Author Topic: Feasibility of reusable TSTO  (Read 21067 times)

Offline Hop_David

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Feasibility of reusable TSTO
« on: 12/13/2012 05:49 pm »
A new thread as Kelvin Zero suggested.

I want you to acknowledge recovering a 2nd stage is much more difficult than recovering a Dragon capsule. That recovering a Capsule with a 25% propellant mass fraction does nothing to demonstrate the same can be done with a stage having a 90% propellant mass fraction.
Nope! Because the vast majority of the volume of a Dragon capsule is empty, and you're talking about landing and empty stage, not a full one.

If all we did was fill up a Dragon capsule with propellant (for the way up... and considering how lightweight we can make propellant tanks, this shouldn't require reinforcements that have significant mass requirements), it would increase its propellant mass fraction to around 75%.

(Googling Falcon 9 mass fraction...)

Lots of question marks. Seems most people venturing numbers have got them by using the rocket equation.

This page suggests a 94% mass fraction for the Falcon 2nd stage.

Assuming your 75% is correct, that leaves a 25% mass fraction that can be devoted to structure, TPS, etc., almost four times as great as the 6% dry mass of the Falcon 9 second stage. So it seems to me the capsule can be made 4 times as robust as the 2nd stage.

Further it is my guess that the 2nd stage's Merlin rocket engine and 4 Draco engines consume a greater fraction of dry mass than the Dragon's engines. But I still haven't found the mass of the Draco engines.

So far as I know the 94% percent figure was from calculated from the existing Falcon 9 delta V budget. When you factor in re-entry and vertical landing, the delta V budget will be greater.

Because the vast majority of the volume of a Dragon capsule is empty, and you're talking about landing and empty stage, not a full one.

You can't be suggesting that the empty stage be a compact truncated cone.  The stage is a cylinder, and it is much more "empty" than the capsule.  Not only is the mass fraction different, the aerodynamics are different.

Clearly, we recover capsules "all the time"; not so with empty stages.

Good summary.
« Last Edit: 12/13/2012 06:06 pm by Hop_David »

Offline Robotbeat

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Re: Feasibility of reusable TSTO
« Reply #1 on: 12/13/2012 07:16 pm »
Don't forget a pressurized vessel is much stronger than an essentially unpressurized one, like a capsule at splashdown.  Thus the capsules need more structure than a pressurized stage would under same situation.
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Offline Hop_David

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Re: Feasibility of reusable TSTO
« Reply #2 on: 12/15/2012 11:03 pm »
Don't forget a pressurized vessel is much stronger than an essentially unpressurized one, like a capsule at splashdown.  Thus the capsules need more structure than a pressurized stage would under same situation.

You see the 2nd stage re-entering as a pressurized volume?

How much propellant do you imagine will still be in the tanks just before re-entry?

Offline cleonard

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Re: Feasibility of reusable TSTO
« Reply #3 on: 12/16/2012 01:14 am »
Don't forget a pressurized vessel is much stronger than an essentially unpressurized one, like a capsule at splashdown.  Thus the capsules need more structure than a pressurized stage would under same situation.

You see the 2nd stage re-entering as a pressurized volume?

How much propellant do you imagine will still be in the tanks just before re-entry?

Yes, there must be some pressure.  A rocket tank is a large version of an aluminum soda can.  When full and sealed they are strong and can't easily be crushed.  When they are empty it's easy to crush them.

Offline Robotbeat

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Re: Feasibility of reusable TSTO
« Reply #4 on: 12/17/2012 02:24 am »
Don't forget a pressurized vessel is much stronger than an essentially unpressurized one, like a capsule at splashdown.  Thus the capsules need more structure than a pressurized stage would under same situation.

You see the 2nd stage re-entering as a pressurized volume?

How much propellant do you imagine will still be in the tanks just before re-entry?
Propellant? Depends on your landing mode. But I expect the pressure to be pretty much the same as flight pressure. It's full of ullage gas, Ie helium.
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Offline RanulfC

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Re: Feasibility of reusable TSTO
« Reply #5 on: 12/19/2012 05:38 pm »
"Pressurization" is a bit of a dicey issue actually. A capsule is "un-pressurized" because it's capable of withstanding the stress' and strains of entry and landing WITHOUT being "pressurized" where as the second stage volume of the Falcon (for example) can't withstand the stress' and strains of launch, let alone enrty and landing WITHOUT being pressurized due to the lighter, less robust structure.

It's a subtle but important difference in design and construction.

Structure is only part of the deal though. My understanding for the Falcon was that the major issue was/is instability during entry. The TPS is on the end of the tanks, yet when it hit atmosphere the stages want to automatically come around to an engine first atitude because of the mass distribution. All the structure AND pressurization won't help if you're tumbling till you hit the materials limit of the vehicle.

To make a viable (as well as "feasible" :) TSTO both stages have to be designed to meet a series of criteria for recovery. Which is going to require working the vehicle to reenter in a stable manner from the start, which currently isn't the case.

One point I tried to make regarding an "Advanced" Falcon-RLV was that sooner or later Space-X is going to have to consider designing and building in such a "capability" in the near future if they are serious about stage recovery.

Randy
From The Amazing Catstronaut on the Black Arrow LV:
British physics, old chap. It's undignified to belch flames and effluvia all over the pad, what. A true gentlemen's orbital conveyance lifts itself into the air unostentatiously, with the minimum of spectacle and a modicum of grace. Not like our American cousins' launch vehicles, eh?

Offline Robotbeat

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Re: Feasibility of reusable TSTO
« Reply #6 on: 12/19/2012 07:35 pm »
Because I'm sure SpaceX hasn't thought about reentry stability for their next-gen Falcon 9 reusability attempt. ;)
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Offline KelvinZero

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Re: Feasibility of reusable TSTO
« Reply #7 on: 12/19/2012 08:28 pm »

Structure is only part of the deal though. My understanding for the Falcon was that the major issue was/is instability during entry. The TPS is on the end of the tanks, yet when it hit atmosphere the stages want to automatically come around to an engine first atitude because of the mass distribution. All the structure AND pressurization won't help if you're tumbling till you hit the materials limit of the vehicle.

Won't this only be a problem if the vehicle starts tumbling? Until then it sounds like the old balancing an upright stick problem.

What about the actual moment of turn over? What speed is it going then?

Offline RanulfC

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Re: Feasibility of reusable TSTO
« Reply #8 on: 12/19/2012 09:23 pm »
Because I'm sure SpaceX hasn't thought about reentry stability for their next-gen Falcon 9 reusability attempt. ;)
They may have thought of it, but they aren't saying anything about addressing it :)

Then again they didn't ever actually "admit" what they planned for trying to recover the stages in the first place now did they? :)

Structure is only part of the deal though. My understanding for the Falcon was that the major issue was/is instability during entry. The TPS is on the end of the tanks, yet when it hit atmosphere the stages want to automatically come around to an engine first atitude because of the mass distribution. All the structure AND pressurization won't help if you're tumbling till you hit the materials limit of the vehicle.
Won't this only be a problem if the vehicle starts tumbling? Until then it sounds like the old balancing an upright stick problem.

As far as I'm aware the "vehicle" starts pretty much tumbling the moment it hits the atmosphere and it IS rather much the old "balancing-an-upright-stick" issue but it is also a design issue because currently you have a vehicle that WANTS to tumble and is given every chance to do so :)

Quote
What about the actual moment of turn over? What speed is it going then?
From the moment it hits the atmosphere from what I gather, and speed is either around Mach-6-10 for the fist stage and Mach-24 for the second. So that's what you have to work with at the moment. The main issue at this point is keeping the TPS aligned with the entry vector when the whole vehicle wants to turn around and point the engines, (and majority of vehicle mass) in the direction of travel. It's not something that you can handle passively with the current design and so far the "suggested" advanced models are not seeming to address the issue.

IF you're going to make an RLV it is going to come down to both structural issues and design issues that both have to be resolved to end up at a "viable" RLV design.

Randy
From The Amazing Catstronaut on the Black Arrow LV:
British physics, old chap. It's undignified to belch flames and effluvia all over the pad, what. A true gentlemen's orbital conveyance lifts itself into the air unostentatiously, with the minimum of spectacle and a modicum of grace. Not like our American cousins' launch vehicles, eh?

Offline ciscosdad

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Re: Feasibility of reusable TSTO
« Reply #9 on: 12/20/2012 02:17 am »
Perhaps they will need to rethink the whole structure and go with a wide base conical design (ref the Boeing designs of the 70's). Gives a nice wide base and a low density to allow for much lower reentry heating.
Not a perfect solution but may be the difference between success and failure given the low margins they have to deal with. Of course, the cost of a completely new vehicle will be horrendous. I suspect SpaceX will seriously consider this type of radical redesign  if their efforts with the Falcon family are not successful.

Offline KelvinZero

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Re: Feasibility of reusable TSTO
« Reply #10 on: 12/20/2012 02:43 am »

Quote
What about the actual moment of turn over? What speed is it going then?
From the moment it hits the atmosphere from what I gather, and speed is either around Mach-6-10 for the fist stage and Mach-24 for the second. So that's what you have to work with at the moment.

I meant the planned moment of turn over.

When you say "what you have to work with at the moment".. do you mean they have been trying to keep the second stage nose first on entry? That it tumbles isn't very surprising otherwise. Sorry I hadn't been following things closely and I might have missed something interesting. I figured it was just a cute animation at the moment.

Offline Robotbeat

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Re: Feasibility of reusable TSTO
« Reply #11 on: 12/20/2012 03:02 am »
There are ways to make it stable nose-first, such as putting the landing rockets and landing gear there.
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Offline KelvinZero

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Re: Feasibility of reusable TSTO
« Reply #12 on: 12/20/2012 07:12 am »
So both sides accept that there is a current problem with stability of the second stage? What is the evidence?
« Last Edit: 12/20/2012 07:28 am by KelvinZero »

Offline Robotbeat

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Re: Feasibility of reusable TSTO
« Reply #13 on: 12/20/2012 07:15 am »
So both sides accept that there is a current problem with stability of the second stage? What is the evidence?
No current problem! They've never attempted recovery of the second stage!!!
Chris  Whoever loves correction loves knowledge, but he who hates reproof is stupid.

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Offline KelvinZero

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Re: Feasibility of reusable TSTO
« Reply #14 on: 12/20/2012 07:28 am »
Ok I will leave it to RanulfC to explain.

(also I didn't understand your answer RB. Are there a different set of rockets for landing??)

Offline Warren Platts

Re: Feasibility of reusable TSTO
« Reply #15 on: 12/20/2012 05:35 pm »
If you put fins on the second stage, that might keep it pointed nose first.

Still there would be sidewall heating issues, even in that case....
"When once you have tasted flight, you will forever walk the earth with your eyes turned skyward, for there you have been, and there you will always long to return."--Leonardo Da Vinci

Offline KelvinZero

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Re: Feasibility of reusable TSTO
« Reply #16 on: 12/20/2012 07:55 pm »
If you put fins on the second stage, that might keep it pointed nose first.

Still there would be sidewall heating issues, even in that case....

Those inflatable heat shields also look promising, and way more extreme than we would probably need. Perhaps you could have a moderate inflating section to change the form a little. I still don't really understand the argument that there is a problem to deal with though. The tumbling doesn't mean anything unless they have been trying not to tumble and failing. A longer body could make balance easier. I thought the reasons for the typical capsule shape were because sharp cones get hotter rather than because entering flat end first was particularly stable. The second stage isn't really 'sharper' if it has less density and the same mass per shield area. Im also not sure how unbalanced it would really be. It still has 15% (or something like that) of fuel remaining. Isnt that still quite significant compared to the weight of engines etc?

(consider this a list of all the things I know I don't know rather than an opinion on reentry vehicle design ;) )

Offline Robotbeat

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Re: Feasibility of reusable TSTO
« Reply #17 on: 12/20/2012 07:59 pm »
An inflatable heat shield was once used to try to return an upper stage from an R-7 (i.e. Soyuz) family launch vehicle. It was an early flight of a certain variant, and I think the metal scrappers got to the returned stage before the recovery team did.
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Offline RanulfC

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Re: Feasibility of reusable TSTO
« Reply #18 on: 12/26/2012 07:10 pm »
Ok I will leave it to RanulfC to explain.
What? And I suppose you'll want me to be coherent while I'm at it too? Yeesh :)
Quote
(also I didn't understand your answer RB. Are there a different set of rockets for landing??)
That's the impression I got also, explain RB?

If you put fins on the second stage, that might keep it pointed nose first.

Still there would be sidewall heating issues, even in that case....

Those inflatable heat shields also look promising, and way more extreme than we would probably need. Perhaps you could have a moderate inflating section to change the form a little. I still don't really understand the argument that there is a problem to deal with though. The tumbling doesn't mean anything unless they have been trying not to tumble and failing. A longer body could make balance easier. I thought the reasons for the typical capsule shape were because sharp cones get hotter rather than because entering flat end first was particularly stable. The second stage isn't really 'sharper' if it has less density and the same mass per shield area. Im also not sure how unbalanced it would really be. It still has 15% (or something like that) of fuel remaining. Isnt that still quite significant compared to the weight of engines etc?

(consider this a list of all the things I know I don't know rather than an opinion on reentry vehicle design ;) )

Well as always the FIRST step is admitting you have a problem ;)

I'm not actually sure the is a "problem" either per-se, However... (there's always a "however" :) )

First let me address the "question" on "blunt" vesus "pointed" in general:
At the speeds being discussed (above Mach-6 but below full orbital speed or about Mach-25) the larger the "radius" of a curve presented the further away from the surface (and "deeper") the shock-wave built up of heated "drag" air will be. Further means less direct heating of the surface usually but a lot will also depend on overall vehicle mass to surface area along with the radius of the surface curve...

Stability is dependent (greatly) on aerodynamics as well as mass distibution within the vehicle which means you can have a "long-and-pointy" shape that couldn't fly straight for any length of time without active control and/or power or a sphere that will ALWAYS "fly" with one particular point facing "forward" under any conditions. In most cases the "standard" capsule designs are pretty much "weighted" in favor of flying in a stable position with the "blunt" end "forward" when moving on a ballistic or un-powered trajectory. This is "usually" off-set from "center" by some amount to provide SOME "lift" during entry both for landing-zone control and to alleviate some of the "g" forces during entry.

As Warren points out you CAN put aerodynamic surfaces on the vehicle (these could be "fins" but can also be some type of "drag" surface such as panels, frames, or inflated "drag" segments) to "force" the vehicle to fly the way you want it to. The MAIN problem here though is that only works once the vehicle gets to a point where your "aerodynamic" forces are high enough to be effective. This usually is NOT the case at the start of a reentry :)

When you have enough aerodynamic "force" to begin effecting the vehicle but NOT enough to be used to effectivly CONTROL the vehicle you either have to have a system capable of providing "powered" over-ride forces, (such as an RCS system) or your vehicle is going to "seek" its natural "balance" from its design. In the case we're talking about that "natural" position for a cylinder-body is going to be with the most mass "forward" which means the body will WANT to go "engines" first rather than "TPS" first.

SpaceX has always been pretty straight-forward with talking about stage recovery as a "planned" exercise in the development process "at-some-point" they never actually SPECIFIED as to when this would happen :)

Part of the "puzzle" over the years is that the general "design" of a standard rocket is a cylinder-body with most of its mass concentrated at one end (engines again) and not a lot of "extra" mass available to spend on recovery devices and means. This pretty much means that without SOME sort of "active" control system the stage will almost always go from a "top-is-forward" position (launch) to an "engines-are-forward" position when it encounters air after the "powered" portion of flight.
Tumbling becomes an "issue" when your vehicle wants to change orientation during the early stages of entry interface as the vehicle "comes-around" to it's more "natural" postition, yet there isn't enough actual "air-flow" to slow and stablize the vehicle as of yet. Due to the constantly changing dynamics of the interface if you're tumbling the vehicle can (and usually does) reach the materials limit very fast and you end up with the vehicle breaking up.

Since SpaceX hadn't "specified" anything speculation was that they "planned" on letting the stage "orient" engines-first for reentry and would then let them take the "brunt" of the entry heating. (Not really good for the engines overall and there was a lot of speculation about active-LOX circulation for cooling or such but since there weren't any details thats about all there was to do :) )
This would allow them to mount the parachutes at the "top" of the stage and then protect them during entry but as noted would be pretty hard on the engines and the design isn't exactly "stable" overall in that position either.

A "squat" capsule shaped vehicle though is going to have a lower and less dynamic "center" of gravity during entry as well as pressure during take off which simplifies the entire problem... However, (see note on that pesky word above :) )

"Aerodynamically" squat equates to higher drag on launch and non-optimum design parameters all around, usually leading to higher overall mass than you really "need" for a Launch Vehicle. It's all part of the "trade-offs" that are done during design.

Now that SpaceX is talking rocket-back, and active "braking" using the engines the overall "look" is that they are planning on carrying enough propellant to not only "reverse" course for the first stage but actually slow it down again enough to actually "reenter" engines first which is as noted probably the most "stable" position for the vehicle. That's not to say that "method" will work so well for the second stage though.

(Might check my figures but if correct "15%" of a full propellant load for the second stage comes out to about 4,140lt of LOX which is @5837kg and 2,610lt of RP1 or @ 2114kg for a total propellant "load" at entry of around 7951kg versus an engine "dry" weight of the Merlin-Vacuum at about 630kg plus nozzle extension. The propellant while massing more will also be more "fluid" {please forgive the pun} which won't necessarly make stabilizing the stage all that easy)

Several factors differ for the second stage which of course the biggest is probably the entry speed will be much higher even IF all the remaining propellant is used to try and slow the stage down. The "fact" that at this point SpaceX is supposed to be looking at terminal retro-thrust and landing means the entry speed will probably remain higher rather than lower. Now you "definatly" have to address the stability issue both during entery and free-flight.

Anyway you look at it, with the current "design" the stage is going to WANT to fly "engines-first" for entery and landing and the only way to get them to NOT do so involves extra mass in propellant, RCS, and probably other systems as well. To get the heating down to a point where the engine bell(s) are going to be able to take the punishment you either are going to end up trying to propulsivly slow down the vehicle, provide a "propulsive" aerospike by running the engine at a lower "power" during the entry, of find a way to make the entire vehicle more "fluffy" to spread out the heating pulse.
(And keep in mind the Merlin can only be "throttled-down" to 70% of full thrust as currently designed)

Randy
From The Amazing Catstronaut on the Black Arrow LV:
British physics, old chap. It's undignified to belch flames and effluvia all over the pad, what. A true gentlemen's orbital conveyance lifts itself into the air unostentatiously, with the minimum of spectacle and a modicum of grace. Not like our American cousins' launch vehicles, eh?

Offline 2552

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Re: Feasibility of reusable TSTO
« Reply #19 on: 12/26/2012 07:53 pm »
Several factors differ for the second stage which of course the biggest is probably the entry speed will be much higher even IF all the remaining propellant is used to try and slow the stage down. The "fact" that at this point SpaceX is supposed to be looking at terminal retro-thrust and landing means the entry speed will probably remain higher rather than lower. Now you "definatly" have to address the stability issue both during entery and free-flight.

Anyway you look at it, with the current "design" the stage is going to WANT to fly "engines-first" for entery and landing and the only way to get them to NOT do so involves extra mass in propellant, RCS, and probably other systems as well. To get the heating down to a point where the engine bell(s) are going to be able to take the punishment you either are going to end up trying to propulsivly slow down the vehicle, provide a "propulsive" aerospike by running the engine at a lower "power" during the entry, of find a way to make the entire vehicle more "fluffy" to spread out the heating pulse.
(And keep in mind the Merlin can only be "throttled-down" to 70% of full thrust as currently designed)

Randy

I think I saw Robotbeat mention somewhere the idea of putting the SuperDracos and their landing fuel behind the heatshield to move the CG forward enough to have a stable nose-first entry. It would probably also avoid having to do a flip-maneuver in the lower atmosphere before landing.

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