Author Topic: Configurations for reusable PSTO shuttle  (Read 3667 times)

Offline sevenperforce

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Configurations for reusable PSTO shuttle
« on: 03/14/2017 02:49 PM »
Didn't really see any dedicated discussion thread on this, so I figured I'd start one.

People get excited about SSTOs (both of the VTVL and winged varieties), but for Earth, using multiple stages is almost always more cost-effective. However, we've never once flown a fully-reusable human-rated shuttle. There's really no obvious configuration for a fully-reusable shuttle that uses staging.

In particular, I like the idea of a parallel-stage-to-orbit design, where all the launch engines fire on the pad together, with the orbiter engines continuing to fire through stage separation (think Space Shuttle). This introduces additional complications, particularly with respect to configuration.

Some considerations:

> 0/0 LAS. Being able to escape in the event of a booster RUD is ridiculously important, from pad to separation. The orbiter either needs to have sufficient thrust to push itself free of an exploding booster, or it needs auxiliary solids for LAS, or it needs full cabin ejection with chute recovery.
> COM/COT. Centers of mass and thrust in a vertically-stacked launch vehicle are simple, but for a parallel-staged launch vehicle, it's a little more difficult. The SSMEs had ridiculously high gimbal ranges to help with this, but with a fully-reusable launch stack things may be more or less complicated.
> Engines and crossfeed. Using the same engines and fuel on the orbiter and the booster simplifies operations and can also allow for fuel crossfeed, but that does introduce some additional complications. If the launch engines are used for orbiter LAS, then they will be large enough to take full advantage of propellant crossfeed.
> Booster recovery. A liquid fly-back booster is one option; a RTLS tail-sitting booster with landing legs a la Falcon 9 is probably better. Splashdowns really aren't ideal for rapid reuse. Asymmetric thrust and COM may be a consideration depending on how the launch configuration looks.
> Orbiter recovery. The orbiter can have wings and landing gear, or it can come back and land vertically with landing legs. However, if it is going to land on its tail, then the crew cabin probably needs to be ejectable using landing-abort solids and chutes, since a tail-first vertical landing is a high-risk maneuver. The ideal solution is to have landing engines which (somehow) can vector or change orientation to allow a vertical landing in horizontal attitude, like sci-fi spaceships. This allows minimal-risk landing and immediate egress.
> Lifeboat. If the cabin is ejectable, then it makes sense to allow it to act as an orbital lifeboat in the case of orbiter damage a la Columbia. However, this means it either needs its own RCS and TPS. It may be possible to integrate its RCS and TPS for use within the orbiter.
> Airbreathing. Should you use a rocket-combined-cycle airbreathing engine on either stage? It's a good question. On the one hand, having airbreathing engines on the booster reduces the weight penalty (since they don't have to go to orbit) and can assist in recovery. Having an airbreathing engine on the orbiter increases weight penalty, but could be helpful for landing since airbreathing engines are more readily vectorable than rocket engines, which must be gimballed. It's also possible to conceive a partial airbreathing engine, like an air augmentation shroud on the booster which wraps around the orbiter's launch engine prior to staging.
> Altitude compensation. If the orbiter engine doesn't have altitude compensation, it will incur a specific impulse penalty. However, aerospike engines are heavy. An SSME pressure-compensated engine is one possibility. Another possibility is to have the orbiter engine interface with the booster body in such a way as to allow a higher expansion ratio after separation. Keep in mind, however, that you may be using the launch engine for recovery as well.
> Cargo. A cargo bay is not necessarily a requirement, but it definitely adds versatility. A cargo bay can also be used to add an auxiliary fuel tank for extended on-orbit operations or BLEO missions. Consider orbital refueling as well.

I have a few ideas for possible configurations, but I'm really interested to see the kinds of things the forum can come up with.

Offline Kaputnik

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Re: Configurations for reusable PSTO shuttle
« Reply #1 on: 03/14/2017 03:30 PM »
Back when X33/Venturestar was going to be the shape of the future, I wondered if a 'Biamese' configuration would have worked. This would have entailed two broadly similar vehicles mounted belly to belly, one acting as the first stage and the other as the upper stage, with propellant crossfeed and a different complement of engines depending on which role the vehicle was used for.
It would probably have been a nightmare to get the CG correct...
Waiting for joy and raptor

Offline edkyle99

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Re: Configurations for reusable PSTO shuttle
« Reply #2 on: 03/14/2017 04:08 PM »
I would start with X-34B X-37B as a hypothetical example.  It is a 5-ish tonne spacecraft that reenters and flies back.  What if a version also provided some appreciable ascent delta-v, allowing use of someone's already-developed recoverable first stage?  The result would be a reusable Agena type stage/spacecraft bus.

 - Ed Kyle
« Last Edit: 03/14/2017 05:19 PM by edkyle99 »

Offline sevenperforce

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Re: Configurations for reusable PSTO shuttle
« Reply #3 on: 03/14/2017 04:47 PM »
Back when X33/Venturestar was going to be the shape of the future, I wondered if a 'Biamese' configuration would have worked. This would have entailed two broadly similar vehicles mounted belly to belly, one acting as the first stage and the other as the upper stage, with propellant crossfeed and a different complement of engines depending on which role the vehicle was used for.
It would probably have been a nightmare to get the CG correct...
With a linear aerospike as was proposed for X33 and Venturestar, uniaxial thrust vectoring by differential throttling is built-in, which would help quite a bit with COM/COG/COT control. Of course, landing the booster on a linear aerospike is less than ideal.

The biamese approach works well, but LAS and orbiter recovery aren't necessarily straightforward.

I would start with X-34B as a hypothetical example.  It is a 5-ish tonne spacecraft that reenters and flies back.  What if a version also provided some appreciable ascent delta-v, allowing use of someone's already-developed recoverable first stage?  The result would be a reusable Agena type stage/spacecraft bus.
I like it. But where do you put it? Slung alongside to allow parallel thrust? And what kind of engine would it take?
« Last Edit: 03/14/2017 04:49 PM by sevenperforce »

Offline edkyle99

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Re: Configurations for reusable PSTO shuttle
« Reply #4 on: 03/14/2017 05:02 PM »
I would start with X-34B X-37B as a hypothetical example.  It is a 5-ish tonne spacecraft that reenters and flies back.  What if a version also provided some appreciable ascent delta-v, allowing use of someone's already-developed recoverable first stage?  The result would be a reusable Agena type stage/spacecraft bus.
I like it. But where do you put it? Slung alongside to allow parallel thrust? And what kind of engine would it take?
It would serve as a serial second stage.  As for engine, I would start by looking at storables, since spacecraft need propellant on-board for months or years at a time.

I'm suggesting an upgrade of the rocket in the first photo, which boosted an Agena that also served as the spacecraft bus in orbit.  These things flew every couple weeks or so.  First, replace the Thor first stage by a Falcon 9-like first stage that is recovered.  Second, replace Agena by something that looks like X-37B.

 - Ed Kyle
« Last Edit: 03/14/2017 05:20 PM by edkyle99 »

Offline sevenperforce

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Re: Configurations for reusable PSTO shuttle
« Reply #5 on: 03/14/2017 05:39 PM »
It would serve as a serial second stage.  As for engine, I would start by looking at storables, since spacecraft need propellant on-board for months or years at a time.

I'm suggesting an upgrade of the rocket in the first photo, which boosted an Agena that also served as the spacecraft bus in orbit.  These things flew every couple weeks or so.  First, replace the Thor first stage by a Falcon 9-like first stage that is recovered.  Second, replace Agena by something that looks like X-37B.
What configurations would be possible in a parallel-stage-to-orbit rather than a two-stage-to-orbit profile?

Mounting a lifting body on top of a booster has always seemed problematic to me. The X-37B launches inside a fairing, but that wouldn't be the case for a manned flight.

Offline Kaputnik

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Re: Configurations for reusable PSTO shuttle
« Reply #6 on: 03/14/2017 06:28 PM »
The advantages of parallel staging are the ability to check engine function prior to launch commit, and the use of all engines on the vehicle contributing to thrust at launch.
The first issue is resolved with improved reliability, and in the context of a recoverable upper stage may not be such a big deal anyway, if intact aborts are possible. On the second point, the requirements of a first and second stage engine are quite different, and creating an engine that does both jobs well is not easy (see SSME, or the excess mass of aerospikes). Further, the thrust requirements of a second stage are so low that the benefit to launch thrust is not generally significant.

Or is there some benefit to parallel staging that I am missing?
Waiting for joy and raptor

Offline DreamlinerFinder

Re: Configurations for reusable PSTO shuttle
« Reply #7 on: 03/14/2017 06:55 PM »
It would serve as a serial second stage.  As for engine, I would start by looking at storables, since spacecraft need propellant on-board for months or years at a time.

I'm suggesting an upgrade of the rocket in the first photo, which boosted an Agena that also served as the spacecraft bus in orbit.  These things flew every couple weeks or so.  First, replace the Thor first stage by a Falcon 9-like first stage that is recovered.  Second, replace Agena by something that looks like X-37B.
What configurations would be possible in a parallel-stage-to-orbit rather than a two-stage-to-orbit profile?

Mounting a lifting body on top of a booster has always seemed problematic to me. The X-37B launches inside a fairing, but that wouldn't be the case for a manned flight.
In theory, it could launch in a fairing la Soyuz?

Offline sevenperforce

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Re: Configurations for reusable PSTO shuttle
« Reply #8 on: 03/14/2017 07:23 PM »
The advantages of parallel staging are the ability to check engine function prior to launch commit, and the use of all engines on the vehicle contributing to thrust at launch.
The first issue is resolved with improved reliability, and in the context of a recoverable upper stage may not be such a big deal anyway, if intact aborts are possible. On the second point, the requirements of a first and second stage engine are quite different, and creating an engine that does both jobs well is not easy (see SSME, or the excess mass of aerospikes). Further, the thrust requirements of a second stage are so low that the benefit to launch thrust is not generally significant.

Or is there some benefit to parallel staging that I am missing?
The primary benefit is, as you note, the additional thrust contribution of using all engines at launch. I don't know off the top of my head that there's a particularly straightforward weight penalty factor for using an aerospike in place of a bell nozzle, but obviously adding thrust reduces gravity drag, which increases dv, which decreases fuel requirements, which further decreases gravity drag, ad infinitum.

The SSME was ridiculously complicated, of course, but didn't a lot of that have to do with the high gimbal range, the high chamber pressure, and the passive TPS? I thought altitude compensation was just the result of altering the nozzle wall angle right before termination.

The idea is to move generally away from the "first stage/second stage" designation and look at the orbiter itself as a launch vehicle which uses the crossfeeding booster for launch assist and external tankage. For example, we can look at the advantages and disadvantages of an orbiter that is larger than the booster, or an orbiter which integrates with an air augmentation shroud on the booster.

An SSTO has to be prohibitively large to have a meaningful payload; for any given size of SSTO, a TSTO can deliver significantly more payload. A PSTO solution, then, can (in theory) be much smaller and still outperform a TSTO. SSTO promises rapid turnaround but the prohibitive size of a usable SSTO makes it a refurbishment nightmare, so a PSTO's much smaller size can potentially deliver better performance and operations.

Mounting a lifting body on top of a booster has always seemed problematic to me. The X-37B launches inside a fairing, but that wouldn't be the case for a manned flight.
In theory, it could launch in a fairing la Soyuz?
Not if you want a fully-reusable system. The Falcon 9 system isn't even fully reusable and their fairings are already a bottleneck.

Offline Kaputnik

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Re: Configurations for reusable PSTO shuttle
« Reply #9 on: 03/14/2017 08:08 PM »
Back of envelope hypothetical PSTO:
GLOW 100t, staging at 4.5km/s, isp of 310s on first stage. Second stage will then be around 25t.
Thrust at launch will need to be around 120t. After staging, T:W could be anything from 0.3 to 1, based on existing vehicles. So the upper stage thrust will be in the region of 10-25t.

Parallel staging means you can shift somewhere between 7-20% of your engine thrust to the second stage, but at the cost of potential compromise to that stage's dry mass and isp. The more conventional alternative would be to uprate the first stage, or add some solids.
Waiting for joy and raptor

Offline sevenperforce

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Re: Configurations for reusable PSTO shuttle
« Reply #10 on: 03/14/2017 08:23 PM »
Back of envelope hypothetical PSTO:
GLOW 100t, staging at 4.5km/s, isp of 310s on first stage. Second stage will then be around 25t.
Thrust at launch will need to be around 120t. After staging, T:W could be anything from 0.3 to 1, based on existing vehicles. So the upper stage thrust will be in the region of 10-25t.

Parallel staging means you can shift somewhere between 7-20% of your engine thrust to the second stage, but at the cost of potential compromise to that stage's dry mass and isp. The more conventional alternative would be to uprate the first stage, or add some solids.
What physical configuration are you modeling? Biamese? If the 0/0 LAS is based around the entire orbiter, then the T:W of the fully-loaded orbiter needs to be greater than 1.

Posed as a more directly answerable question, we could say this: for a fully-reusable crew shuttle, is there a PSTO with safety comparable to a TSTO but smaller overall launch vehicle size?

One option would be to take a scaled-down Shuttle with an internal tank and hydrocarbon engines and strap a crossfeeding liquid booster over each wing. Launch vertically (with no COM problems), crossfeed fuel to the orbiter. The boosters separate and RTLS to land on landing legs; the orbiter continues to space and returns for a gliding landing.

Offline hkultala

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Re: Configurations for reusable PSTO shuttle
« Reply #11 on: 03/14/2017 08:42 PM »
On TSTO with balanced staging point the second stage engine is so weak(about 10 times weaker than 1st stage), that the thrust advantage of second stage is too small, and more may be lost by the comphromize nozzle and bigger second stage tanks needed for parallel staging and higher drag than what the higher liftoff thrust gives.

Parallel staging only makes much sense when the staging point is not balanced but the staging is happening very early - for example, because first stage is using very low isp SRBs, like STS.


However, Soyuz is still using parallel staging with liquid engines - but is that mostly to allow starting the second stage engines on ground? Original R-7 had no third stage so all the engines were started on ground, no equipment to start ay engines on air were needed.

Offline Kansan52

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Re: Configurations for reusable PSTO shuttle
« Reply #12 on: 03/14/2017 08:51 PM »
Kinda sounds like the original all reusable system envisioned by NASA until trimmed back to the STS.

Offline Kaputnik

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Re: Configurations for reusable PSTO shuttle
« Reply #13 on: 03/15/2017 07:23 AM »

Parallel staging only makes much sense when the staging point is not balanced but the staging is happening very early - for example, because first stage is using very low isp SRBs, like STS.

Agreed, and this means that the second stage in a PSTO will be extremely large (compared to a manned capsule)- which makes the OPs requirement of an integrated LAS extremely impractical.
Waiting for joy and raptor

Offline sevenperforce

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Re: Configurations for reusable PSTO shuttle
« Reply #14 on: 03/15/2017 01:18 PM »
Parallel staging only makes much sense when the staging point is not balanced but the staging is happening very early - for example, because first stage is using very low isp SRBs, like STS.
Agreed, and this means that the second stage in a PSTO will be extremely large (compared to a manned capsule)- which makes the OPs requirement of an integrated LAS extremely impractical.
It's the same problem that any SSTO design has with LA/ES. Getting a crew clear of an exploding vehicle is not a simple task.

The Shuttle was originally fitted with ejection seats, but these would have been of limited use in actual flight. They are useless in re-entry accidents like Columbia, and it's unlikely that they would have saved the crew of Challenger given the cabin's proximity to the disintegrating SRBs. A far better option is to use a crew cabin ejection system as in the F-111 Aardvark and the B-1 Lancer; in this system, charges blow the orbiter apart while the cabin is boosted away on its own, effectively becoming a separate crew capsule. This would have saved both Challenger and Columbia, but wasn't possible without extensive redesign of the Shuttle Orbiter structure.

SSTO designs don't have enough dry mass budget to incorporate escape cabin ejection, but a PSTO would.

One idea I quite like is to borrow from the SpaceX playbook and use liquid pusher engines which can also support propulsive landing. If the cabin is placed at the COM of an empty orbiter, the LES engines can be used to land propulsively in a horizontal attitude.

As far as configurations are concerned, one promising idea is the "launch ring" -- a toroidal-cylindrical body that shrouds the tail of the orbiter, carrying fuel tanks and auxiliary rocket engines. The body serves as an air augmentation shroud for both the orbiter engines and the auxiliary engines, boosting thrust and specific impulse during the initial ascent phase. The ring would stage early, as soon as the added thrust from air augmentation no longer counteracted the added drag on the ring, and use its engines to make a controlled, if ungainly, RTLS and propulsive landing.

Offline sevenperforce

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Re: Configurations for reusable PSTO shuttle
« Reply #15 on: 03/16/2017 06:34 PM »
What options would be available if someone wanted to reboot the Saturn S-ID concept?



For example, adding fuel tanks to the skirt with modern, deep-throttling restartable engines (I'm looking at you, Raptor) and crossfeeding to the core. The skirt could fly itself back and land propulsively; the core would make orbit easily. It's not actually a terrible thing to make the orbiter fairly large, since a physically larger orbiter (if strengthened) can re-enter in a normal attitude and will have low peak heating due to a very draggy cross-section.

Challenge would be getting it down to land in a reliable manner suitable for a passenger shuttle.

Offline envy887

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Re: Configurations for reusable PSTO shuttle
« Reply #16 on: 03/16/2017 07:13 PM »
What options would be available if someone wanted to reboot the Saturn S-ID concept?

For example, adding fuel tanks to the skirt with modern, deep-throttling restartable engines (I'm looking at you, Raptor) and crossfeeding to the core. The skirt could fly itself back and land propulsively; the core would make orbit easily. It's not actually a terrible thing to make the orbiter fairly large, since a physically larger orbiter (if strengthened) can re-enter in a normal attitude and will have low peak heating due to a very draggy cross-section.

Challenge would be getting it down to land in a reliable manner suitable for a passenger shuttle.

You pretty much just described ITS with a shorter booster.

Offline sevenperforce

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Re: Configurations for reusable PSTO shuttle
« Reply #17 on: 03/16/2017 07:44 PM »
What options would be available if someone wanted to reboot the Saturn S-ID concept?

For example, adding fuel tanks to the skirt with modern, deep-throttling restartable engines (I'm looking at you, Raptor) and crossfeeding to the core. The skirt could fly itself back and land propulsively; the core would make orbit easily. It's not actually a terrible thing to make the orbiter fairly large, since a physically larger orbiter (if strengthened) can re-enter in a normal attitude and will have low peak heating due to a very draggy cross-section.

Challenge would be getting it down to land in a reliable manner suitable for a passenger shuttle.
You pretty much just described ITS with a shorter booster.
If the ITS booster only had four engines, was shaped like a toroidal cylinder, tapered at the top, and crossfed to a single-engine upper stage.

In other words, nothing like the ITS whatsoever.

Offline envy887

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Re: Configurations for reusable PSTO shuttle
« Reply #18 on: 03/16/2017 08:52 PM »
Forget the booster. The ITS ship is similar in size, shape, mass, and thrust to the S-1C. Replace the fixed ring of 6 RVacs with a detachable ring of 10 or 12 SL Raptors and you have a modern S-1D. Plus it already has reentry and landing designed in.

The toroidal tank is a good idea, but it only needs to be big enough to fly the ring back... Maybe 50t of methalox.

Offline Dante80

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Re: Configurations for reusable PSTO shuttle
« Reply #19 on: 03/17/2017 01:14 AM »
Speaking of parallel stages...does anyone remember this bad boy?  :D



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