The latest political nuisance for NASA is that senators and congressmen from Utah are trying to pressure NASA to promise that any new heavy-lift launcher it designs will include derivatives of the shuttle’s solid rocket boosters (SRBs). They’re even claiming that the recent NASA authorisation bill that President Obama signed into law in October legally requires it. Hogwash!

The recent authorisation bill does direct NASA to proceed (subject to funding) with development of a heavy-lift launcher to support future space exploration. However, nowhere does it require use of the shuttle SRBs. The farthest it goes toward that is to specify that NASA should use existing technology from the shuttle and the now-defunct Ares launchers “to the extent practicable”.

Whether it is necessary, “practicable”, or even desirable to use SRBs in a new man-rated heavy-lift launcher depends very much on who you ask. It seems clear who the Utah legislators have been asking – ATK, the Utah-based company that makes the shuttle SRBs and was expecting to make derivatives of them for Ares. Observers with less of a financial stake in the decision might disagree.

Saturn V precedent

Claiming that solid rockets are necessary for a heavy-lift launcher is obvious nonsense. The US’s previous heavy-lift launcher, the Saturn V, used no solid rockets and lifted a bigger load than the new launcher is required to carry.


What’s “practicable” depends very much on who analyses the problem. Usually the devil is not in the details, but in the assumptions.

One key assumption involves the engine used to power the launcher’s first stage, which is used to push the rocket off the ground. The launcher will probably weigh 2000 tonnes or more at takeoff (the Saturn V weighed about 3000). That requires a lot of thrust, which can be attained with either a few big engines, or a large cluster of smaller ones.

The US has no really big liquid-fuel rocket engine in production today, and historically has been averse to large engine clusters. That leads people to think that SRBs are about the only answer.

Clusters of engines

But there are alternatives. The simplest would be to put the Saturn V’s big first-stage engine, the F-1, back into production. It’s an old design, and not as fuel-efficient as more modern ones, but it worked well and had plenty of thrust. (Or, of course, you could buy big Russian engines. But Congress really doesn’t like that idea.)

The other option is to bite the bullet and use big clusters of engines. The RS-68, currently in production for Delta IV rockets, could probably do it with seven or eight first-stage engines. (The Saturn IB had eight engines in its first stage, and SpaceX’s Falcon 9 has nine.)

This does seem like more work than just using the SRBs. Is there a good reason for it? Yes. Large solid rockets have never been a very good way to build launchers that might have crews on top, especially because of the problems in getting the crew away from a failing launcher.

Parachute danger

For one thing, shutting solid rockets down in an emergency is difficult. Contrary to popular belief, it’s not impossible, but it’s a somewhat violent process and that creates its own problems. (Solid boosters were chosen for the shuttle on the assumption that they could be shut down on command, but the decision wasn’t reconsidered when that proved to create impossible structural loads.)

For another, while solid rockets can be fairly reliable if you spend enough money on them, when they do fail, their failures are often catastrophic. (Ailing liquid-fuel engines, by contrast, usually just shut down when they fail – this happened several times on the Saturns.)

Moreover, a disintegrating solid booster creates its own special hazard to an escaping crew capsule: a huge shower of chunks of flaming fuel, the heat from which can destroy the capsule’s parachutes.

Finally, all of this is on top of the problems that solid fuels create even when everything works perfectly: vibration, pre-launch handling hazards, and atmospheric pollution.

There is plenty of reason to avoid using solid boosters in a new heavy-lift launcher, and there are feasible ways to do it. Steering clear of them would present a political problem: the loss of jobs in Utah. It’s worth it.

Henry Spencer is a computer programmer, spacecraft engineer and amateur space historian