What Elon Musk would do over in the Starship design

Irene Klotz: In hindsight, if you were doing this all over again is there anything you would’ve done to make the path shorter?

Elon Musk: Hindsights 20/20 and I think there’s a lot of design choices that we would’ve made differently… I guess if I could go back in time I would probably not use hypergolic propellants because they are very expensive and difficult to handle. I wouldn’t use a blunt-body reentry vehicle. These are all design specifications that worked fairly well for low-Earth orbit, but are not well suited for going to the moon and back, or going to Mars.

That was from an Aviation Week podcast episode: Elon Musk Interview. The blunt-body was also the design of choice for the Air Force in the early days of the ICBM. The Air Force contracted General Electric’s design for the Atlas blunt-body Mk I and Mk II nosecone. The design choice was based on theory:

Allen and Eggers dismissed the pointed nose shape, a carry over from rifle bullet design, at the start, instead focusing their calculations on a blunt, hemispherical shape, recommending that “not only should pointed bodies be avoided, but that the rounded nose should have the largest radius possible.”

General Electric Mark 2 heatsink blunt body reentry vehicle, used for Atlas ICBM

The rationale was that a blunt body would reduce speed and dissipate heat in the shock wave. Reentry may produce temperatures of 12,000 degrees F while the strongest materials could only withstand half that. While the blunt body design had some successes, it was largely abandoned for ablative design where the nosecone looks more like a bullet. During reentry the materials produce a porous carbon char or resin that ultimately protects the the vehicle. This was noticed by Goddard back in the 1920s observing meteor reentry, but wasn’t readily anticipated using physical theory. Ultimately, the Army discovered this through serendipity, and used it early on for the Jupiter IRBM:

The ablative approach came from a fortuitous result of research begun in 1953, investigating materials to replace graphite for jet vane application during the development of the Redstone missile (jet vanes were used for directional control instead of gimbaling the engine). The trouble was one of quality control because while a source of the right grade of material was found, the manufacturer’s poor quality control meant that only twenty-five percent of the jet vanes were acceptable. In an attempt to find a replacement, researchers tested several materials including a jet vane made of commercial grade fiberglass-reinforced melamine. Exposure to the Redstone rocket motor exhaust eroded the vane as expected but much to the surprise of the researchers, one-quarter inch beneath the surface the material was not only undisturbed but the embedded thermocouples revealed no heating had taken place.

The Army then continued testing different materials and shapes using rocket motors to simulate reentry until it landed on the design that eventually replaced the Air Force’s blunt-body design. This episode shows how important it is to have overlapping design paths which can take different approaches and test them empirically before having “all the data” or the 100% design solution.

Operational Jupiter ablative reentry vehicle.

For more on vehicle reentry, see David Stumpf’s “Reentry Vehicle Development Leading to the Minuteman Avco Mark 5 and 11.” Air Power History Vol 61, Issue 3 (Fall 2017): 13-36.