The basic Viking parachute design that was used to land vehicles on Mars in 1976 was the same design used to deliver the Curiosity rover to Mars in 2012. While it has been more than 40 years since the initial mission, many of the technologies haven’t changed, until recently.
View: Photos of the Day: Testing NASA’s Supersonic Decelerators
“We’ve been growing our expertise and knowledge, and improving the technologies in the areas associated with planetary exploration and landing spacecraft,” says Ian Clark, particularly, NASA’s Low Density Supersonic Decelerator, for which Clark is the principal investigator.
“We want to grow our landers bigger, and as we cast our eyes to the horizon and think about landing humans on Mars, we need the capability of slowing down even larger masses than what we’ve been able to do thus far,” adds Clark. “That really means having more drag produced, so we’re developing technologies to grow that capability.”
The two main technologies being developed as part of the Low Density Supersonic Decelerator Project (LDSD) are supersonic inflatable aerodynamic decelerators (SIAD) and a new supersonic ring sail parachute.
Before a spacecraft can land safely, it must slow from six kilometers per second to 500 meters per second, depending on its mass. Both the SIAD and the parachute are used to do this.
As an inflatable drag device, the SIAD is intended to be deployed around Mach 4, providing increased drag on the entry vehicle so it can reach the conditions at which the supersonic parachute can be deployed.
The parachute is 100 feet in diameter, and is constructed mainly of Kevlar, nylon, and Technora. “It’s about two and half times the area of the biggest parachute we’ve ever used on Mars, and is a little bit of a new design,” explains Clark. “It’s something that we think will allow us to grow our landing capabilities from a one ton capability towards two to three tons in the near term.”
However, the technologies being developed will be extensible to even larger payloads.
“When thinking about humans on Mars, we start thinking about payloads in the 15-ton class,” adds Clark. “That’s more mass than anything we’ve been able to land on Mars to date.”
The SIAD is a woven textile-based inflatable drag device that “looks like an inflated donut,” says Clark. Attached directly to the front of the landing vehicle, the SIAD inflates in a fraction of a second to a fairly low pressure of around four PSI. “Even at the low pressure, it acts like a rigid structure and provides more surface to react against the atmosphere to produce more drag to slow the vehicle down.”
The SIAD grows the test vehicle from about 4.7 meters (15 feet) to about six meters (20 feet) in diameter, and includes a secondary donut around the periphery, called a burble fence, that is used for aerodynamic stability. For testing purposing, the SIAD will be inflated using off-the-shelf automotive gas generators.
“You [inflate] it very quickly because you don’t want this asymmetrical, very floppy, flexible structure existing, where it could be disturbing the motions of the vehicle,” says Clark. “If it only inflated on one side and stayed like that for any length of time, it could start perturbing and even tumbling the vehicle.”
Testing to Scale
In order to gain confidence that the devices will operate the way they need to en route to Mars, the sytems have to be tested at the scale and in the conditions it will be used.
To do that, NASA has built a full-scale replica of a Mars entry vehicle to which the technologies are attached. The vehicle resembles a flying saucer and will be hoisted off the beach in Hawaii, out over the Pacific Ocean, using a large scientific balloon.
At 34 million cubic feet, the balloon will be the size of a football stadium when it reaches altitude. Once the balloon reaches 120,000 feet, the vehicle is released and a large, solid rocket motor fires to accelerate the test vehicle to 180,000 feet at about four times the speed of sound.
“At that altitude the atmosphere of Earth is very similar to the atmosphere of Mars,” explains Clark. The atmosphere is very thin, with a low density. “At that point we would be going as fast as we would be towards Mars when we use these devices. Once we get to those conditions, we would inflate the SIAD to slow the vehicle down to around Mach 3 and we’ll be able to deploy the parachute, which will deploy at Mach 2.5.” From there, both the devices will slow the vehicle down and carry it to the Pacific Ocean.
Before the team begins testing the technologies, they must make sure that the test system is adequate. “With these [initial] tests, all we’re trying to do is shake out the test vehicle, the balloon launch, all of the operations,” says Clark. “When you’re trying to qualify a technology, there are many different questions that need to be addressed, but it’s difficult, if not impossible, to do a test that answers all your questions.”
While the team has had a number of obstacles, the biggest challenge has been developing tests to validate their technologies, as the sheer size of the devices and the conditions they will encounter extend beyond what they have previously been capable of replicating.
“We have grown the device to sizes and environments beyond the test capabilities that existed,” adds Clark. “So we had to come up with solutions of how to do the testing that was necessary.”
One of these solutions was modifying a rocket sled test bed to test the structural stability of the supersonic parachute. A relatively cheap task, the test used welded steel on a standard gage railroad track and surplus solid rocket motors.
“We hoist the parachute to about 4,500 feet underneath a helicopter. It is released while connected to the rocket sled by a 4,000 foot rope,” explains Clark. Then, as the parachute is released from the helicopter and begins inflating, the rockets on the sled accelerate horizontally down the track, pulling the parachute vertically towards the ground with 120,000 pounds of drag force.
“It sounds like a Rube Goldberg device, but it was actually a very creative solution,” says Clark. “We had to find creative ways of using existing materials, existing coatings, and existing manufacturing techniques.”
The first opportunity to use these new techniques for their intended purpose will most likely be the next Mars mission in 2020.
As far as landing humans on Mars, the technologies being developed are some of the first steps. “Sending humans to Mars is a very long term endeavor,” adds Clark, who explains that “the trip to Mars is longer than the eight months it takes a spacecraft to go from Earth to Mars. It’s the years preceding, all of the design, development, testing, and retesting.”
Filed Under: Aerospace + defense