The future of the Mars Sample Return (MSR) mission may be uncertain, but don’t tell that to boffins at Sandia National Laboratories that tested heat shield prototypes by blasting them with rays of focused sunlight so strong Archimedes would blush.
Sandia reported yesterday that it had conducted successful tests on heat shield samples for both MSR and the Dragonfly mission which will send an autonomous rotorcraft to the surface of Saturn’s moon, Titan.
Both missions require their spacecraft to endure the extreme heat of atmospheric entry on alien worlds, relying on phenolic impregnated carbon ablator (PICA) coatings to survive.
According to NASA, which was responsible for the material’s invention in the 1990s, PICA is the lightest weight ablator able to withstand reentry conditions on Earth, Mars and Titan – and it’s made from a carbon-based composite material derived from wood pulp cellulose.
SpaceX and NASA have both made extensive use of PICA, so it’s not a new material and it’s not like it hasn’t been tested before. What’s fascinating is how Sandia goes about testing ablative materials: Not by blasting samples with an arc jet or lasers, but using the Sun’s passive power.
Witness the true power of Archimedes’ death ray
There’s a simple reason for conducting the tests on MSR’s heat shields at Sandia’s National Solar Thermal Test Facility (NSTTF): It would be the first mission to return rocks from Mars to Earth, and it will weigh more on departure than on landing.
“The heavier the payload and the bigger the entry vehicle, the hotter the vehicle gets during atmospheric entry, and the better the heat shield needs to be,” Sandia engineer and NASA test director Ken Armijo said.
The NSTTF testing facility has a decided advantage over lasers and arc jets that make it an appealing option to test the heat shields for new types of spacecraft: It’s big enough to handle pieces of material up to three feet wide – far bigger than one can hit with a laser or arc jet, which are typically limited to targets 8 inches in diameter.
“We can basically fit whole pieces of planes up there if we wanted to and blast them with concentrated sun beams,” Armijo said – all by just focusing a bunch of mirrors on a target.
Among the many experimental facilities at the NSTTF is a 200-foot (61 meter) tall tower surrounded by a field of 212 heliostatic mirrors, not unlike solar concentrators used to generate electricity. The facility can focus sunlight so that it is magnified to up to 3,500 times the amount that might hit a beach towel on a sunny day, for example. Simultaneously it blasts its target with nitrogen gas to simulate entry into oxygenless atmospheres like those on Mars and Titan.
Because blasting spacecraft material with solar energy from mirrors means expending less energy to achieve its goal, testing PICA tiles for the MSR mission and Dragonfly is also considerably cheaper at Sandia than at some other facilities. According to Armijo, testing at the NSTTF solar tower only costs around $25,000 a day. Expensive, but not compared to the more than $100k needed to fire up an arc jet or laser facility for similar use. The costs for NSTTF, Armijo told The Register, are only for personnel, operations and facility upkeep.
Depending on the type of test being conducted, solar testing at the NSTTF can also reduce energy expenditure by anywhere from 15 kilowatts to 60,000 kilowatts per test – equivalent to as many as 20,000 laundry dryer cycles, Sandia noted.
Setting up material for an NASA test of ablative spacecraft cladding atop Sandia NSTTF’s solar tower – click to enlarge
The NSTTF has been around since 1979, and has performed ablation tests on a variety of materials over the years, including the nose cones for NASA’s space shuttles, Armijo told us.
Testing of the type the PICA tiles were subjected to is mainly designed to test solar flux, a measurement of solar energy received at a particular point. Along with evaluating hypersonic and re-entry flux, the facility can also test communications under high flux, and can help understand how spacecraft will respond to different orbital maneuvers.
Anything to cut some budgetary corners
Whether the cash saved by passive solar trials of the MSR’s plating at Sandia will be enough to save the program is anyone’s guess. Testing was conducted before the latest round of trouble with the ultra-ambitious project emerged, but not before fears about its feasibility led NASA to delay certain elements.
NASA most recently said it wanted to proceed with the MSR despite concerns, and plans to significantly slash its costs by turning to the commercial space sector for help. NASA administrator Bill Nelson said in April that the space agency had to slash between $5 and $7 billion off of MSR’s cost in order to avoid having to cannibalize other programs.
“Mars Sample Return will be one of the most complex missions NASA has ever undertaken,” Nelson said earlier this year. “We need to look outside the box to find a way ahead that is both affordable and returns samples in a reasonable timeframe.”
As of October, NASA said it was awarding contracts worth up to $1.5 million to eight companies to further develop their proposals, with awards going to SpaceX, Rocket Lab, Blue Origin and others.
Keep in mind that the proposals themselves are still pretty far-fetched. SpaceX’s, for example, would involve using Starship to get the samples from Mars back to Earth. Although SpaceX has made some considerable headway with Starship’s development of late, it still has yet to successfully land one for reuse on Earth, much less Mars. Here’s to hope. ®
