Ablative TPS - a short history

Early NASA missions (Gemini, Apollo, Mars Viking) employed new ablative TPS materials that were tailored for the specific entry environment. However, after Mars Viking, NASA-sponsored ablative TPS development essentially ceased as the research focus shifted to reusable TPS in support of the Space Shuttle. As an example, the Pioneer Venus and Galileo missions employed fully dense carbon phenolic that was developed by the United States Air Force for ballistic missile applications. Over the past 30 years NASA adopted a "risk averse" philosophy relative to TPS, i.e., use what was used before, even if it was not optimal, since it had been flight-qualified. An unintended consequence was that the ablative TPS community in the United States slowly disappeared.

The Stardust and Genesis missions were exceptions in that they employed new ablative TPS simply because those missions could not be accomplished with existing, flight-proven TPS materials.

To illustrate, Figure 1 shows a chronology of NASA entry missions that have employed ablative TPS. As seen, in over 40 years, NASA entry probes have only employed a few ablative TPS materials. The red symbols indicate materials still available. The black symbols indicate materials no longer manufactured, and the blue symbols indicate materials that may have to be re-qualified due to the unavailability of heritage precursor materials. It should be apparent that half of these materials are (or are about to be) no longer available.

Figure 1 also indicates the broad range of peak heat fluxes that these various missions encountered. Note the logarithmic scale of the ordinate. Figure 2 provides a better representation as it illustrates both peak heat flux and stagnation pressure for these missions. In addition, it includes values for the TPS mass fraction¹ for each mission. It should be apparent that NASA entry probes have successfully survived entry environments ranging from the very mild (Mars Viking ~25 W/cm² and 0.05 atm. to the extreme (Galileo ~30,000W/cm² and 7 atm.)

Jupiter Missions

Lessons Learned from Galileo

The Galileo probe to Jupiter was the most challenging entry mission ever undertaken by NASA. The probe employed a 45 deg blunt cone aeroshell and it entered the Jovian atmosphere at a velocity of ≈ 47.4 km/s. The forebody TPS employed fully dense carbon phenolic ( = 1450 kg/m³) that, at the time, was the best ablator available. The entry environment was very severe and estimates of the peak heating (combined convective and radiative) were on the order of 35 kW/cm² with a total integrated heat load of ≈ 200 kJ/cm². It is important to note that the above numbers include the effects of blockage due to ablation species.

To enable qualification testing of the TPS, NASA Ames developed and built test facilities that included a new arc jet test facility called the Giant Planet Facility (GPF) and a laser test facility to understand the spallation characteristics. The GPF arc jet operated on an H₂-He gas mixture and was capable of producing very high heat fluxes (convective and radiation) on test samples.

Figure 3 presents the convective and the radiative heating environment for many of the missions, including the Galileo and the Pioneer-Venus probes and also shows the operational environment for the GPF facility. The arc jet testing was augmented by testing with continuous wave (CW) carbon dioxide lasers which were capable of even higher heat fluxes, albeit with small spot sizes on target.