Fuel Choice Doomed Shuttle
(Editor's Note: Julius Braun, longtime EAA Chapter 35 member, retired from the U.S. Army as a Brigadier General following 36 years of service. During that time, he conducted extensive work in research and development of missiles and rockets, working at one point with rocketry pioneer Wernher von Braun. His remarks that follow first appeared in the Express News on April 27, 1985, following the Challenger tragedy).

The blame-fixing congressional and select-committee hearings aimed at finding a suitable scapegoat to publicly castigate for the space shuttle Challenger’s unfortunate tragedy are probing in the wrong place.

There may have been faulty booster seal designs, a flawed decision tree and undue political pressure to meet a tight launch schedule. However, the true roots of the disaster go back 15 years when a very basic choice was made by non-technical legislators who mandated that NASA use large, unproven solid propellant rockets as first-stage boosters instead of tried and proven liquid propellant rockets.

Prior to the shuttle, manned space vehicles and their booster rockets could only be used for one mission. The spacecraft were usually damaged beyond repair on returning to earth and the boosters were destroyed when they fell back. Original shuttle proposals envisioned a winged, liquid-propellant, rocket powered, reusable booster about the size of a Boeing 747. It would glide to a landing after the shuttle was separated from the booster. Estimates of the cost to develop such a large liquid booster were almost as great as those for the cost for the shuttle.

Large solid-propellant booster rockets were then proposed as a cheaper solution. It was felt the program costs were increasing beyond the amount that Congress would allocate. These proposed solid boosters were to be reusable. After the propellant was consumed and the boosters separated from the shuttle, they were to be parachuted into the ocean, recovered, refurbished, reloaded and used again.

Numerous arguments were presented to influence the choice of a booster system. There were honest differences of technical judgment as to the relative merits of the two types of propellants.

The greatest support for recoverable liquid-propellant rockets was from the Wernher von Braun team at Marshall Space Flight Center in Huntsville, Ala., a team that was obsessed with reliability and safety above all for man-carrying space vehicles. They went on record in unanimous opposition to any system that couldn’t undergo the intense and repetitive ground testing they felt was required before a flight system could be certified as safe for use in manned flight.

On the other hand, support for incorporating extremely large solid-propellant rocket boosters came from many quarters, primarily from an expanding and hungry solid-rocket industry that saw the shuttle as a choice opportunity to break into the manned space program. An intense lobbying campaign was mounted to convince decision makers that solid rockets should be used.

Once the arguments for solids over liquids began to coalesce, systems analysis studies were conducted to prove that large solid rockets were more economical than liquids. As a basic assumption, most studies were anchored on an unrealistic 60 shuttle launches per year by 1986. Forecasts of development and operational problems with solid rockets were restated by the Huntsville advocates of liquids (who had gone through all this before) but were brushed aside as having little consequence in the overall decision-making process. Solid-rocket advocates claimed that the nation could not afford a large recoverable liquid-propellant booster development program. The decision went to the solids.

This is not to say that large solid rockets don’t have their place. They are well-suited for certain military applications where response time can be critical. They are somewhat simpler and smaller than liquids but trade that off in greater demands on the rest of the vehicle system, especially in such items as precise control of thrust, lower efficiency, burning termination, excessive vibration, a mandatory uniform and benign prelaunch environment, plus other negatives.

The solid-propellant Minuteman and Poseidon missiles are stored under closely controlled temperature and humidity conditions. When handled and operated out in the open, as are the shuttle boosters, things can and do go wrong.

An overriding advantage of liquid over solid systems is that flight propulsion systems can be certified as satisfactory before flight. The shuttle’s solid boosters are literally built up from sub-assemblies just prior to use. Their first hot run is always an actual launch. And once ignited they can’t be shut off except by venting or by being blown open with explosive charges. Even that only diminishes thrust; burning continues to propellant exhaustion.

Motors identical to those used in a shuttle launch are test-run lying horizontally. The fully loaded boosters are rarely static-tested upright in an actual launch posture, with their more than one million pounds of hard-rubber propellant flexing and sagging downwards. The internal stresses among the four separate segments of fuel that make up the total propellant charge almost guarantee erratic burning and local hot spots.

The large solids are adequate, but far from optimum, methods of augmenting the liquid propellant engine thrust of unmanned launch vehicles such as the Titan III and Thor Delta, where safety is secondary to payload-lifting ability. However, with their inherent deficiencies, they have no place in manned flight.

The probability of another catastrophic failure of the shuttle’s solid boosters as they are presently designed is built into them. They may operate successfully for another 24 launches, but once again the potential failure modes will fall into place and another disaster appears inevitable. A totally new solid motor design using a single-piece rocket body might eliminate the fault-prone sealed joints but would still have all the other limitations of the present solid-propellant motors.

Let’s re-examine liquid-propellant boosters for the shuttle, then using proven technology, build them under an accelerated schedule and get the program back on course.

Julius Braun