More musings on the SLS

September 21, 2011

Follow-on Saturn V designs from the 1970s (click to enlarge)

Dr. Jerry Pournelle, whom I respect a lot (even when I disagree with him), has a well-argued criticism of the Space Launch System here. I recommend it to you. Jerry has been involved with, studied, and talked to those involved with the space program for over fifty years. Jerry makes some very good points, I think.

Why are all three stages liquid hydrogen/LOX fueled? As Jerry notes, hydrogen has great specific impulse but has come problems. First, it has to be kept at extremely low temperatures – it liquifies at -423 degrees F. Second, it’s not dense, making for very large storage tanks. In fact, liquid hydrogen required five times the tank volume than high-density kerosene; but hydrogen has three times the energy of kerosene, so if the structure can be kept light enough, maybe the weight cost isn’t so important.

That’s not why the Shuttle flew with liquid hydrogen, though. Remember the 1970s, folks. Like today, everything was supposed to be “green.” LOX/LH2 rockets create nothing but water. You might get some trace from burnoff of the engines and the areas around the engines, but that’s it.

The Solid Rocket Boosters (SRBs) are a different story. If you go back and study many of the original shuttle designs, from all the companies that studied it, most had flyback boosters. The idea of the external fuel tank, the main engines being attached to the orbiter, and then requiring solid boosters to get the thing off the ground were relatively recent additions.  Now, flyback boosters were a reach in 1975, true; but so was the shuttle itself. I don’t know if it could have been built or not. Lots of companies seemed to think so.

The solid boosters are in many ways giant fireworks rockets. The fuel is powdered aluminum and ammonium perchlorate is the oxidizer. Perchlorates are supposed to be bad stuff in water supplies, but ATK says theirs is crystallized and can’t get into water supplies. However, the -chlorate part means chlorine, and it combines with the products of the main engines to produce hydrochloric acid. It’s said there is not enough produced to be a big problem. OK. Still, NASA can say the vehicle mainly produces water and is therefore very “green.”

The Saturn V used kerosene in the first stage and LH2 in the two upper stages. It seemed to work fine. I just hate to have that much liquid hydrogen sitting around. It’s a lot more dangerous and difficult to handle than kerosene.

Jerry also points out that the solid boosters had to be built in Utah for political reasons – I don’t recall, he is probably right. But that meant they had to be built in pieces to be shipped to Florida. If they were built in Louisiana like the first stage of the Saturn, they could have been shipped by water and could have been built in one segment, or at least fewer; that would have required fewer joints, which means fewer potential places for problems. The Challenger crew would most likely still be with us. The SRBs for the new vehicle have been described as five-segment instead of the four-segment versions used with the shuttle. Isn’t that more of a problem instead of less?

It so could be a super-Saturn and do a very fine job. Apparently, instead, we have to not only use the technology we’ve developed over the last thirty years, but make the same mistakes as well.


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