Rocket Team Notes Curious Design Feature

Space launch vehicles are very complex machines, and one of the deceptively challenging features of a space booster is the payload fairing. Payload fairings have to protect the delicate spacecraft, both on the ground and in flight. They must be lightweight and have to fall off safely at just the right point during ascent.

One of the less obvious challenges in fairing design is that of static charge buildup. Rockets climbing through certain weather conditions can build up an electrostatic charge on the outside surface. Such high-voltage charges can be very damaging to solid-state electronic components and, thus, they have to be dealt with, or, preferably, avoided entirely. For quite some time, the condition that was known to produce such charge buildup was a cloud layer at least 5,000 feet thick and with a temperature below freezing at some point in the cloud.

Payload fairings, even ones made of metal, generally have some degree of insulation placed on the outside of the surface to protect the payload against aerodynamic heating. Such insulation invariably is an insulator and, therefore, is subject to collecting a static charge.

I was part of a review team on a new rocket when we noted a very curious design feature.

A very thin layer of conductive material had been electrodeposited on the outer surface of the payload fairing in order to bleed off any static charge buildup. That conductive layer was prone to being rubbed off under some environmental conditions -- conditions that had to be avoided during launch.

The fairing's conductive layer could not withstand a cloud thickness greater than 3,000 feet. This actually meant the layer would be destroyed by a condition that was less than the condition it was designed to protect against. Thus, for the new rocket, the limitation became not 5,000 feet of cloud with a freezing layer, but 3,000 feet, freezing layer or not.

This was rather like having a seatbelt that disengaged when the car started moving, or a 5mph impact-resistant bumper that fell off at 4mph. We asked if this could be true, and the limitation was confirmed. The design remains in my mind the clearest case of an engineering fix being worse than the problem it was intended to correct.

Soon thereafter, the launch limitation for all vehicles became 3,000 feet of cloud with a freezing layer in the cloud.

This entry was submitted by Wayne Eleazer and edited by Rob Spiegel.

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