Alternative Shuttle Development Universe

[Byeman]

For Archibald.

What if NASA concentrated on an unmanned fly back booster and not the shuttle orbiter a'la DARPA/Boeing XS-1 (https://www.darpa.mil/program/experimental-space-plane
Just the Booster of the original two vehicle concept.

NASA could have achieved 100% reusability on the booster. Would have to put off sortie missions like Spacelab (or do MOL like missions) or payload retrieval (which was only used 4 times). Various upper stages and configurations could have been used, An Apollo type spacecraft could have been employed and eventually transitioned to lifting body. For satellite deployment, there would be no need for crew. Whatever flew on the back could have been adaptable to different missions. Satellite servicing still would have been possible.

TPS would not have been as important. No SRBs. Mass increases would have been less impactful.


This would have been an interesting history arc.

 

[publiusr]

Perhaps simplified F-1s?

With winged boosters fine throttling is less needed than in vertical landing...wider variety of upper stages?

 

[Archibald]

(presently scratching my head, mostly in disbelief, and wondering how I should consider that...)

...

(some bouts of nervous laughter later)

...

 

[Archibald]

...Let's try this:
- unmanned flyback booster, but derived from the S-IC
- second stage: expendable S-IVB
- kinda Saturn INT-20, except half reusable.
- on top of the S-IVB: as you said: expendable stages or "manned spaceships" (let's call them that way)
- in-line or side-mount ? in-line stays closer from vanilla Saturn V so...
- MOL and Spacelab are better done by "true" space station - or nothing at all.
- Apollo is off-the-shelf and works fine, no need to re-invent it as Orion or... Starliner (the former overweight, the latter flawed)
- Lifting body: why not ?
- Could cool down the vociferous side of NASA wanting sat retrieval and refurbishment (throw them a bone)
-The above also apply to MOL and Spacelab supporters, if any left. Give them a small reusable hab inside the lifting body.
- Also throws a bone to "winged spaceship / spaceplane" supporters.
- Would make the system 2/3rd reusable : kinda reusable-expendable-reusable stages so another bone thrown at "RLV crowd"

 

[Byeman]

Perhaps simplified F-1s?

With winged boosters fine throttling is less needed than in vertical landing...wider variety of upper stages?

No, still needed a better engine. Better ISP and throttling. It is still a winged asymmetrical vehicle going through max q.

Also, this is not about now but back in the 70's.

 

[Byeman]

...Let's try this:
- unmanned flyback booster, but derived from the S-IC
- second stage: expendable S-IVB

Both are too big. We aren't sending a 250klb (payload aka orbiter) into orbit. Just the 65klb 60' long x 15' payload, that could have an upper stage(s). Upper stage would be like a Centaur V size with engine thrust like an MVac or J-2.

- MOL and Spacelab are better done by "true" space station - or nothing at all.

MOL like Spacelab, not the MOL mission. No need for that.

Spacelab pallet missions like Atlas, LITE, ASTRO, etc would be better off as free flyers

SPARTAN free flyers would be more robust, have longer duration missions and use comm to send down data.

Use SRVs to return material science samples (most experiments could fit in them).

Life science would be done on the Spacelab/MOL missions.


The whole point is to do something like Falcon 9. Put on the money in the booster with its more benign launch and entry environment. Eliminate SRBs, refurb facilities and recovery ships. ET is shrunk down to an upper stage ECLSS, crew habitability, OMS, radiators, crew recovery, etc only when a crew flys.

 

[Archibald]

Well, this fellow (ha ha) once started a thread...

Landing a Saturn S-IB, Falcon 9 style

Landing a Saturn S-IB, Falcon 9 style

forum.nasaspaceflight.com

I see your point. Would Saturn IB be a best place to start ?

-back in the day (1959-1960) both Convair and Martin thought about wrapping an "aircraft package" around their respective Atlas and Titan. Wing, winglets, jet engines, undercarriage, and the kitchen sink.
-So let's strap a similar "aircraft package" around Saturn S-IB. Which mean Chrysler space division would have to do it.
-Interestingly enough, back in 1960-62 Chrysler wanted to try recovering that S-IBB: except with Ryan Rogallo wing. Bad idea, a Rogallo wing that size would be impossible to deploy in flight.
- So our flyback booster would be mix of the winged Atlas / Titan / S-IC proposals.
- Good knews: since the H-1s burned kerosene, they could share some with the flyback turbojets.
I have a few tons of documents and pictures related to this on my HD. Will post some later.

 

[Byeman]

I see your point. Would Saturn IB be a best place to start ?

No, too over built, But that likely is near the size

-So let's strap a similar "aircraft package" around Saturn S-IB. Which mean Chrysler space division would have to do it.

Not necessarily and see above.

- Good knews: since the H-1s burned kerosene, they could share some with the flyback turbojets.

a. Need better engines than H-1
b. Maybe don't need jet engines.

 

[Archibald]

You want that thing to glide back and landing ? Well... thinking about it, the X-15 flying as fast as 2 km/s and 110 km high, glide landed all the way from Las Vegas, back to Edwards... but it had a pilot.

Note that automated landing experience existed from 1950's big cruise missiles like Navaho & X-10, Regulus II and a few others.

Also Autoland from the british Trident airliner.

Autoland - Wikipedia

en.wikipedia.org

The first such landing in a BEA Trident was achieved at RAE Bedford (by then home of BLEU) in March 1964. The first on a commercial flight with passengers aboard was achieved on flight BE 343 on 10 June 1965, with a Trident 1 G-ARPR, from Paris to Heathrow with Captains Eric Poole and Frank Ormonroyd.

And guidance from the Ryan 147 Firebee drones. Also Surveyor radio guidance.

 

[Byeman]

You want that thing to glide back and landing ? Well... thinking about it, the X-15 flying as fast as 2 km/s and 110 km high, glide landed all the way from Las Vegas, back to Edwards... but it had a pilot.

Shuttle had the capability from orbit, except for landing gear deploy. Astronauts chose to fly the HAC.

 

[martinbayer]

Well, this fellow (ha ha) once started a thread...

Landing a Saturn S-IB, Falcon 9 style

Landing a Saturn S-IB, Falcon 9 style

forum.nasaspaceflight.com

I see your point. Would Saturn IB be a best place to start ?

-back in the day (1959-1960) both Convair and Martin thought about wrapping an "aircraft package" around their respective Atlas and Titan. Wing, winglets, jet engines, undercarriage, and the kitchen sink.
-So let's strap a similar "aircraft package" around Saturn S-IB. Which mean Chrysler space division would have to do it.
-Interestingly enough, back in 1960-62 Chrysler wanted to try recovering that S-IBB: except with Ryan Rogallo wing. Bad idea, a Rogallo wing that size would be impossible to deploy in flight.
- So our flyback booster would be mix of the winged Atlas / Titan / S-IC proposals.
- Good knews: since the H-1s burned kerosene, they could share some with the flyback turbojets.
I have a few tons of documents and pictures related to this on my HD. Will post some later.

Way better to start with a clean sheet design to learn the basics of effective reusability.

 

[Michel Van]

Oddly there was Study for reuse the Saturn Ib first stage, with wings & jet engine and Cockpit for Astronauts
called EAG 4391 by English Electric
yes British study

Source
British secret project 5 by Dan Sharp

 

[Archibald]

Way better to start with a clean sheet design to learn the basics of effective reusability.

Depends from the budget allocated to NASA. In our universe they got $5.15 billion to build the Shuttle, and ended with a 20% overrun at $6 billion and something.
Flyback booster all alone should be cheaper. The fully reusable Shuttles were in a $10 billion ballpark, and the "massaging" of the costs could not drop it below $7 billion. No idea which slice of the difference related to the flyback booster.

As for engines: the perenial issue is NASA binary thinking. "Sophistication" as in "staged combustion" is only worth with hydrolox. Kerolox and other low-energy propellants are not worth sophistication, so "gas generator" and that's it.

Of course with perfect hindsight (Merlin 1D) we know that 348 seconds can be achieved through pushing gas-generator to the limits. Otherwise it is 320 seconds best case. In vacuum of course.

Thinking about it...

Right Side Up: A History of the Space Transportation System

“In short, the Space Shuttle is so inefficient because it is built upside-down.”--Robert Zubrin Chapter 1: Preflight Technicians swarmed around the gleaming white delta-winged shape, mostly around the nose and tail, but some at strategic points along the length, at the engine bays, the...

www.alternatehistory.com

But their booster was manned (from memory).

 

[martinbayer]

Depends from the budget allocated to NASA. In our universe they got $5.15 billion to build the Shuttle, and ended with a 20% overrun at $6 billion and something.
Flyback booster all alone should be cheaper. The fully reusable Shuttles were in a $10 billion ballpark, and the "massaging" of the costs could not drop it below $7 billion. No idea which slice of the difference related to the flyback booster.

As for engines: the perenial issue is NASA binary thinking. "Sophistication" as in "staged combustion" is only worth with hydrolox. Kerolox and other low-energy propellants are not worth sophistication, so "gas generator" and that's it.

Of course with perfect hindsight (Merlin 1D) we know that 348 seconds can be achieved through pushing gas-generator to the limits. Otherwise it is 320 seconds best case. In vacuum of course.

Thinking about it...

Right Side Up: A History of the Space Transportation System

“In short, the Space Shuttle is so inefficient because it is built upside-down.”--Robert Zubrin Chapter 1: Preflight Technicians swarmed around the gleaming white delta-winged shape, mostly around the nose and tail, but some at strategic points along the length, at the engine bays, the...

www.alternatehistory.com

But their booster was manned (from memory).

For this particular alternate reality, I actually meant to postulate to start with a clean sheet design for an uncrewed winged rocket powered reusable VTHL booster to be combined with more or less existing expendable upper stages to learn the basics of effective reusability in a less harsh than the fully orbital environment - my apologies for not being explicit enough (a strange position that I honestly never expected to find myself in...). Even at the time, in my view crews in rocket boosters would have been a costly and unnecessary design complication.

 

[Byeman]

For this particular alternate reality, I actually meant to postulate to start with a clean sheet design for an uncrewed winged rocket powered reusable VTHL booster to be combined with more or less existing expendable upper stages to learn the basics of effective reusability in a less harsh than the fully orbital environment - my apologies for not being explicit enough (a strange position that I honestly never expected to find myself in...). Even at the time, in my view crews in rocket boosters would have been a costly and unnecessary design complication.

Easy what I was proposing. Sorry for the duplicate.

 

[TheKutKu]

Wouldn't an Inline upper stage be more manageable across a variety of missions and configurations than an assymetrical one? The interstage would be attached to the upper stage during stage separation, then jettisoned like on S-II.

A symmetrical launcher with two side-mounted fly-back boosters around a sustainer/core could also lower development cost, since each booster could be smaller (but would probably have a higher operating cost since there are more boosters).

 

[martinbayer]

Wouldn't an Inline upper stage be more manageable across a variety of missions and configurations than an assymetrical one? The interstage would be attached to the upper stage during stage separation, then jettisoned like on S-II.

A symmetrical launcher with two side-mounted fly-back boosters around a sustainer/core could also lower development cost, since each booster could be smaller (but would probably have a higher operating cost since there are more boosters).

Now while I'm generally a fairly open minded guy when it comes to alternative geometric stage arrangements, the Space Shuttle has conclusively proven that asymmetric couplings can work pretty well, thank you very much.

 

[Archibald]

There it is ! Langley's Shuttle II and its unmanned, glideback booster. Through a bazillion of studies since 1974, they had determined Mach 3 was the optimal staging velocity for the booster to return gliding - that is, no powered flight with heavy and voracious jet engines.

 

[Deleted member 4286]

...Let's try this:
- unmanned flyback booster, but derived from the S-IC
- second stage: expendable S-IVB
- kinda Saturn INT-20, except half reusable.
- on top of the S-IVB: as you said: expendable stages or "manned spaceships" (let's call them that way)
- in-line or side-mount ? in-line stays closer from vanilla Saturn V so...
- MOL and Spacelab are better done by "true" space station - or nothing at all.
- Apollo is off-the-shelf and works fine, no need to re-invent it as Orion or... Starliner (the former overweight, the latter flawed)
- Lifting body: why not ?
- Could cool down the vociferous side of NASA wanting sat retrieval and refurbishment (throw them a bone)
-The above also apply to MOL and Spacelab supporters, if any left. Give them a small reusable hab inside the lifting body.
- Also throws a bone to "winged spaceship / spaceplane" supporters.
- Would make the system 2/3rd reusable : kinda reusable-expendable-reusable stages so another bone thrown at "RLV crowd"

 

[Deleted member 4286]

View attachment 724023

 

[Byeman]

too big

 

[Archibald]

Yeah, and so is the Shuttle II booster. Although it would be easy to shrink it. Note that its props are hydrolox: any issue with that ?

 

[TheKutKu]

There it is ! Langley's Shuttle II and its unmanned, glideback booster. Through a bazillion of studies since 1974, they had determined Mach 3 was the optimal staging velocity for the booster to return gliding - that is, no powered flight with heavy and voracious jet engines.

Mach 3 isn't very fast, the expendable second stage would need to be large to make up for it (indeed here the orbiter is heavier than the booster), and a third stage would be all but required for higher orbits.

Sadly there are no runways in the atlantic and pacific oceans, or island chains at a variety of latitude that could cover launches to several azimuts. Inland launch sites have an advantage on that point, so do Falcon-9-like Retropropulsive boosters... And the landing sites of the Shuttle's Trans-atlantic abort would require much larger and faster boosters, which would make reuse more difficult for logistical and structural/thermal reasons; and these aren't viable for Vandenberg launches.

Would it be possible to turn the booster into a floatplane? That's, after all, the common answer to a lack of runway over a body of water. Floats - with a landing approach - would make the splashdown softer on the rocket (compared to a ballistic then chute-slowed booster like the SRB, obviously not compared to a runway landing) and prevent the actual structure and engines from being immerged too much. They also scale better than parachutes for large boosters, while seaplanes and ground-effect-planes can weight hundreds of tons. The booster would then be towed back to Port Canaveral, the whole process would not last longer than a Falcon 9 drone-ship landing recovery. The floaters would certainly not be light, but a first stage can handle a much higher dry mass than an upper stage without lowering the payload mass too much..

But at this point maybe adding jets would still be lighter... And the thermal protection of the floaters would be quite difficult to make and maintain. Considering the vastly larger number of jet-powered fly-back boosters studies, the trade-off probably aren't worth it at all.

Sorry for the spitballing; there's this picture, from the Aerospace project review blog, of a McDonnel Expendable Second Stage 1972 proposal that is quite fascinating, of course, this one is a derivative of NASA's "fully reusable shuttle", so the booster is too large for this thread, and the second stage needs to be thrust-assisted by solid boosters to make up for the lack of ground-started orbiter.

 

[Archibald]

There are (less sexy) pictures over there.

Series Development: A 1969 Plan to Merge Shuttle and Saturn V to Spread Out Space Program Cost

The story of spaceflight told through missions and programs that did not happen - that is, the great majority of them.

spaceflighthistory.blogspot.com

As applied to the Space Shuttle, series development could take either of two forms. In the first, the Space Shuttle's fully reusable piloted Booster would be developed and brought into service, then development work would begin on its fully reusable piloted Orbiter.

Until the Orbiter became available, the suborbital Booster would lift off from Cape Kennedy, Florida, carrying on its back an unmanned payload attached to an expendable upper stage based on an existing stage design — the Saturn V S-IVB third stage was one attractive candidate. The upper stage would ignite high over the Atlantic, boosting the payload to Earth orbit — or beyond. The astronauts, meanwhile, would pilot the Booster back to a runway at Cape Kennedy, where it would be refurbished, mated with a new upper stage and payload, and flown again.

 

[Byeman]

Mach 3 isn't very fast, the expendable second stage would need to be large to make up for it (indeed here the orbiter is heavier than the booster), and a third stage would be all but required for higher orbits.

Sadly there are no runways in the atlantic and pacific oceans, or island chains at a variety of latitude that could cover launches to several azimuts. Inland launch sites have an advantage on that point, so do Falcon-9-like Retropropulsive boosters... And the landing sites of the Shuttle's Trans-atlantic abort would require much larger and faster boosters, which would make reuse more difficult for logistical and structural/thermal reasons; and these aren't viable for Vandenberg launches.

Would it be possible to turn the booster into a floatplane? That's, after all, the common answer to a lack of runway over a body of water. Floats - with a landing approach - would make the splashdown softer on the rocket (compared to a ballistic then chute-slowed booster like the SRB, obviously not compared to a runway landing) and prevent the actual structure and engines from being immerged too much. They also scale better than parachutes for large boosters, while seaplanes and ground-effect-planes can weight hundreds of tons. The booster would then be towed back to Port Canaveral, the whole process would not last longer than a Falcon 9 drone-ship landing recovery. The floaters would certainly not be light, but a first stage can handle a much higher dry mass than an upper stage without lowering the payload mass too much..

Why? Just do an RTLS as the original concept was going to have the booster do.

 

[Yellow Palace]

Note that its props are hydrolox: any issue with that ?

I think it's more-or-less inevitable, in fact, given the attitudes of the time.

Based on the 1971 MDAC Expendable Second Stage study, from back when the Shuttle Booster was going to be a flyback stage and the Orbiter carried its own tanks, and a 65klb payload, you're looking at something like three-eighths the total vehicle mass

• Booster - new LOX/LH2 engine(s), total 1,770 lb thrust, 960klb propellant
• Upper stage - 2 x J-2 or equivalent, 290 klb propellant

That's a decent sized rocket, but not at all unreasonably so. The booster probably gets 3 x OTL SSME, or 4 smaller ones. Playing around with sizes, something like a 320klb engine would work with six on the first stage and one vacuum-optimised engine on the upper stage.

One F-1 derived engine would also work for the booster, but I think that LOX/LH2 would be preferred for 'efficiency', as it was with the OTL Shuttle.

If/when a small orbiter for this system was funded, it would carry maybe 10klb to orbit with 2-3 flight crew. Really a space station ferry craft; I'm assuming you'd get some kind of crew module for the payload bay.

Why? Just do an RTLS as the original concept was going to have the booster do.

The study I'm referencing assumed that the booster would recover downrange - Seymour-Johnson AFB in North Carolina was mentioned. For a low inclination launch I'm guessing you'd want somewhere in the Bahamas.

 

[TheKutKu]

(for reference, these are the documents

https://ntrs.nasa.gov/api/citations/19730006138/downloads/19730006138.pdf

to, with incremental URL

https://ntrs.nasa.gov/api/citations/19730006150/downloads/19730006150.pdf

on NTRS, I think)

 

[Archibald]

Sweet geez... it's a 12 volume study ! I downloaded all 12 pdf files. Way too big for this forum, I'll try my chance at NASAspaceflight.

 

[Yellow Palace]

Ah, they've clearly made the whole thing available then - I only have Volumes I, Volume II Book 1, Volume VIII and Volume IX. The titles of the documents are, for anyone wanting to check it out,

• 19730006138 Vol. I - Executive Summary
• 19730006139 Vol. II - Technical Summary - Book 1 -Expendable Second Stage/Reusable Booster System Definition
• 19730006140 Vol. II Technical Summary - Book 2 - Expendable Second Stage System Definition
• 19730006141 Vol. II Technical Summary - Book 3 - Booster Vehicle Modifications and Ground Systems Definition
• 19730006142 Vol. III - Wind Tunnel Test Data
• 19730006143 Vol. IV - Detail Mass Properties Data
• 19730006144 Vol. V - Operations and Resources
• 19730006145 Vol. VI - Interface Control Drawings
• 19730006146 Vol. VII - Preliminary Design Drawings
• 19730006147 Vol. VIII - Preliminary CEI Specification - Part 1
• 19730006148 Vol. IX - Preliminary System Specification
• 19730006149 Vol. X - Technology Requirements
• 19730006150 Vol. XII - Design Data Book

Several of these are probably only important if you actually want to build one. But I don't know how big your garage is, so go for it. Note that Volume XI is unaccounted for, and there may be (it's not clear) additional parts to Volume VIII.

 

[Archibald]

Thanks to that study I've realized that the "Jim solution" might have happened: under very peculiar circumstences.

NASA needed the Flyback-booster + S-II for a very special mission: hauling the Reusable Nuclear Shuttle in orbit ( = a 120 mt NERVA space tug, if you prefer).

Imagine the following three stages stack : flyback S-IC + expendable S-II + RNS

The RNS could have been delivered fully (hydrogen) fueled to orbit. Next, with its 825 seconds specific impulse it could have made a rountrip to lunar orbit and back.

And in lunar orbit it would have dropped a Chemical Space Tug turned Lunar Module: called LM-B. And that LM-B could make a roundtrip to lunar surface and back with a crew module.

So: Flyback S-IC + S-II + RNS + LM-B + crew module. Could have replaced Apollo with a partially reusable architecture.

 

[Byeman]

Still too expensive. Just trying to serve the same shuttle payload requirements with flyback booster and expendable upperstage.

 

[publiusr]

Any thoughts to making a winged booster favor one propellant more heavily?

Second stage engine feeds off the winged fly-back as well as the fly-back itself.

 

[TheKutKu]

...Let's try this:

- second stage: expendable S-IVB

Problem is that, with NASA's goals: 60 launches a year, 65k lbs per launch, S-IVB is not ideal

First, performances, Shuttle needed ~9,230 m/s to orbit [1]; S-IVB was ~120t heavy with ~109 tons of propellant with ~1.8t of residuals. Instrument unit (~2 tons, could probably be reduced in this case) not included [2], J-2 had 104t thrust and 421s isp, so a TWR of 0.7 at ignition (shuttle was around 1 at SRB separation); with a 65k lbs/29.5t payload, the S-IVB has ~5,140 m/s of delta-V, 5,330 m/s with a J-2S (w/ 436s isp); requiring the booster to give around 4,000 m/s of delta-V to close with the ~9,230 m/s (probably a bit, due to the higher gravity losses of the second stage compared to the shuttle), for an expendable first stage, it's fine, for a reusable booster, it's not ideal.

That's almost ~1,500 m/s faster than the Shuttle's SRB separation (~mach 4 at 50 km) and 500 m/s faster than the fastest F9 recovery (~mach 7 at 70 km), it's closer (but slightly slower) to a Falcon heavy center core recovery (~mach 9 at 100-120 km), and it seems that SpaceX has given up on those after repeated recovery failures. It's increased reentry heating, maintenance, and starting to get really outside of what was the precedents of the X-15.

It also requires a larger return range (Falcon heavy droneships used to be at 1,000-1,200 km from the cape, compare 35 to 650 km for a Falcon 9; 230 km for SRB splashdown); at this point it may be a good idea to give up on booster RTLS and do downrange landing (Bermudas are only at 1,600 km from the cape; Mid-atlantic or Puerto Rico for other azimuths), something which requires more facilities, possibly in other countries.

Note that is in the case of a serial-ignition, if the S-IVB is somehow ground-started, then there will be more performance problems. Furthermore, since the TWR is already not very high, stretching it may not be worth it,and a second engine may be desirable...

Second is the production rate, S-IVB is Saturn hardware, with facilities designed for 6 launches a year... It never had a stellar production rate, and it'd be really insufficient for NASA's goal of 60 launches a year. The infrastructure cost on top of the modifications for Booster-launch may not make it particularly attractive over a new stage. I'm also sure that large cost gains could be made with a new stage (Remember: the Space Shuttle ET was more than twice cheaper than an engine-less S-IIC despite being larger [3])

So could it work? Probably, but if it happens, it'll only be after the typical shuttle-era budget cut and compromises, and NASA's first choice will definitely be toward a new stage and not S-IVB.


[1] Space Propulsion Analysis and Design,1995
[2] Apollo by the numbers, sp-4029
[3] "Stages to Saturn"; 1969 NASA Authorization H.R. 15086; GAO/NSIAD 1993-93-115,adjusted to inflation

 

[Byeman]

Any thoughts to making a winged booster favor one propellant more heavily?

Second stage engine feeds off the winged fly-back as well as the fly-back itself.

it isn't parallel staged.

 

[publiusr]

Mach 3 isn't very fast, the expendable second stage...from this picture, from the Aerospace project review...

View attachment 724376

That can't seem to make its mind up whether it is an upper stage or parallel stage.

Those solids so close-to the fly-back worry me.

Shuttle showed cross-feeding works...might the upper stage be empty and--after Max-Q be fed from the booster?

Upper stage has helium to stay taut that it exchanges with the booster for fuel...flyback returns helium for re-use?

I thought 21st Century LVs would have an unusual appearance...never guessing Rocketship XM would become real.

 

[Byeman]

Shuttle showed cross-feeding works...might the upper stage be empty and--after Max-Q be fed from the booster?

The shuttle did no crossfeeding. Crossfeeding is where engines use propellants from multiple tanks at the same. Not from a separate tanks

 

[publiusr]

The re-use of helium possible for pressurization--fly-back returns that valuable gas perhaps? Upper stage alone using hot gas somehow?

 

[Byeman]

The re-use of helium possible for pressurization--fly-back returns that valuable gas perhaps? Upper stage alone using hot gas somehow?

no, can't scavenge from the tanks. Always need He with LH2.

 

[Scott Kenny]

Mach 3 isn't very fast, the expendable second stage would need to be large to make up for it (indeed here the orbiter is heavier than the booster), and a third stage would be all but required for higher orbits.

If they could have tolerated Mach 4 that's the rough speed of the Shuttle at SRB separation.

SRBs popped at ~44km altitude at something over 1,300 metres per second

 

[martinbayer]

The shuttle did no crossfeeding. Crossfeeding is where engines use propellants from multiple tanks at the same. Not from a separate tanks

Trying to split hairs there - transferring liquid propellant (fuel, oxidizer, or both) from one system element to another *IS* crossfeeding, independent of whether both elements have engines or not. Show me a definition that supports your narrow view.