Chair Force Engineer

Friday, September 18, 2009

Turn on the lite

One curious recommendation of the Augustine Commission is their preference for "Ares V Lite," a proposed heavy-lift rocket which would fill the gap between Ares I and Ares V and be capable of launching the manned Orion spacecraft. When compared to the baseline Ares V, the "Lite" version can only lift 140 tonnes to the reference orbit instead of 160 tonnes. Bear in mind that even the "Lite" rocket has more performance than the legendary Saturn V.

With so little information in the public about Ares V Lite, it begs the question of that makes this new rocket so "Lite" when compared to Ares V? Because the Ares V baseline had switched to a 5.5-segment SRB (with a dummy spacer, to allow for an even longer core,) my guess is that "Ares V Lite" will use 5-segment SRB's similar to the one tested by ATK last week.

It's also possible that the "Lite" core stage is shorter, with the SRB attach points moved further aft on the SRB. This is necessary so the SRB cross-member can pass between the LOX and hydrogen tanks through the intertank structure.

The upper stage probably didn't shrink by much, because the propulsion requirements for escaping low earth orbit are similar (depending on whether Altair and Orion have changed in mass.) The biggest change is whether the upper stage is still expected to accelerate from Mach 12 to orbit, or if the staging velocity has changed. My recommendation to the Ares team is to invest any mass savings from the total system into systems which will reduce the upper stage's on-orbit boil-off, allowing it to loiter for a longer period of time.

My biggest question to the Ares V team is about the number and type of core engines. The baseline Ares V had six RS-68B engines. But if heating from the SRB's is an issue for the core engines, this is all subject to change. A regen-cooled RS-68R, or expendible engines based on the Space Shuttle Main Engine, might be better suited to surviving the thermal environment. They would also improve the overall performance of the rocket, at the expense of reduced thrust. Switching to regen-cooled engines results in a smaller core stage (either shorter or narrower, depending on the whims of the team at NASA-Marshall) carrying less fuel. Depending on how much lighter the core stage gets, the reduced thrust of expendible SSME's might not be an issue.

Ares V Lite might be moot if NASA doesn't see the $3B/yr budget increase requested by Augustine 2.0. Nevertheless, it would be interesting to see what Marshall cooked up, and whether it could live up to its performance estimates.