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Saturn V Heavy

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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:

--- Quote from: Allan F on July 25, 2013, 03:49:49 PM ---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.

--- End quote ---
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.

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