"Ballutes" - a cross between balloons and parachutes - may one day act as lightweight heat shields and braking mechanisms for cargo or even people landing on the Earth or Mars from space. The concept recently received a boost from NASA, which is funding a proposal to develop the idea.
Most spacecraft use solid heat shields to protect themselves when they slam into the atmosphere from space. Those used for a single landing have "ablative" heat shields that gradually erode in the atmosphere, while the space shuttle, which is used over and over again, boasts a reusable heat shield.
But in the last few decades, engineers have toyed with the idea of flexible, lightweight heat shields. Because they would weigh less than conventional solid shields, the weight they save could be used to carry more science instruments or other gear.
Ballutes, which use pressurised gas to inflate cushions on the bottom of their payloads before entering the atmosphere, could also slow the payloads down for a soft landing. Robotic missions to Mars have dipped into the upper reaches of the planet's atmosphere to slow down and circularise their orbits. A ballute would provide even greater surface area, increasing the atmospheric drag and maximising this "aerobraking" effect.
"People are getting more and more comfortable with the notion of inflatables," says Eric Wetzel, director of engineering for the firm Andrews Space in Seattle, Washington, US. In July, for example, Bigelow Aerospace launched a test module for a space hotel that is inflatable rather than solid, like the International Space Station (see Inflatable spacecraft blows itself up).
Preliminary ballute designs have been used on bombs as they are dropped through the atmosphere.
Now, Andrews Space has received $600,000 from NASA to try to develop a ballute for space missions. Their proposal uses a novel design in which the ballute material actually releases gas to cool itself down in the atmosphere, allowing a thinner - and crucially, more lightweight - material to be used.
The ballute would inflate around a space capsule's underside and sides before entering the atmosphere. Once in the atmosphere, polymers within the layers of the ballute would heat up, releasing gas that would seep out through the fabric's pores.
The gas provides an extra buffer for the ballute. So as it rams deeper into the atmosphere, layers of the gas are stripped away, taking heat along with them. This cools the vehicle down just as sweating cools people, Wetzel told New Scientist.
As part of their contract, which was administered by NASA's Small Business Innovative Research programme, researchers at Andrews Space are testing four different materials and four different polymers that would provide the gas buffer. They plan to simulate the fierce heat of re-entry on different swatches of material using a giant blow torch, or arc jet, possibly at NASA's Ames Research Center in Mountain View, California, US.
In 2005, they tried unsuccessfully to test a ballute in the atmosphere by dropping it from a height of 30 kilometres. It was supposed to use a rocket to accelerate to 2.8 times the speed of sound, then inflate to a width of about 1 metre, using a paintball pressurised-air canister. But it never got the chance to inflate because the rocket did not fire as planned.
The company hopes to re-try the test in the coming months in eastern Oregon and eventually hopes to do test launches with a more powerful rocket that could reach much higher altitudes.
If the design works, the company believes a ballute that would inflate to a diameter of about 20 metres could be used to return crew from the International Space Station.
But ensuring ballutes are reliable enough for such use will be a challenge. Unlike solid heat shields, ballutes have to be activated before landing - and if they do not, their payloads, or crews, could be destroyed.
A 2003 paper by scientists at Ball Aerospace and Technologies Corporation, ILC Dover and NASA notes several other drawbacks, including the difficulty of developing adhesives and stitching that can withstand the high temperatures of atmospheric entry. Because the ballutes may also be stored for long periods of time before being deployed, they could become creased or damaged in flight, which could prevent them from operating as designed.
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