes and their performance was substantially below that of HTOL aircraft of similar size.
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Offline john smith 19

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Re: F9 Second Stage Reusability
« Reply #395 on: 04/17/2017 03:54 PM »
The 'secret sauce' might be some dual use capability. Since second stages must already be strong enough to support Falcon Heavy payloads,
That's sort of right, but reminds me of something else.

Shotwell said there are 2 F9 designs.  One is a regular F9 and can serve as the booster for an FH. The other is the design that serves as an FH core. Presumably it can also serve as a regular F9. But now you mention it both core stages would have to be stronger than the base F9 stages to carry the maximum 64 tonnes, unless F9 is built with a lot of structural margin, which I don't think is the case.
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Offline john smith 19

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Re: F9 Second Stage Reusability
« Reply #396 on: 04/17/2017 04:09 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.
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.
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.
Except it's the only official description of how SX thought they were going to solve the problem before Musk stated that upper stage reuse was completely off the table for F9 based hardware.
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Good to know.
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Offline rsdavis9

Re: F9 Second Stage Reusability
« Reply #397 on: 04/17/2017 04:41 PM »
The 'secret sauce' might be some dual use capability. Since second stages must already be strong enough to support Falcon Heavy payloads,
That's sort of right, but reminds me of something else.

Shotwell said there are 2 F9 designs.  One is a regular F9 and can serve as the booster for an FH. The other is the design that serves as an FH core. Presumably it can also serve as a regular F9. But now you mention it both core stages would have to be stronger than the base F9 stages to carry the maximum 64 tonnes, unless F9 is built with a lot of structural margin, which I don't think is the case.

What does the different S1 structural versions(F9 S1 vs FH S1) have to do with the S2?
If the S2 can take 64 tons on the top on the ground then it doesn't make any difference in flight.
A heavier payload does not increase the load during flight. The load is just the thrust in flight.
What am I missing?
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Offline envy887

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Re: F9 Second Stage Reusability
« Reply #398 on: 04/17/2017 04:53 PM »
The 'secret sauce' might be some dual use capability. Since second stages must already be strong enough to support Falcon Heavy payloads,
That's sort of right, but reminds me of something else.

Shotwell said there are 2 F9 designs.  One is a regular F9 and can serve as the booster for an FH. The other is the design that serves as an FH core. Presumably it can also serve as a regular F9. But now you mention it both core stages would have to be stronger than the base F9 stages to carry the maximum 64 tonnes, unless F9 is built with a lot of structural margin, which I don't think is the case.

What does the different S1 structural versions(F9 S1 vs FH S1) have to do with the S2?
If the S2 can take 64 tons on the top on the ground then it doesn't make any difference in flight.
A heavier payload does not increase the load during flight. The load is just the thrust in flight.
What am I missing?
FH has 3x as much thrust but not nearly 3x as much mass to accelerate, so it can hit much higher g loads, especially near burnout. Heavier payload and higher acceleration mean much higher loads on the second stage: F=m*a

More acceleration also means higher speeds in the dense part of the atmosphere, so aero loads are higher as well.

Offline garidan

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Re: F9 Second Stage Reusability
« Reply #399 on: 04/17/2017 05:05 PM »
I think SpaceX is not willing to spend a lot on developing S2 reusability, it makes no sense on the medium term and the value to recover is a lot less than S1.
You have to consider as well that this "attempts" will happen during paid missions (apart the demo flight of falcon heavy, not by chance the first to try a S2 recovery) and as it was for S1, they cannot increase risk to the main mission, that is deliver payloads.
So I guess they will capitalize on S1 experience and try to recover ithe same way as S1,  tail first, with pica reenforcement on the tail and, if and when they get one S2 intact in the sea, they will add legs, far lighter then the S1 ones being the weight so much less.
So the first attempt, at a minimum, requires (a lot more) thermal protection on the tail and a way to trim down the engine nozzle, too big and too "thin" for reentry, and certainly not a big loss in reuse terms.
If the engine with nozzle trimmed short is too much powerfull for an overslam landing (I'm not so sure), they can add 2 or 3 superdracos just for the last burn, I don't know how much weight it adds.

But for the first attempt I guess there will be only a trimmed nozzle, just to test if it together with additional thermal protection allows for an intact S2 to reach above the sea.

For the first attempt the nozzle could be shortened just from the start, if S2 performance is not part of the certification. If flyback and reentry proves itself good, than a way to trim the bell after payload delivery will be studied: I think a hot bell that thin is not hard to cut short, and it's easier to cut it than jettison that part without putting at risk the reliability of S2 engine.

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