Saturn V Heavy

[Peter B]

While doing a bit of YouTube browsing I happened upon footage of a Delta IV Heavy launch - impressive sight.

Then my imagination wandered...to the idea of a Saturn V Heavy: place an S-IC on either side of a Saturn V, and (a) imagine the sound at lift-off with 15 F-1s at full power, and (b) imagine the payload.

I assume there would be severe technical problems, particularly related to heat and the amount of water needed at launch, and the thrust shortly before booster cut-off. Perhaps the latter could be reduced using a crossfeed system such as is planned for the Falcon Heavy.

Would there be other technical issues?

[VQ]

Probably a couple. Structural issues, especially with the vibration problems they had on the Saturn V, would be tough. And with a heavier payload, gravity drag would be more significant at the beginning of the burn of the second stage (I think its initial acceleration was already under 1g).

I would be interested to know how much mass to orbit you would add by just slapping on 2, 4, or 6 S1C's and imagining the structural upgrades had negligible mass.

[Allan F]

Wouldn't it be better to use SRB's, possibly the cancelled 5-segment boosters derived from the Space shuttle? As I understand it, the Saturn V was barely able to lift itself out of the launchpad. I understand the problem with solid fuel rockets, but their large initial thrust would be very helpful.

[ka9q]

Well, as long as the S-ICs had thrust-to-weight ratios greater than unity (and they certainly did) then it can't worsen the thrust-to-weight ratio of the entire stack at liftoff.

That said, I can also see a lot of obstacles, one being the extremely high thrust/weight ratio towards the end of first stage flight (with all three S-ICs burning). The inboard engine on the S-IC in the Saturn V had to be shut down early to limit acceleration to 4 g, so you can probably imagine (or calculate) what it would be with three S-ICs all firing together with nearly empty propellant tanks.

With parallel staging you usually want the boosters to burn out and be jettisoned well before the core stage so that thrust is reduced and acceleration is limited as mass is shed, and also so you can get rid of the weight of the boosters themselves. Hence you have the Shuttle SRBs burning out after only a couple of minutes while the SSMEs burn all the way from the pad into orbit. As I seem to recall, the booster models of the Titan III (III-C, -D, -E, Titan 34, etc) didn't even ignite the core stage until burnout of the two SRBs.

SpaceX has an interesting idea with their Falcon 9 heavy. Although the three parallel stages are essentially identical (same tanks, same size and number of engines), they exchange propellants during flight. I haven't read the details but I'm pretty sure propellants are pumped from the two booster stages into the core stage during boosted flight, with all three stages burning. This causes the two boosters to deplete their propellants before the core stage, and to leave the core stage with a (mostly?) full set of tanks at booster burnout and separation.

This is a really interesting way to solve the problem; it's effectively like strapping two shorter-burning boosters on the side of a really big one that can also drop the empty part of its own tank midway through flight to lessen weight.

[Peter B]

Wouldn't it be better to use SRB's, possibly the cancelled 5-segment boosters derived from the Space shuttle? As I understand it, the Saturn V was barely able to lift itself out of the launchpad. I understand the problem with solid fuel rockets, but their large initial thrust would be very helpful.

I suppose it might be better to use the SRBs in terms of thrust to weight. But as you suggest there's the old "can't switch 'em off" problem if something goes wrong.

Also, my musing was set back in the late 1960s when the Saturn V was in use but the Shuttle-era SRBs were still but a glint in their designers' eyes.

As for the effect of using two extra S-ICs, here's my attempt at some sums.

Saturn V mass at lift-off: 6,200,000 pounds;
S-IC thrust at lift-off: 7,500,000 pounds;
Acceleration at lift-off: 7,500,000 / 6,200,000 = 1.21 G.

Now the normal S-IC had a fully fuelled mass of 5,000,000 pounds. I assume that the "booster" version's mass would increase slightly due to the need for a nose fairing and all the paraphernalia to attach it to the main rocket, so let's say 5,020,000 pounds. Now we have:

Saturn V Heavy mass at lift-off: 16,240,000 pounds;
3 x S-ICs thrust at lift-off: 22,500,000 pounds;
Acceleration at lift-off: 22,500,000 / 16,240,000 = 1.38 G.

So even at lift-off it would be accelerating slightly faster than a straight Saturn V.

[Peter B]

...I can also see a lot of obstacles, one being the extremely high thrust/weight ratio towards the end of first stage flight (with all three S-ICs burning). The inboard engine on the S-IC in the Saturn V had to be shut down early to limit acceleration to 4 g, so you can probably imagine (or calculate) what it would be with three S-ICs all firing together with nearly empty propellant tanks.

I agree. That's why I suggested propellant crossfeed in the original post.

SpaceX has an interesting idea with their Falcon 9 heavy. Although the three parallel stages are essentially identical (same tanks, same size and number of engines), they exchange propellants during flight.

Yep, beat you to it - I mentioned that above.

I haven't read the details but I'm pretty sure propellants are pumped from the two booster stages into the core stage during boosted flight, with all three stages burning. This causes the two boosters to deplete their propellants before the core stage, and to leave the core stage with a (mostly?) full set of tanks at booster burnout and separation.

This is a really interesting way to solve the problem; it's effectively like strapping two shorter-burning boosters on the side of a really big one that can also drop the empty part of its own tank midway through flight to lessen weight.

Both the Wikipedia page on the Falcon Heavy and SpaceX's own Falcon Heavy page are unclear whether the booster stages use crossfeed to top up tanks or to directly feed the engines of the core first stage, although SpaceX says that crossfeeding can be switched off, which suggests it would be used to top up the tanks.

But yes, the idea is that when the boosters' tanks are depleted and they're cast off, the core first stage will have full tanks.

That means the booster stages would deplete their fuel in two-thirds of the normal burn time, which means for an S-IC a booster burn time of about 110 seconds. At that point:

Saturn V Heavy mass at booster engine cutoff (BECO): 6,816,000 pounds;
3 x S-IC thrust at BECO: 22,500,000 pounds;
Acceleration at BECO: 22,500,000 / 6,816,000 = 3.3 G.

That figure surprised me - I expected it would be much higher. Anyway, at that point you effectively have a fully fuelled Saturn V, already at perhaps 20-30km altitude and a similar distance downrange, and travelling at perhaps 1500-2000 metres per second. I'll leave it to people who know how to do the sums to work out these things.

Obviously, though, the idea would be to increase the size of the payload, taking into account VQ's point about the effect this would have on the upper stages of the rocket.

[ka9q]

That surprises me too -- I also expected a higher acceleration at booster burnout. But figure what it would be if all three S-ICs burned together, depleting their tanks at the same time. On the bright side, gravity losses would be lower...

Given how many eggs would be in one basket here, it seems wiser (as well as less design work) to just launch several independent Saturn Vs and do an earth orbit rendezvous. That is if you can figure out how to store cryogenic propellants in orbit for any length of time.

[Peter B]

Given how many eggs would be in one basket here, it seems wiser (as well as less design work) to just launch several independent Saturn Vs and do an earth orbit rendezvous. That is if you can figure out how to store cryogenic propellants in orbit for any length of time.

I suppose the issue would be determined by what you intend using the rocket for. For example a Saturn V Heavy would allow you to loft a much bigger Skylab than was actually launched in 1973, or in a modern context allow the construction of the ISS in a lot fewer launches. But if you wanted to do some sort of extended lunar mission, I agree that multiple Saturn V launches would probably be a better way to go.

[Everett]

What what do you know, the Encyclopedia Astronautica has a huge page listing every Saturn variant ever considered.

http://web.archive.org/web/20100102212524/http://astronautix.com/lvs/saturnv.htm

(great site, has more information then you could ever want on every flown, designed and paper project in both the US and the USSR.)

They actually did a lot of studies for follow on saturn versions. Basic ideas were strap-on solids from various ICMB programs, stretching the 1st, 2nd and 3rd stages various length (interestingly, the S-II stage was rarely stretched), and upgraded engines. They included the F-1A, which was ground tested, the J-2S, which was ready for flight but never flown, the HG-3, which would lead to the SSME, and various aerospike engines (never left the drawing board, as far as I can tell.)

Of more interest here, they never considered a "Saturn V heavy," although they did consider strapping four core stages together, for 500 tons into LEO (I don't have any idea how that would work structurally, let alone aerodynamically, or how they planned to attach the payload.) They also considered add 4 strap-on 260 inch liquid boosters, each with 2 F-1's. They aslo considering using strap-on 260 inch solids, the most powerful rocket ever tested. SInce they were an inconvenient length for stretched first stages, they planned to put propellent tanks above the solids, which would be drained first. Oddly, those tanks would be emptied long before SRB sep, at least according to my amateur math (so probably wrong  , the S1C would be only partially fueled at that time. They never considered shuttle SRB's, as they didn't exist yet. They also usually tried to get it to fit under the roof of the VAB.

Also, according to my 'math,' you could actually fit 7 260 inch boosters around the S1C, whether solid or liquid. Also according to my math, a S1C, not stretched but with flat bulkhead tanks and F-1A's, with 7 260 inch solids with tanks above them for all the fuel used by the first stage before SRB sep, a SII-8 (planned for the Saturn C-8, which was canceled before the Saturn V was put into production) with flat bulkhead tanks and HG-3's for the second stage, and a S!V-C (again, cancaled early on) with flat bulkhead tanks and an HG-3 for the third stage, it can just barley lift my 468 ton nuclear thermal stage into orbit.

Neglecting increased gravity losses from the low upper stage thrust.