Reverse Velocity Rocket Sled Test Bed for Inertial Guidance Systems

The High Speed Rocket Sled Test Track at Holloman AFB, New Mexico is a precision, highly-instrumented facility which has found extensive use in the quantitative error evaluation of state-of-the-art guidance systems. The sleds, containing the guidance system tested and extensive monitoring instrumentation, are subjected to a positive and negative acceleration profile tailored to both simulate missile environment and maximize separability of test system errors. While thrust is provided by rocket engines, deceleration has traditionally been accomplished by water brake: a sledborne scoop which disperses water from a channel between the two rails. With the evolution of high speed flight computers has come a trend toward real time guidance system error compensation. This has resulted in enhanced visibility of errors such as high order nonlinearities and effects due to erroneous compensation. The isolation of system errors is so strongly a function of the sled acceleration profile which drives those errors that the success of a specific profile to aid error separation can be simulated prior to actual testing. This was shown for the case of an anticipated sled test project which was to involve high (>50 g) accelerations and was expected to propagate unusually highly-correlated errors: the result was that no level of water brake deceleration provided the velocity change necessary for error separation. Thus the reverse velocity sled concept was proposed and analyzed in 1979. This technique uses a reverse thruster to provide a sled brake of desired magnitude and time duration, the optimal retro thrust resulting in a considerable reverse sled velocity. This sled is presently under design and construction and is to be thoroughly tested in calendar year 1982. The aerodynamics of the reverse velocity sled being built are such that the sled will return to the launch point. This factor leads to new dimensions in this test concept. Firstly, start and stop points can be measured to extreme accuracy, because of their proximity, permitting excellent determination of non-cancelling errors such as asymmetries and even-powered nonlinearities. Secondly, traversal of the Test Track in two directions decouples Track survey errors from test guidance system errors. Thus, if the test system performance conforms to a deterministic model, then the test system can be used to help identify, and then remove, many errors in the Track reference itself. This paper reviews the efforts accomplished to date, including an analysis of the above-mentioned benefits of sled return to the launch point.