When the space shuttle’s external tank (ET) was being filled with the cryogenic fuel components LOX and LH2, moisture from the surrounding atmosphere could condense onto the ET’s sprayed-on foam insulation (SOFI). This condensation could drip onto components, such as the LOX feed line bellows, where it would freeze, forming a potentially dangerous ice block.

Ray Tracing Model shows traverse distance from beam center as a function of spot intensity.
This condensation and thus much of the ice accumulation could be eliminated by heating the SOFI surfaces with IR energy. A ray trace math model was developed to predict the intensity of IR heat energy that could be projected from a halogen lamp, or cluster of lamps, to a target surface to prevent ice accumulation. The target in this case was the area on the shuttle external tank above the LOX bellows. The ray trace model developed for this application (see figure) can easily be extended to accommodate other optical and acoustic ray tracing applications, where the approximation of ideal reflecting surfaces and incoherent waves is appropriate and where atmospheric attenuation can be neglected.

The math model and software were developed in order to assist in the design of an IR heat projection system consisting of halogen light bulbs and parabolic reflectors. Due to pad safety requirements, the system was deployed at a distance of approximately 135 m from the ET surface to be heated.

Features of the software are specifically directed toward the ET SOFI heating application using projected heat lamps. The basic math modeling technique, using Monte Carlo integration and geometric ray tracing, can lead to many other applications in optics as well as acoustics, or anything involving wave energy propagation, with the constraint that waves are incoherent, reflecting surfaces are ideal, and atmospheric attenuation can be neglected.

This work was done by Robert Youngquist of Kennedy Space Center, and Steven Klinko and John Lane of ASRC Aerospace Corporation. U.S. persons may obtain this software at no cost from the Open Channel Foundation website at http://openchannelfoundation.org/projects/Ray_Tracing_Math_Model/ . KSC-12835