Depends on how the LRBs fit into the architecture.
"Technically" they simply replace the SRB's in most concepts. (
Page 3, and Figure 6 and 7) The only way to 'stack' the orbiter was either to remove the engines the stage it was stacked on, (which would have to be based on the ET) or have an LRB "stage" that was big enough to fit both the Orbiter and ET... And THEN comes the hard part of figuring out how to start the engines in-flight
Stacking the Orbiter on top of an interstage and some sort of LRB would avoid the inherent risks from having the crewed vehicle on the side of the stack, although it would need some imagination to fit the SSMEs in there.
"Fitting" the engines isn't really a problem but keep in mind you need to feed them with propellant which means you STILL need the ET which works out something like the
Saturn-Shuttle concept. If you're willing to do a major, (almost total) rebuild of the Orbiter and turn it into pretty much a straight glider, (see above paper, figure 21) then you can mount the engines to the bottom of re-built ET, maybe podded for recovery. (Same paper, figures 1 through 4) But then you need a monolithic LRB the size of a Saturn V, to develop the new ET, rebuild the Orbiter.... I don't see it happening with any of the budget NASA had.
2xF-1 scale engines on each LRB would get somewhere to the thrust, can’t work out how big the booster would be however. The H-1 was dunked in the ocean a few times and it was refurbishable - before the $$ dried up, so no reason to believe why with a bit of investment wouldn’t get the F-1 (probably an F-1A) there too.
This
paper, from 1989, has the F1 rejected due to two F1's per booster not having enough margin for an 'engine-out' situation, (baselining 4 each "new" engines per LRB) and from what little is give it isn't clear if this is the standard F1 or the F1A. Glad to see someone else noting the work done on the H1 recovery/refurbishment experiments
I'm not so sure the F1 could be so easily since it was a very different beast of an engine. And keep in mind there was work on an F1B a few years ago which planned a "cheaper" F1A (essentially) because the F1 was a pretty expensive engine. Now Rocketdyne in the mid-90s was pitching an 'advanced' version of the RS-27, (who's ancestor was the good-old H1
) called the
RS-X, (using RS-27 and Atlas engine parts) which the brochure showed in a 'cluster' configuration that could be two, three, or maybe four engines. But again even the RS-27 had changed so much from the original H1 I'm not sure how the recovery/refurbishment experiments would still apply to an even more advanced engine.
My read from the paper is:
Four (4) of their 'notional' LRB engines with a sea-level thrust of 685,000lbf nets a total single LRB liftoff thrust of 2,740,000lbf leaving and 'engine out' thrust of 2,055,000lbf
Two (late Apollo) F1's at 1,553,200lbf each get a total of 3,106,400lbf which (obviously) drops by half in an engine-out situation.
A single F-1A/B had a takeoff thrust of 1,800,000lbf so two would have 3,600,000lbf and again a less than the 2 million lbf they seem to assume as a requirement for an engine out situation.
Running the number the RS-X only has 424,880lf so four would only amount to 1,699,520lbf so you'd need at least six of them, getting a total of 2,549,280lbf, which would drop to 2,124,400lbf with an engine out. So if you can FIT six engines you still meet the margin
That said, the Orbiter would still be on the side of the stack - without the adoption of a boat tail style returnable vehicle for the SSMEs on the bottom of the ET - but then you’re developing two reusable vehicles.
Well there was a studied concept for a "Titan Boost Assist Module" (by the Air Force of course who needed some extra "boost" for their polar orbit missions) which suggested they figured they could strap two sets (four engines total) of Titan II engines and some custom tanks to the base of the ET with "minimum" changes to construction
But that wasn't 'reusable' in any sense and NASA was pretty adamant about NOT using Titan derived components and especially the toxic propellants it used. In fact that paper mentions that a Titan-esq LRB was considered early on because it was the only propellant combination that allowed an LRB of 'similar' size to the current SRB which was the most aerodynamic and fit the known stress parameters. Later wind-tunnel work suggested that requirement wasn't as 'strict' as they thought
Oddly though...
That's one of the articles that I had read in the past.
This
article, which I'll cite again, is very interesting because if you compare the more 'official' one you'll note that it doesn't take much "reading-between-the-lines" to see that official paper makes some unsupported assumptions, dismisses some options to easily with no explanation, (methane is rejected as not being 'compact' enough when compared to LH2...???) and 'gently' points out the preferences at which NASA and the study contractors were aimed. While the general recommendation would seem to be the development of a pressure fed kerolox LRB in fact they argue against that but point out that the tankage of the pump-fed kerolox system would not be 'strong' enough to survive recovery. So a recoverable engine/pump pod would need to be developed and since we have to go to so much trouble, well, KSC (and systems operations) would prefer to NOT use another 'fluid' on the pad so here's this idea for a pump-fed hydrolox LRB...
I don't know if Steven Pietrobon, (I'm familiar with him from the NASAspaceflight-dot-com forums) had read this or other LRB papers but I'm guessing so because his paper makes a straightforward case for something that NASA obviously never even considered. His LRB is functionally as similar to the standard SRB as possible, (figure 2, table 3) and actually come in about 44 metric tons lighter with superior performance and enhanced payload to orbit.
He suggests a keroxide version of the RD-170 he calls the RD-17X with a thrust of 1,920,992lbf at sea-level, or two modified RD-180s running on keroxide with the same thrust.
"But don't you need at least 2 million pounds of thrust?"
Yep, that's what the official paper says and the SRBs put out about 2,800,000lbf on lift off so why the difference?
Because when designed for it a pump-fed, throttling engine CAN be run at higher as well as lower thrust levels and if the LRB can be throttled up to 112% your thrust goes up to 2,168,957lbf.
Of course someone will point out that any 'engine out' will cut this in half if you split it between two engines as suggested which is well below the threshold the 'official' paper allows. But lets keep in mind that that assumes loosing and engine you can still get to orbit which you actually can't do with the SRBs nor most of the LRB designs since getting a powerful enough American engine is a problem. LRBs allow you to abort before nominal 'burnout' where as SRBs do not, and most of the LRB abort modes at lower altitude are arguably more survivable. And of course there's the issue with there being no really big keroxide engines to play with hence the need to 'redesign' an existing engine.
Well, there was ONE possible answer, the Beal Aerospace
BA-810 developed and tested in the mid-90s. Sure it was designed to be pressure fed and 'only' had a thrust of 809,300lbf, was ablative and expendable but if nothing else the Merlin and RS-62 engines have shown evolution can be pretty straight forward. Pump-feed increases your performance and there are actually some historical work done that shows you can actually make a pretty compact and straightforward turbopump/engine combination, (see the
RMI XLR040 "Super-Performance" rocket motor for example) using the specific characteristics of keroxide. (Including using it for cooling) There might be ONE slight problem though:
The truck is pretty damn close to the test stand but you get the idea.
A bit more prespective:
Not sure you could fit more than one on a 13ft diameter booster
Randy