Chair Force Engineer

Tuesday, June 17, 2008

Staging Strategies

In comparing Ares V to its spiritual predecessor, Saturn V, the difference between the two main staging methods becomes apparent. Saturn V was a three-stage vehicle, with each staging event occurring in serial. Ares V will be a "2.5 stage" vehicle, with one of the two staging events occurring in parallel.

Each approach to staging has its ups and downs. In brief, the staging comparison can be summed up thusly:

Serial staging:
++Most effective shedding of unused tankage mass
++More situations for a survivable abort
--Taller vehicle
--No way to verify that upper-stage engines work prior to staging-event

Parallel Staging:
++Shorter vehicle
++Confidence that engines work at liftoff; no ignition event during staging
--Less efficient shedding of unused tankage mass
--Problems in one parallel stage tend to quickly snowball into problems affecting the entire vehicle

I wanted to expound on these differences in a little bit of detail. Consider the difference in staging events between Saturn V and Ares V. On Saturn, the stages would burn out and drop off in succession. All of the tankage and structure associated with Stage 1 was shed prior to Stage 2 ignition. On Ares V, there are really three stages. The first stage consists of the SRB's and all of the core propellant that is burned during SRB-powered flight. The second stage is the remaining core propellant and structure. The third stage is the serially-staged Earth Departure Stage.

The inherent inefficiency of Ares V is that when the SRB's burn out, the core is unable to shed the remaining "first stage" mass: the portion of structural mass which would have contained the expended core propellants. When the SRB's drop off, Ares V is carrying a lot of dead weight along the way.

The same argument can be made ad nauseum about any rocket: it will be more efficient to have infinite drops of unused structural mass. But practical considerations, including complexity and reliability, limit most launchers to two or three stages. A simple optimization routine demonstrates that there is little more to be gained for every stage beyond three.

The other problem I really see with parallel staging is the ability for problems in one booster to grow into vehicle-threatening problems. The Challenger disaster is case in point: a fairly small amount of flame escaping through the aft O-Ring led to structural failure of the SRB attach strut, followed by structural failure of the external tank, followed by destruction of the orbiter by aerodynamic stresses.

The parallel staging argument actually works in favor of Ares I as a safer alternative to Delta IV Heavy for crew launch. If any of the three core boosters loses an engine, the vehicle is lost. There's no redundancy, since there's no propellant cross-feed system. (Besides, the loss of either outboard engine would probably make the vehicle uncontrollable.)

On the other hand, serially-staged vehicles have a potential safety advantage: if one of the lower stages leaves you a bit short on velocity, you merely ignite an upper stage engine to abort. Saturn V had several abort modes which relied on the third-stage engine or the Spacecraft Propulsion System. I'd assume that Ares I would also be capable of an abort using the Orion main engine.

With all this being said, it should be asked if any of these lessons can be applied to Ares V. The answer is "yes," but only if NASA is willing to sacrifice schedule and admit that there were mistakes in the previous course of action. As it stands, Ares V is taller, heavier, and possessesmore thrust than Saturn V. Yet the performance isn't appreciably better (depending on who you listen to for the Saturn V's performance.) It might be possible for Ares V to get by on a smaller core and carry additional propellant in a pair of drop tanks that would be drained during the SRB burn, and discarded during SRB staging. But this would add complexity to a very complex vehicle.

If Mike Griffin and company want to out-do Von Braun and his compatriots, they'd be wise to follow him more closely. Dump the SRB's entirely, and split the Ares V core into two serial stages. Stage 1 would use high-thrust LOX-Kerosene engines, while Stage 2 would be extremely similar in size, thrust, and Isp to the Saturn V second stage (complete with five J-2X engines.)

Building a new first stage similar to Saturn V's first stage will be challenging, but will probably be easier than when the Saturn V first stage was developed. The RD-171 engines currently available have slightly more thrust than the F-1, and significantly higher Isp. Another bonus is that the first stage has a fairly low staging velocity (around Mach 3,) so splashdown recovery might be feasible.

For all-out performance, I do have a preference for serial staging. And in looking upon the Saturn V, the genius of the Von Braun team becomes apparent with each passing day that today's NASA spends re-baselining the Ares V.