One of the greatest engineering challenges in spaceflight is to protect a spacecraft from the searing heat of re-entry, as the fate of the Space Shuttle Columbia so tragically illustrated. So, one of the most intriguing technical twists in NASA's planning for its return-to-the-Moon Constellation programme may come later this month when the US space agency announces its heat shield selection for the Orion crew exploration vehicle's (CEV) crew module.

NASA looks set to choose an updated version of the material that protected Apollo programme crew capsules. Today's Avcoat - the Apollo material was created originally by Avco - would feature a modified chemical formula to stay in line with environmental legislation passed since the end of the original Moon landings programme.

By September 2008 work at NASA's California-based Ames Research Center had demonstrated that the new Avcoat was lighter than the 1960s version and performed well, according to NASA's Constellation programme's transport architecture chief and Jet Propulsion Laboratory systems engineer Brian Muirhead.

Apollo 13 Command Module
On 17 April 1970 USS Iwo Jima crewmen hoist the Apollo 13 Command Module aboard ship. The Apollo 13 spacecraft splashed down at 12:07pm on 17 April in the South Pacific Ocean

But it was never the first choice. Avcoat was the challenger to the heat shield baseline material, Phenolic Impregnated Carbon Ablator (PICA), which had been touted as the Moon mission thermal protection system (TPS) by Lockheed Martin Space Systems even before the company became Orion prime contractor in August 2006.

The company had proposed PICA as the TPS solution during the procurement process, citing its use with NASA's Stardust mission, for which Lockheed had built the return capsule. Stardust was NASA's mission to retrieve a sample of comet tail particles, launched in 1999 and returned to Earth in 2006.

The story of the Orion CEV's heat shield that has since emerged is a reflection of the Constellation programme's own history with changing requirements, design reversals and a constant struggle to save mass.

By December 2006 Lockheed had agreed with NASA that it would also use the Stardust mission's PICA and that the heat shield would be segmented in structure, not monolithic like Apollo's Avcoat.

However, the heat shield was always separate to the main $3.9 billion Orion CEV design, development, test and evaluation contract that Lockheed won in August 2006. A month later, Boeing was awarded a 16-month $14 million contract to examine PICA.

But NASA Ames was leading the work and in that September it was also looking at a super-lightweight ablative for what was then termed the Block 1 International Space Station variant CEV.

While today NASA does not refer to Orion and blocks, in the beginning there was a block 1A for crew transport to low Earth orbit, a block 1b for pressurised cargo to LEO, and for the Moon missions the block two.

In April 2007 mass issues for Orion were becoming public and the TPS was identified as one of the subsystems being targeted for mass savings.

By May 2007 the focus on the heat shield had reached a point where Boeing was awarded $10 million to look at its own PICA variant. It still had the original 16-month PICA contract, Ames was continuing with its studies and NASA awarded Textron Systems $24 million to look at Apollo's Avcoat as part of the "alternative block two TPS materials and heat shield systems advanced development" project. In the 1980s Textron had purchased Avco, Avcoat's developer.

Like the Apollo programme, which had experienced primary and secondary heat shield development studies, NASA was only to select the alternate material if the primary technology failed to meet its needs. In the 1960s Avcoat had been the primary heat shield and a back-up programme was dropped when the Avco material was shown to do the job.

Fourteen months after its initial PICA contract, Boeing delivered to NASA a prototype segmented PICA heat shield in November 2007. Under the original timetable the handover of the heat shield to Lockheed should have taken place in January 2008.

But instead, in February of that year Lockheed was expecting an Ames/Boeing PICA preliminary design review in April, which could have led to the TPS technology being handed over for CEV integration.

But this handover did not occur and by July 2008 it emerged that the heat shield had become a far worse threat to Orion's mass limit. To cope safely with lunar return NASA had realised that the TPS might have to be 20% larger. Testing to resolve this was expected to continue into 2009.

Then in September 2008 Muirhead told Flight International about Avcoat's success, it became clear that the new date for heat shield approval was early next year and Orion would not have its own preliminary design review until mid-2009, up to 16 months late.

The heat shield is intrinsically linked to the issue of Orion's mass. Selection of the TPS material may signal a resolution to that. But with Orion's design freeze not taking place until the critical design review, and not likely to happen until the end of 2010 at the earliest, this significant technical decision is not necessarily the beginning of the design review's end.

Source: Flight International