Start Transient Leak Detection System (STLDS)


Cryogenic propulsion systems use extremely cold propellants and high pressures to achieve the thrust-to-weight ratios required for space vehicles. Leakage from these systems during operation can cause changes in pressure within an enclosed fuselage (such as the Space Shuttle vehicle [SSV]) that are significant enough to be recognized as the initiation of a potentially catastrophic leak. Monitoring for these unexpected changes in pressure can allow detection of a leak in time to abort a launch and possibly prevent damage to or loss of the vehicle.

Driving Event

The Space Shuttle Main Propulsion System (MPS) and Main Engine (SSME) power heads are enclosed within the aft fuselage of the Orbiter vehicle. There is no way to detect the initiation of significant and potentially catastrophic MPS/SSME system leakage within the aft fuselage during SSME thrust buildup (6.6 seconds to T-0). During SSME test firing at the Main Propulsion Test Article (MPTA) (SF6-01), a high-pressure hydrogen leak developed after ignition that resulted in an overpressure of the aft fuselage severe enough to blow off the heat shield simulators.

Lesson(s) Learned

  • Integrity of the SSV MPS/SSME following SSME start must be derived as acceptable based on historical data and satisfaction of prestart requirements.
  • Because there is no real-time leak detection following SSME start, it is possible to allow lift-off with a significant leak occurring.


Dedicate real-time onboard monitoring of the aft fuselage environment, using delta pressure and possibly delta temperature measurements to confirm the stability of the MPS/SSME postignition/preliftoff. Implement monitoring with enough redundancy (three of four) to allow an abort decision if conditions warrant.
The following is an SSV-specific example:
  1. Determine the minimum amount of LO2, GO2, LH2, or GH2 leakage required to affect the aft fuselage pressure, temperature, density, or vapor distribution/optics (or a combination of these parameters) enough to be detectable by available technology. To be of use, candidate technology would have to provide high-sample-rate data to allow recognition of, and reaction to, a hazard developing between 6.6 seconds and T-0.
  2. Then, determine if this amount of leakage is sufficiently below a level that would affect SSME ignition "confirm" or initial main stage "verification." If derived leakage values are determined to be low enough not to result in a shutdown caused by degraded performance, implementation may be warranted.
The Propulsion System Integration Group (PSIG) started an evaluation of this approach for Space Shuttle Program implementation in April 2004 (Discussion Item 040407-D05, attached). Initial analysis was promising, but the effort was overcome by other events and was never completed.

Note that high-sample-rate delta pressure measurements are a candidate for this task.

Program Relation

Space Shuttle Program

Program/Project Phase


Mission Directorate(s)

  • Aeronautics Research


  • Cryogenic Systems
  • Propulsion
  • Launch support systems
  • Vehicle concepts
  • Risk management
  • Reliability
  • Level II/III requirements definition