Chair Force Engineer

Tuesday, June 10, 2008

Liquid Courage

When NASA adopted the Ares launch vehicles in its lunar mission strategy, it wedded itself to a new five-segment solid rocket being developed by ATK. That fateful decision from three years ago has created particularly thorny challenges for NASA. Much effort is being spent on mitigating thrust oscillations on Ares I, while Ares V may necessitate building another new SRB with 5.5 segments.

It would have been worth asking whether a liquid-fuel booster would be a better choice than the new solid rocket. The most deceitful sales pitch behind ESAS is that the SRB was "shuttle derived." The truth is that any change in the dimensions of a solid rocket is non-trivial. As for liquid-fuel boosters, changes in length are comparatively easy (while changes in diameter involve somewhat more cost, schedule and risk, but are not unheard of. After all, the Saturn I upper stage was lengthened, widened, and re-engined to form the upper stage on the Saturn IB and Saturn V.)

What if a widened Atlas core, using two RD-180's, replaced the five-segment SRB in NASA's plans? For starters, crew launch could be accomplished with commercially-purchased Atlases using a widened Centaur-derived upper stage and minimal modifications at the Cape's LC-41. The new boosters could then be applied to Ares V, and stretched or shortened as mission needs would dictate.

Liquid fuels will always possess certain advantages over their solid brethren. The LOX-kerosene combination has better specific impulse, lower structural mass requirements, and lower density than solid propellants like the ones on the shuttle. The result is a more voluminous booster for the same mission, but one that weighs less.

The weight consideration is an important one. The behemoth Ares V weighs so much that new "super-transporters" will be built to replace the 40+ year old crawlers which transport the shuttle to the pad. The super-transporters will not come cheap. My personal preference would be placing a gross weight restriction on Ares V that would fall within the limits of the current crawlers. I would also use two Ares V's per mission, to get around the performance limitations on a smaller Ares V.

No design trade is without its drawbacks, and the Achilles Heel of liquid boosters is the limited thrust when compared to solids. It takes roughly 5.5 RD-180 engines to match the thrust of the Shuttle's two SRB's (and the new Ares SRB will have even more thrust than the Shuttle SRB.) It can reasonably be expected that three or four evolved Atlases would be necessary to replace the two SRB's in Ares V. More boosters leads to higher failure rates (even if the RD-180 pairs are replaced with RD-171's, which are functionally equivalent but possess fewer parts than the RD-180 pair.) There's a good reason why the Soviets needed four liquid boosters on their Energia-Buran shuttle system, with each booster powered by a single RD-170 (functionally equivalent to the RD-171 or two RD-180's.) The only upside is that the lower liftoff mass will mean that less thrust is needed. Still, it's not enough of an offset to get away with only two boosters for the payload masses that NASA wants.

The solid-liquid trade study is one that couldn't have been adequately analyzed during the 60 days of the ESAS study, and will likely end up as an interesting footnote in the Ares story. The question is whether the Ares story will fall into the genre of historical nonfiction, or fantasy and tragedy. If the latter is true, perhaps liquids were the answer after all. But the decision to not cap the weight of Ares V (even at the expense of payload) is one that taxpayers shouldn't forget if the massive rocket, and its shiny new infrastructure, ever get off the drawing board.

POST SCRIPT: It just so happens that the Delta IV core, at just over 5 meters in diameter, is almost exactly as wide as the SRB's aft skirt. The wide-bodied Atlas will be slightly bigger at 5.4 meters. The SRB cutouts on the mobile launch pad would seem like an ideal fit for EELV-derived boosters. But they will likely need to be moved farther out from the center of the pad, as there's only seven meters separation between the two.