Author Topic: Configurations for reusable PSTO shuttle  (Read 1863 times)

Online 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...
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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 »

Online 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 »

Online 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?
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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 à la Soyuz?

Online 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 à 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.
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Online 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.
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Online 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.

Online 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.

Online 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.

Online 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.

Online 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



Online sevenperforce

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Re: Configurations for reusable PSTO shuttle
« Reply #20 on: 03/17/2017 05:16 PM »
Speaking of parallel stages...does anyone remember this bad boy?  :D
A beauty to be sure. Of course, nowadays we don't really need to have the booster glide in for a landing when we can simply have it land on its tail. The only trouble, then, is getting the right thrust balancing with crossfeed and a safely recoverable crew-carrying shuttle.

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.
It's an interesting idea that could stand to be explored, for sure.

To start with there, are a few different ways that the engines could be plumbed, all with their own advantages and disadvantages. The original S-1D proposal lacks any tankage on the skirt, plumbing solely from the main tank, but this won't allow a propulsive landing. It would still be a possibility if we added something like superdracos for the landing, which could do double duty in a launch assist role, but that might be more trouble than it's worth.

Adding just enough tankage to the skirt for a propulsive landing (as you suggest) would require crossfeed, as the engines would be plumbed from the core tank during ascent and then plumbed from the auxiliary tankage during landing. The complexity of crossfeed can be avoided if the auxiliary tankage is made slightly larger and plumbed to two designated landing engines for both ascent and landing, with the remaining skirt engines plumbed solely from the core tank for ascent, without crossfeed.

The next option is by far the simplest, as far as plumbing is concerned: the tank in the skirt carries enough fuel for the skirt engines during both ascent and landing. This means there are no fuel lines crossing between stages, which is obviously advantageous for system simplicity and reuse.

The final option is to crossfeed the core engine from an even larger skirt tank, so that the core tank is still full at separation. This results in earlier staging, a smaller terminal stage, and the maximum performance optimization, but it does involve a burgeoning first stage and added plumbing complexity. Another advantage here is that the core stage will have a T/W ratio greater than 1 at launch, giving it 0/0 LES functionality.

Possible configurations:



There are other considerations as well, like control. The Saturn V had a fixed core engine and gimbals on the outer four engines, but this would need gimbal on the core engine. The skirt engines could be either partially or completely fixed, depending on which ones would be used for landing.

Another issue is altitude compensation on the core engine. One option is to use a SL Raptor and simply take the performance hit; after all, even the SL Raptor has a specific impulse better than the MVac. Another option would be to use a lengthened engine bell which can be used at sea level, like the SSMEs. The final option would be to add a lower-thrust vacuum-optimized OMS cluster to assist during orbital insertion and circularization.

Finally, this overall staging configuration lends itself really ridiculously well to air augmentation, since there is already a metal fairing wrapped around the core engine. A simple aluminum shroud boosts static thrust by 15% and goes up to a 50% boost around Mach 1. If we did go with the initial option (skirt engines plumbed directly from the core tank and auxiliary liquid tanks+thrusters for the landing) then these could be used as part of the launch assist to get up to a reasonable fraction of Mach 1.

Curious to see what overall configuration results in the lowest overall launch vehicle size but can still take a crew of 7 to the ISS and bring them back. I'm thinking it could be significantly smaller than the S-1D; methalox has a more oxygen-rich mixture ratio than kerolox, and the Raptor takes densified propellants, so the higher specific impulse is really going to shine.

Online envy887

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Re: Configurations for reusable PSTO shuttle
« Reply #21 on: 03/18/2017 01:33 AM »
Curious to see what overall configuration results in the lowest overall launch vehicle size but can still take a crew of 7 to the ISS and bring them back. I'm thinking it could be significantly smaller than the S-1D; methalox has a more oxygen-rich mixture ratio than kerolox, and the Raptor takes densified propellants, so the higher specific impulse is really going to shine.

Hmmm.... yeah. S-1D is monstrously over-sized for that. Raptor is too large for a upper stage sized solely for taking 7 to the ISS, or for landing anything remotely that size, it can't throttle low enough. Even Merlin only throttles down to 32 tonnes-force. So an auxiliary landing thruster setup is pretty much required. The upper stage could maybe use a production version of the 1,000 kN demo Raptor engine.

But because the upper stage always ends up heavier, the 1.5 STO does worse in my calculations than a TSTO. It always needs 20 to 40% greater GTOW mass to deliver the same payload and return. The larger tank drag area doesn't reduce the TPS requirements enough to offset the extra fuel and thrust mass needed to put that tank it in orbit.

Online sevenperforce

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Re: Configurations for reusable PSTO shuttle
« Reply #22 on: 03/20/2017 04:11 PM »
S-1D is monstrously over-sized for that. Raptor is too large for a upper stage sized solely for taking 7 to the ISS, or for landing anything remotely that size, it can't throttle low enough. Even Merlin only throttles down to 32 tonnes-force. So an auxiliary landing thruster setup is pretty much required. The upper stage could maybe use a production version of the 1,000 kN demo Raptor engine.
Raptor has half the thrust of the F-1 to begin with, so depending on whether it used a 4+1 configuration as with the S-1D or a 2+1 configuration like the Atlas D, we would be looking at launch mass between 30% and 50% of an S-1D. Raptor will be able to downthrottle to 20% of max thrust.

Quote
Because the upper stage always ends up heavier, the 1.5 STO does worse in my calculations than a TSTO. It always needs 20 to 40% greater GTOW mass to deliver the same payload and return. The larger tank drag area doesn't reduce the TPS requirements enough to offset the extra fuel and thrust mass needed to put that tank it in orbit.
Well, that depends greatly on whether crossfeed is employed. And staging would typically happen high enough that tank drag is no longer a problem.

This presents a fairly straightforward multivariable optimization problem, actually. Probably easier to approach iteratively, at least initially. I'll try taking the numbers from the ITS system as a base reference point.

Consider a PSTO launcher powered by three Raptor engines (two parallel and one sustainer), sized for carrying ten passengers to orbit. Dragon 2's dry mass is 6.4 tonnes; this includes the trunk and SuperDracos but we'll allow that for conservatism. Scaling gives us 9.14 tonnes, plus 200 kg per passenger (to allow for reasonable additional payload), so our minimum payload is 11.14 tonnes. I'll call it 12.

Three Raptors boast 9,150 kN of SL thrust, allowing us a launch mass of 666 tonnes with a brisk 1.4 T/W ratio. The ITS, which stages at 2.4 km/s, requires a hefty 470 tonnes of propellant for RTLS, corresponding to a reserve dV of 3.3 km/s. If we go with an S-1D or Atlas approach and crossfeed fuel from the core to the parallel engines, holding only enough fuel in the skirt for landing, then we'll need m0/m1 = 2.7 or so on the return alone. We don't really know the T/W ratio of the Raptor but if we guess at 200, that gives us an engine mass of 1.55 tonnes per Raptor. I can already tell that our vehicle T/W ratio at touchdown, even at minimum throttle, would be well over 100, which is utter nonsense.

So instead, let's suppose we fuel the parallel Raptors from larger tanks on the skirt alone, with no crossfeed either way. Let us allow 1.5 km/s of gravity and atmospheric drag; this means that 2.4 km/s at staging costs us 3.9 km/s. Our vehicle drinks up 464 tonnes of fuel to get there. Two thirds of this (309 tonnes) will be borne by the skirt.

Propellant fraction on the ITS booster is 0.96. This will be lower for us, due to the square-cube law, but it's fine for an approximation since that includes engine mass. Crunching the numbers, and getting tanks large enough to carry 309 tonnes of propellant plus landing reserves requires a dry mass of 14.9 tonnes plus 3.1 tonnes of engine. Given that the two Raptors cannot downthrottle below 124 tonnes combined thrust, that's a toasty 7.75 T/W. It would definitely need some dedicated landing thrusters.

What about the second/sustainer stage? At launch it massed 314 tonnes. With 12 tonnes of payload and 1.55 tonnes of engine, that's about 12 tonnes of tankage and 288 tonnes of fuel. At 2.4 km/s staging, this has dropped to 133 tonnes of fuel. The SL Raptor will give 361 s of specific impulse in space, so it could reach orbit with 34.5 tonnes of residuals, enough for about 1 km/s of landing reserves. With one SL raptor and a dry mass of 25.55 tonnes, that's a landing TWR of 2.4, so it would need dedicated landing thrusters as well.

Offline Rocket Science

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Re: Configurations for reusable PSTO shuttle
« Reply #23 on: 03/20/2017 04:55 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
I agree Ed about a X-37B like 2nd stage, when I suggested it as part of my "Flyback Falcon 9" several years back...
https://forum.nasaspaceflight.com/index.php?topic=27477.0
« Last Edit: 03/20/2017 04:56 PM by Rocket Science »
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Online sevenperforce

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Re: Configurations for reusable PSTO shuttle
« Reply #24 on: 03/20/2017 05:19 PM »
I agree Ed about a X-37B like 2nd stage, when I suggested it as part of my "Flyback Falcon 9" several years back...
https://forum.nasaspaceflight.com/index.php?topic=27477.0
I feel like the wings on the second stage would make it dangerously draggy on the front end during ascent, given that we'd invariably be using propulsive landing on the first stage.

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