At an isolated facility in the Mojave Desert, protected by two security checkpoints and patrolled by armed guards, Nasa is conducting an experiment to help shape humanity’s future in space. Inside a vast harshly-lit hangar, surrounded by monitors, cables and test equipment, I watch as technicians attach wires to a giant inflatable doughnut.
It appears that either staff at the agency’s Dryden Flight Research Center have got their hands on some weird alien artefact or are constructing a surrealist art installation. In fact this six-metre wide air-filled ring is a small section of a spacecraft landing system known as a Hiad – Hypersonic Inflatable Aerodynamic Decelerator.
“You’ve seen one of the doughnuts,” says Neil Cheatwood, who leads the Hiad project. “Imagine a stack of doughnuts forming a pyramid – like the hats the [cult ‘80s] musical group Devo used to wear.” (Devo’s said headwear even has its own Wikipedia page)
Cheatwood is attempting to tackle a fundamental problem with landing astronauts on Mars or returning them to Earth from other worlds: current spacecraft designs would not make it to the ground in one piece. They would be coming too fast and getting too hot, smashing into the ground or burning up in the atmosphere. The existing technologies used for heatshields – also known as aeroshells – are not up to the task.
Capsules returning to the ground from orbit, such as the Russian Soyuz or the privately funded SpaceX Dragon, are protected by heat shields designed to withstand entering the atmosphere at around 7.5km per second. But astronauts coming back from the Moon are travelling at 11km per second; from Mars they would be going at a phenomenal 14km per second.
Rather than building ever-wider and thicker spacecraft heatshields, the engineers behind Hiad use giant conical stacks of inflatable doughnuts. These can act as both a heat shield and a brake by increasing the drag on the spacecraft as it enters the atmosphere.
“The drag is the force of molecules hitting your vehicle, with a larger vehicle you can imagine it is like having a larger net. This catches more molecules and it slows you down earlier in the trajectory,” explains Cheatwood. “When it comes to heat shields, size really does matter.”
The advantage of an inflatable system is that you do not have to build a massively wide rocket to accommodate a fixed heatshield. For a return from Mars, for example, the Hiad could be packed away in a suitcase-sized compartment and inflated up to 26 metres across – large enough to slow the spacecraft down in even the thin outer atmosphere.
Although the concept was originally conceived in the 1950s, at the dawn of the space age, effective Hiads are only possible with modern materials. Nasa is currently experimenting with a sandwich of thermally resistant textiles, which includes an outer layer to take the heat and insulating layers for cooling. Only a few millimetres thick, it is similar to the fabric that is used for firefighter outfits or thermal blankets but capable of enduring temperatures of more than 1200C (2192F).
So how would this work in practice? Imagine you are returning from Mars: It has been a productive 18-month mission but as you speed at 50,000km/h across the Solar System, the home world is starting to get alarmingly big in the window.
Nearing the Earth, you activate the Hiad inflator system to deploy a vast conical structure in front of your spacecraft. Flying pointy-end first, it not only slows you down, but dissipates some of the heat of re-entry. Once your capsule speed has reduced to a more reasonable 7.5km per second, you could jettison the Hiad and safely parachute to splashdown.
However, this is not some far future technology for a hypothetical mission to Mars. Hiads may be needed far sooner to complement the aeroshells used to protect rovers or landers during unmanned missions to other planets.
“If I wanted to land the Curiosity rover at a higher altitude on Mars, we really can’t do it with the existing aeroshell,” says Cheatwood. “The sequence of events we have to go through from entering the atmosphere to sky-crane ignition, we just run out of time.”
Or, to put it another way, a multi-billion-dollar mission ends up in tiny pieces, scattered across the Martian soil.
The same applies with trying to land a heavier rover. Given the size of existing interplanetary rockets, right now the one-tonne Curiosity rover is the largest thing Nasa can land on Mars. “If we wanted to land a bigger rover or humans, we just can’t do it with the size of the existing aeroshell,” says Cheatwood. “If you double the mass inside it, it falls much faster and you run out of time.”
Cheatwood has been working on Hiads for more than 10 years and, as well as wind tunnel tests, has flown several smaller versions of the system on sub-orbital ‘sounding’ rockets to see how they perform in space. Nasa has even developed an online game, so you can try it for yourself.
The elaborate test rig that I have come to see at Nasa’s Dryden facility is designed to subject each inflatable ring to the stresses and strains they might face in space. You almost feel sorry for the doughnut, as the technicians attach dozens of wires anchored to its surface to hydraulic jacks in the floor. They are preparing to pull, stretch, wrench and twist the thing.
I wonder if future astronauts will ever trust their fate to such an apparently flimsy bit of technology? Cheatwood worried about this too – until he talked with Apollo 17 moonwalker Harrison Schmitt.
“Schmitt said, ‘If the engineers had said that was the right thing to do, that’s what we’d do’,’” recalls Cheatwood. “From the technology point of view you could fly humans with this and that’s what we’ll be doing if we go to Mars.”
So, if you are booked on one of the first flights to Mars, remember to pack (giant inflatable) doughnuts.