Saturn INT-20 / Saturn Shuttle / flyback S-IC idea


Senior Member
6 June 2006
Reaction score
Saturn INT-20 was a S-IC with S-IVB as second stage. The S-IVB being 360 tons lighter than S-II, acceleration was terrific.

Boeing suggested to remove a F1; even with four engine, payload matched that of Saturn INT-21.

In 1971, with the reusable booster cancelled, Boeing promoted a flyback S-IC. The S-IC would be turned into a giant aircraft, flying back to the Cape after jettison.
(As shown in Orionblamblam APR)

Result was the Saturn Shuttle.

Problem with the Saturn Shuttle = the vehicle length. The external tank ontop the S-IC sounds ok, but having a 100 tons orbiter on the side… bad.

My idea consists of INT-20 / Saturn-shuttle hybrid.

The Shuttle / external tank is replaced by a S-IVB with an Apollo CSM / LAS. This S-IVB is mounted on the side of the S-IC, a bit like the external tank is the current shuttle.

Result is something like that (perdon the crude thing)

Thanks to the weight of the "flyback hardware" added to the S-IC, the fifth S-IC can probably be added again.

The idea driving this concept ? some fun with alt-history…
NASA obtains the Saturn-Shuttle in 1971, but the orbiter / external tank are cancelled in 1974-76 as too expensive, leaving NASA with the flyback S-IC only.

Of course one can delete the CSM ontop of the S-IVB and replace it by a large payload to LEO. No idea of the payload...

I’m curious about this flyback S-IC… any chance it would have worked ? It is three times bigger than the actual orbiter, but its speed is also much lower (2.5km per second)

Or is it just like trying to fly an A380 at mach 5 ? I mean, utterly mad ?

Feedback welcome…
Not bad, Archibald
but you don't need to put the S-IVB sideway on Flightback

the Nosecone is design to rotate inside
to make place for the Engine of second stage
found a new Boeing version for Saturn S-IC

Saturn S-ID
its defector first stage of Saturn V but 4 F-1 engine are dropt
like Booster on Atlas, the 4xF-1 pod lands on parachute
so can be recycle for next launch

thanks to Scott Lowther for the Info
A true 1.5 SSTO.

Thruth be told, the Saturn V was some amzaing machine. Just think about what follow = the S-II and S-IV were kind of expendable SSTOs, thanks to an outstanding T/W ratio.

They were close from the fatidic 92% limit (8% structure, 92% propellants = SSTO)

If you add the 1.5 SSTO to the list... darn, every Saturn stage can reach orbit !

Alas, they have to be expended, because if you add TPS + recovery systems, you degrade the T/W ratio, and say goodbye to SSTO.

Unless you "reuse" the stages... in orbit!... bring back the wet workshop concept, and apply it to this S-ID, to the S-II and S-IVB.

Or, even better, rebuilt the Saturn V in orbit (!!) and fire every stage again (sounds crazy)
Boeing proposed for INT-20
the Centaur as Thrid stage !

with that you can launch 40000 pound (18 tons) to moon/GEO Transfers orbit

but you get more payload
if you use a Saturn S-ID with S-IVB and Centaur ;D

Advance Mission Study
Saturn Mission Payload Versatility
Boeing October 11, 1967

were to get ? HERE :,5870.msg49782.html#msg49782
Just a few pics of my model INT-20

The model uses the Airfix Saturn V as a basis, with my own resin parts for the F-1 engines, S-IVB thrust structure, S-1C/S-IVB interstage, and Apollo CM and SPS nozzle.




Three Saturn Vs – INT-21 Skylab ( with the Skylab 2 Saturn 1B behind ), INT-20, and Apollo 11

cool Model MartG

those who wonder why INT-20 has only 4xF-1
there were several version proposed:
one with three F-1 engines in the first stage, one with four, and one with five.
The payload for a four-engine S-IC/INT-20 wasn't too far off the five-engine version, and you threw away one less engine The five-engine version had to shut down one or two of the engines early anyway, because the acceleration late in the first-stage burn approached the 6G limit.

The modifications were made using a conversion kit, and a four-engine S-IC could be converted back to a five-engine one, providing a lot of manufacturing and operational flexibility.

Fun fact: all late production F-1s were certified for 2250 seconds and 20 starts.

Edit: fixed INT-20 reference, was erroneously INT-21.
Top Bottom