On October 12th, Doug spent the day at the 2014 iteration of the NASA Jet Propulsion Laboratory’s (JPL) Open House. You can read the first Lofty installment HERE, but there’s more! It was a day full of space-nerd goodness, and one of the highpoints was Site 18: “Flying Saucers for Mars.”
This particular site was dedicated to a project known to researchers by the acronym LDSD, the Low-Density Supersonic Demonstrators. Low-Density is a descriptor for Mars’s atmosphere, and Supersonic is an indication of the speed range where the balloons and parachutes are useful. To cut to the chase, we’re talking parachutes—parachutes for Mars—and how they work in a low-density atmosphere and at supersonic speeds.
Tommaso Rivellini, one of the EDL (Entry, Descent, and Landing) engineering leads for the Mars Curiosity lander, describes the problem as this in his article “The Challenges of Landing on Mars”:
Upon arrival at Mars, a spacecraft is traveling at velocities of 4 to 7 kilometers per second (km/s). For a lander to deliver its payload to the surface, 100 percent of this kinetic energy must be safely removed. Fortunately, Mars has an atmosphere substantial enough for the combination of a high-drag heat shield and a parachute to remove 99 percent and 0.98 percent respectively of the kinetic energy. Unfortunately, the Martian atmosphere is not substantial enough to bring a lander to a safe touchdown.
Kinetic energy is the energy of motion, and the wispy atmosphere of Mars—roughly 1% as dense as Earth’s atmosphere—is just thick enough for a parachute to do its job. So, unlike with Earthbound parachutes, that job doesn’t include gently lowering the lander to the surface. The atmosphere on Mars simply isn’t dense enough for a parachute to bring the mass of a spacecraft to the surface.
Our current Mars parachute designs date to the era of Viking Martian landers in 1976, and those parachute systems have reached their performance limits with the Mars Science Lander (MSL). More popularly known as Curiosity, the size of the one-ton MSL is often compared to a Mini Cooper automobile.
In order to deliver landers to Mars that are larger than Curiosity, or to land in a mountainous region—Mars has the largest mountain in the solar system in the 69,459 foot tall Olympus Mons and four other mountains which are taller than comparably puny Everest—NASA needs new parachute designs. LDSD steps in.
LDSD is suite of deceleration technologies being investigated by NASA. The project is being lead by principal investigator Dr. Ian Clark. Clark earned his PhD in Aerospace Engineering at Georgia Tech, and he has been awarded the prestigious Presidential Early Career Award for Scientists and Engineers.
The first LDSD testing mechanism that Clark discussed was a rocket sled used to test the SIAD-R (Supersonic Inflatable Aerodynamic Decelerator). This particular device isn’t a parachute. It’s more like an inflatable bladder that encircles the outer edge of a spacecraft’s aeroshell. This device is meant to slow the spacecraft from supersonic speeds (ranging from Mach 2 – 3.5) to subsonic speeds. A look at the videos with this post will give you an idea of the origin of the “flying saucer” part of the “Flying Saucers for Mars” title of this exhibit.
Clark indicated that the rocket sled, which he vividly described as a siege tower, was powered by Cold War-era solid rocket motors that had formerly been used as a part of a missile defense system for Los Angeles. Though he didn’t say it by name, Clark could only be talking about the Project Nike sites that ringed Los Angeles. It’s wonderful to think about these Cold Warriors being used for science as opposed to their original purpose.
The LDSD program also included the testing of a more traditional looking parachute, complete with a billowing canopy and long control lines. In keeping with the rigorous nature of its intended use, the parachute design also required some extreme engineering so that it might be tested in a manner that approximates its use. Because of the low density of the Martian atmosphere, the parachute has to be enormous to generate the necessary amount of drag to slow the spacecraft down. In this case, the parachute that was tested was thirty-four meters (roughly 110 feet) in diameter. A parachute this size is too large for a wind tunnel, and so it has to be tested outside. The parachute test rig resembled a Rube Goldberg device as much as something designed by NASA. For this test, a helicopter carried the parachute canopy aloft. Lines from the canopy (the line was nearly a kilometer in length) were connected via a wench/puller to yet another rocket sled. Once the helicopter released the canopy of the supersonic parachute, the rocket sled was ignited to tug on the parachute to simulate the forces to which it would be subject on Mars. In this test, the peak force generated by the rocket sled and transferred to the parachute was over 90,000 foot pounds. Although the parachute did develop a single tear, the test was deemed a success.
The second flying saucer (the test device really does resemble a saucer) of the LDSD program took part in an extremely ambitious test that was conducted this past summer. An enormous experimental balloon—it has a volume of more than 1million cubic meters and, according to Clark, when fully expanded it’s the size of the Rose Bowl—carried the test device to an altitude of 120,000 feet. Once the balloon reached this height, it released the saucer, and the fun began. A solid rocket motor fired, accelerated the saucer to Mach 4, and propelled it to an altitude of 180,000 feet. It’s necessary to conduct the test at this altitude, because this is the zone where Earth’s atmosphere most resembles that of Mars. At this point, the SIAD device expanded and began slowing the saucer from its top speed of Mach 4. At Mach 2.5, onboard sensors deployed the new supersonic parachute design. In this test, the supersonic parachute failed to fill completely with air, thus pointing out another design flaw. But, this is why testing is done, to find the weaknesses in a design. So it was a successful failure.
The total cost of the LDSD program is about $200M. Considering the price of the Curiosity mission was about $2.5B, this is a small price compared to the cost of real failure.