Cuddihy's Cut

Cuddihy's Cut on the events of the day....


Monday, May 15, 2006

Fake but accurate? and, LH2 vs RP-1.

NASAWatch posted the official cancellation of the reusable RS-25 for the CaLV. Presumably, this means that NASA is indeed going to the RS-68 with a 10 m rocket body, as originally reported in a now retracted article.

It seems like the RS-68 is indeed the new choice for the 'SDLV', which, as others have now pointed out, has very little left that is 'shuttle derived.'

In fact, as Chairforce Engineer has stated, the SDLV now bears more in common with the Saturn V rocket than the Shuttle. All that's left, in my opinion, is to shift from the absurdity of a LH2 powered first stage to a Kerosene (RP-1) first stage. Then, at last, we would be back to where we started. Von Braun proven right again.

Lately it's become pretty clear to me that using LH2 for a lower stage is pretty much a fool's errand. I recently had to run some launch numbers for class (capstone) project that required launching multiple heavy satellites to a high LEO orbit (500 km and 1000 km). The only two launchers I was allowed to consider were the Atlas V and Delta IV. Imagine my shock when I discovered just how bad the Delta is for LEO launch. It's so bad that Boeing doesn't even publish true launch curves for LEO in the the payload user's guide. I had to reconstruct them from published launch capacity to 185 km circ, estimated LEO launch mass, Is, and mass fraction.

I calculated the following numbers for payload to LEO(metric tons)*:



Rocketpayload to 500km(circ)payload to 1000km (circ)
Atlas V 40210.3t9.1t
Atlas V 52213.5t12.0t
Atlas V 55215.8t14.0t
Delta IV 5,25.6t5.1t
Delta IV 5,48.1t7.2t
Delta IV Heavy17.4t16.2t

* these numbers included an inclination penalty to launch into a 64.3 degree orbit from the Cape, if anybody's looking at the actual payload curves.


Amazing, isn't it? the smallest Atlas V (402), with no solids on it, demolishes all but the Heavy, three core Delta IV. Which makes it all the more curious that the DOD still has contracts for Boeing to launch satellites to LEO with medium Deltas. WTF?

The Delta doesn't really gain any advantage until you start looking at higher altitudes. This is entirely due to the higher Isp RL-10B-2 (Isp 462s) vs. the RL-10A-4 (Isp 450s). It has nothing to do with the first stage.

Virtually any introductory course to rocketry will stress that higher ISP is 'better'. Most people liken it to miles per gallon in an automobile, an especially inapt comparison IMHO. By that measure, the first stage of the Delta IV, boasting the Isp 420s RS-68, should destroy the 'wasteful' Atlas V, w/ Isp 311s RD-180. The RD-180 has higher thrust to weight (70 for RD-180 vs 40 for RS-68) and overall thrust. But, of course, RP-1 weighs a lot more than LH2.

So what explains the discrepancy? The big problem as I understand it s that the Delta IV is thrust limited. In other words, the RS-68 has to limit its thrust while in the atmosphere to keep from structurally damaging the rocket itself. This has the effect of dramatically increasing the burn time of the first stage engine. As the Ambivalent Engineer makes clear in an excellent discussion on staging, more burn time for the first stage means higher gravity losses incurred for the same total impulse. So basically, Boeing developed this super-whamodyne LH2/LOX common core booster with a big honkin RS-68, for perceived higher efficiency, only to have to use it suboptimally because--oops, the first stage has to go through the atmosphere. It's a bit like dropping a high-torque truck engine into a small racecar, but forgetting that all that torgue goes to waste if the low-gear transmission and 10 inch wheels can't deal with it.

The Atlas V, on the other hand, with a much heavier mass at takeoff, is structurally stronger to begin with and runs at nearly its full thrust rating for the entire first stage burn. It incurs smaller gravity losses and the first stage is able to add a higher velocity increment as a result, which increases the payload on the second stage, despite having a lower 'efficiency' engine than the Delta IV.

Add to that the difficulty and expense of working with LH2, and the massive amounts of it you need on a lower stage, it makes one wonder--why use it on the CaLV lower stage at all?

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