# Why weren't SRBs used in the design of the Saturn V?

Were solids considered for the Saturn V? If so, why was the idea discarded?

I would guess this is due to a number of reasons:

• Inability to throttle
• Technology readiness in the era
• Launch escape
• Low $I_{sp}$ and von Braun/ABMA's experience? Large SRBs do have a boatload of manufacturing quirks, though. – Deer Hunter Dec 3 '14 at 22:17
• I suspect it would suffer similar problems to canceled Ares V. From NSF: "...exhaust plumes of six RS-68 engines, combined with the two SRBs, interact to reduce the efficiency of the engines, and cause extreme heating on the base of the core stage". – TildalWave Dec 3 '14 at 22:47
• To some extent it's perfectly possible to "throttle" a solid rocket booster. See for example Why is there a hole in solid rocket engines? which discusses how grain geometry can be used to influence the thrust versus time curve. While this doesn't solve the problem of closed-loop thrust control, it does do open-loop thrust control, which would have been sufficient for thrust control during the first stage burn; compare Saturn V Launch Simulation about 4/5 down which has a graph of acceleration vs time. – user Oct 17 '16 at 9:30
• Yes, shuttle srbs were designed to reduce thrust while the vehicle was passing through max q regime. – Organic Marble Oct 17 '16 at 10:49
• Solid rockets were used, but only small ones for stage separation. – Uwe Dec 30 '16 at 15:28

According to the Aerojet wiki, they were considered.

Aerojet solid fuel technology was under consideration for use in Saturn first stages. In the 1963, the U.S. Air Force gave Aerojet General \$3 million to start construction of a manufacturing and testing site several miles southwest of Homestead, Florida. . . . .Between Sept. 25, 1965 and June 17, 1967, three static test firings [of the AJ-260 rocket] were done. SL-1 was fired at night, and the flame was clearly visible from Miami 50 km away, producing over 3 million pounds of thrust. SL-2 was fired with similar success and relatively uneventful. SL-3, the third and what would be the final test rocket, used a partially submerged nozzle and produced 2,670,000 kgf thrust, making it the largest solid-fuel rocket ever.[15]

There was a problem with the third test. Near burnout, the rocket nozzle was ejected, causing propellant made of hydrochloric acids to be spread across wetlands in the Everglades and a few crop fields and homes in Homestead. Many residents of Homestead complained about the damage done, which included paint damage to their cars and killing thousands of dollars worth of crops.[16]

By 1969, NASA had decided to go with liquid-fueled engines for the Apollo’s Saturn V rockets. . .

More details here:

The largest solid rocket motors ever built were Aerojet's three 260 inch monolithic solid motors cast in Florida.[18] Motors 260 SL-1 and SL-2 were 261 inches in diameter, 80 ft 8in long, weighed 1,858,300 pounds and had a maximum thrust of 3.5M pounds. Burn duration was two minutes. The nozzle throat was large enough to walk through standing up. The motor was capable of serving as a 1-to-1 replacement for the 8-engine Saturn 1 liquid-propellant first stage but was never used as such. Motor 260 SL-3 was of similar length and weight but had a maximum 5.4M pounds thrust and a shorter duration.

• Nice answer. Would be great to find von Braun's POV. – Deer Hunter Dec 4 '14 at 7:23
• That quote about Aerojet just doesn't add up. If it was being considered for the Saturn rockets, it would be NASA, not the Air Force that was commissioning things. In any case, the Saturn V design, using F1 liquid-fuelled engines, was already announced in early 1962, a year before Aerojet got its funding. And, by 1969, you'd better be sure NASA had decided what engined to use in the Saturn V -- they'd already been flying the things for two years! – David Richerby Dec 12 '14 at 20:04
• @DavidRicherby: Adding strap-on SRBs to a Saturn-V might be a way of getting the Saturn 8's payload capacity to orbit back if LOR didn't pan out. – Joshua Apr 25 '16 at 21:51

MSFC studies did include in-house analysis of Saturn V solid booster modifications. There are several reports from 1965 detailing what was known as a Saturn MLV 'Modified Launch Vehicle' program. Within these documents are design, operation, cost and management plans for enhanced, larger Saturns.

The first such solid booster-augmented Saturn V design was the Saturn MLV-V-4(S), essentially a standard Saturn V (as it had the same stage lengths) with a strengthened S-IC first stage that also mounted four 120-inch diameter SRBs. It is interesting to note that this design offered very similar payload capacity to the standard, booster-free model. This could be a case of propellant offloading.

Regardless, those guys at Marshall knew there was some potential there, and set about detailing the parameters for the improved Saturn MLV-V-4(S)A and V-4(S)B. The A variant would feature a first stage with a 337-inch stretch, but keep standard F-1 engines because the solids would help them lift the heavier first stage early on (otherwise TWR was below 1). The B variant would improve the second stage to carry up to 970k to one million pounds of propellant. LEO payloads were to be 160.9 tonnes for the A and around 172 tonnes for the B. Solid boosters allowed for the Saturn V core stack to be upgraded less extensively to surpass 160t LEO performance. SRB-free Saturns MLV-V-3 and V-3B needed much longer second and third stages, F-1A engines, and either the advanced HG-3 or 400klbf-class toroidal hydrogen engines.

Moving on from MLV, we have it's bigger brother in the form of the ELV concepts. ELV, or 'Evolved Launch Vehicle,' would push the Saturn V to the limits of its design possibilities, perhaps beyond practicality, and even past sanity. Of the three largest models, two would rely on solid fuel boosters to lift their weight off the pad. Smallest (not at all small) was the Saturn ELV V-25(S)U, which is interestingly also called the 'Earth Launch Vehicle' given it was to send people to mars in the 1980s. It would do so by lifting the NERVA stages required for TMI into 300nmi orbits for docking. Each stage would have a total mass of 249 tonnes, so its maximum LEO capacity would be just a bit beyond that. In this case, the solid boosters were 156-inch diameter strapons (four of these) around an F-1A powered first stage stretched 498 inches.

The V-4/260 is the largest booster-assisted Saturn V design I can find. A V-25(S) core with a stretched upper stage, surrounded by four 260-inch solids. The boosters and first stage would generate 55 million pounds of thrust together, and the SRBs were so large they carried extra kerosene and LO2 for the core to increase burn time. Payload was to be 363 tonnes to LEO, so high that only the V-4X(U) could beat it (four stretched Saturn Vs joined together under a single 520 tonne payload).

These rockets, perhaps not including the later madness, were actually considered and you can find a lot down to even acoustics and fuel fill data.

However, there are two main reasons why they weren't flown: first, there was the great cost of new launch facilities; second; there was the lack of missions and funding after Apollo.

Firstly, developing a launch vehicle also means developing the ground systems for it. Adding to one means adding to existing hardware whether it flies or not. The scale of the moon rocket made things just that much bigger and more difficult, meaning more costly. Adding SRBs meant new launch pads, new flame tranches (and so new testing for sound and heat stresses, etc.) and a new crawler to support the even greater weight of the rocket. This is important. Solid boosters must be fuelled prior to transport, meaning the crawler doesn't just carry more stage mass if the boosters were liquids, but it also carries dense fuel. The boosterless MLVs would just use the existing trenches, saving money they would have used to make their more complex stages, but also saving time. In addition, payload gains could be so great that the 410-foot tall crane height of the VAB would have to be raised. If so, you have so shell out money to raise the roof of that structure. If not, you can't use your product to the best of its ability - the only ability that justifies its existence, lifting more than the original.

Secondly, following the repeated demonstrations of the standard Saturn V'S capability to launch nearly 50 tonnes to the moon, US manned space flight began to focus on LEO. The moon rocket showed its worth orbiting an entire space station in one hit. But in truth, Apollo lunar sorties were to only mission that really required the booster at the time. Skylab was originally planned as a wet workshop launched with the smaller Saturn 1B. With no need for more Skylabs, mission planners sought new ways to put the Saturn V to work, including slapping a Centaur as a fourth stage for massive interplanetary probes - but none of these were waiting either. Funding for NASA dried up between Apollo and STS, so no Development money could be spared for solid booster augmented Saturns. The issue was, the normal one was already too big.

Cheers if you get through all that, and I hope it provides at least something helpful to you!

Al.

• Forgot to add - launch escape could have been hampered by increased vehicle acceleration at low altitude, where the rocket would be powerful enough to shove through the air but the capsule would now have to lift away under more resistance. The MLV-V-4(S) was to have had a TWR at launch of up to 1.45, significantly more than the 1.18 as standard. Also, from what I know, the tech was feasible, given that the 156-inch boosters would have been UA-1205s or 1207s from the Titan family. – Alastair Haslam Dec 30 '16 at 15:17

Solid fuel rockets of the 1960s used an oxidizer rich in chlorine, the fumes of which were widely dangerous with a 260-inch engines. The military had been using something cheaper in ww2, but switched to per chloride as it gave greater range to rockets fired from planes, tanks, landing craft. The current discussion is to use ammonium nitrate as the oxydizer and wax and 15% aluminum powderfor newer solid fuel boosters. The exhausts would not be as toxic

• This seems to be a less detailed rehash of the earlier answer. – Nathan Tuggy Oct 17 '16 at 0:12