Preparing Planetary Parachutes

In previous posts I’ve talked a little about the challenges of landing on other planets. A planet’s unique composition and gravity have a major effect on a spacecraft’s entry into and descent through the atmosphere, as well as its landing on the surface – more simply, the spacecraft’s EDL. One central problem associated with interplanetary missions that has always fascinated me is the problem of testing EDL systems without being able to simulate the target planet’s environment. (Pictured, an artist’s concept of the Mars Pathfinder mission’s terminal descent and landing.)

Take Mars for example. The planet’s atmosphere is one percent as thick as the Earth’s and Martian gravity is one-third that felt on Earth. It’s impossible to simulate these conditions. Individual systems can be tested separately in approximate environments, but the only time the whole system will go through the stages of EDL is when it arrives at Mars.

In looking up results from Martian EDL systems tests, I kept turning up information on parachutes. I’ve never thought any aspect of spaceflight was simple, but I also never thought parachutes were very interesting. Turns out they are. For the moment, I’m going to limit myself to Mars landings and the Mars Science Laboratory (MSL) in particular. Set to launch in November 2011, MSL will use the brilliant yet frighteningly complex Sky Crane landing system to deliver the rover Curiosity to the surface.

But if the Sky Crane is going to have a chance to do its job, the whole MSL payload is going to have to enter the atmosphere and slow to a speed where its retrorockets can take over. That is the job of its parachute.

MSL’s EDL will be the most demanding of any missions sent to Mars. Curiosity is significantly heavier than anything sent before; at nearly 2,000 pounds, the rover dwarfs its predecessors Spirit and Opportunity who weigh a little over 400 pounds each. As a reference point for how much Martian missions have changed in a little over a decade, the first rover on Mars was the diminutive 24-pound microwave-on-wheels-looking Sojourner that landed in 1997. (Left, a schematic of the Sky Crane’s final descent and delivery of Curiosity to the Martian Surface.)

Previous missions used the topography of Mars to simplify landings; landing points in low-altitude areas like valleys take advantage of and prolong the drag generated during the parachute-assisted descent. MSL, however, is out to prove that a payload can be delivered to the surface at higher points on the planet. This will open up more of the Martian surface for exploration for future missions. To this end, the MSL descent vehicle is designed to generate some lift during its descent, but it will also have a shorter descent time. (Right, Sojourner.)

The heavier rover and Sky Crane system necessitate a larger descent module, which combines with the elevated landing site to place increased demands on the parachute. It will have to slow a heavier mass in a shorter time than has ever been attempted on another planet. MSL’s parachute will deploy 4 miles (6.5 km) above the surface when the payload is traveling at nearly 1,000 miles per hour (447 metres per second). The chute will have a 3.41 mile (5.5 km) descent to slow the payload’s fall to 187 miles per hour (84 metres per second).

The parachute has 3.41 vertical miles (5.5 km) to slow the descent vehicle’s speed by 813 miles per hour (363 metres per second). According to the EDL profile, this will happen 85 seconds. A lot has to happen very fast.

Before going into more detail about the design and testing of MSL’s parachute, some background into the parachutes’ overall flight qualifications might be useful.

Parachutes on interplanetary missions must meet four basic flight qualifications: volume, drag, strength, and stability. The parachute must fit into its casing without going over its weight allowance when packed, it must provide enough drag to safely deliver the payload to the planet’s surface, the material must be strong enough to withstand the air rushing into it during the initial descent, and the parachute must oscillate as little as possible during descent to ensure the payload has a smooth fall to the surface. (Picture, the parachutes are packed for an Ares rocket test flight. Packing a parachute is a delicate operation, largely done by hand by skilled workers.)

It stands to reason that a larger parachute will provide more drag because of its increased surface area, a thicker material will be stronger, and a multiple parachutes (like Apollo’s three parachutes) will increase stability. But all these factors add weight and volume, and any design failing to meet two basic flight qualifications is not worth pursuing.

For MSL, engineers settled on a single disk-band-gap (DBG, pictured) parachute. 80 suspension lines connect the parachute to the bridle, which in turn connects to the descent vehicle. This is a tried and true design and makes up the majority of parachute types sent to other planets and increases the payload’s odds of a straight descent path. Its name comes from its configuration. The main disk- or dome-shaped canopy has a hole in the top to relieve the air pressure. Below the main canopy is a gap, followed by a fabric band. The band increases the parachutes lateral stability. It controls the direction of incoming air, and a thicker band ensures a more centered the airflow and a more stable overall parachute. The gap above the band lets air vent out. This prevents the main canopy from rupturing under pressure and allows the band to do its job. Without the gap, any lateral winds the parachute may encounter could pull the whole configuration sideways.

The choice of a single parachute was intended to simplify the mission’s EDL; engineers could focus on maximizing the performance of a single system instead of working out interdependencies between multiple systems. While simpler, a single parachute is also a risk; the flip side of simplicity is that MSL has no backup. NASA decided to move away from the redundancies that have been a staple since the organization’s inception.

Apollo’s three parachutes is an example of redundancy – if one failed the crew would still manage a soft splashdown. It was also part of reason behind the twin rovers Spirit and Opportunity. If one failed, there was a backup. A parachute failure on MSL would be unrecoverable, though a common fate that befalls missions sent to Mars. (Left, Apollo 15′s splashdown using a ringsail parachute design. The failed parachute did no harm to the crew. 1971.)

MSL’s parachute is the largest ever sent to another planet with a 64.6-foot (19.7 metre) diameter. For a comparison, the next largest parachutes used on Mars were the 52.9 foot (16.15 metres) diameter parachutes that delivered Viking 1 and 2 to the surface in 1976. Rovers have typically used smaller chutes - Sojourner arrived via a 40.6-foot (12.4 metre) parachute, and Spirit and Opportunity were each delivered by a 49.5-foot (15.09 meter) parachutes.

Regardless of size, each mission to land on Mars has used the same mode of parachute deployment and MSL is no exception. The parachute is packed into a case for the interplanetary journey. At a preset altitude from the target planet’s surface, a mortar fires the chute. It unfurls and inflates, increasing the drag of the payload and slowing its descent.

Going into testing for MSL’s parachute, NASA had a wealth of experience to draw from – the deployment method was known as were the flight characteristic of a DGB parachute in Martian atmosphere. The challenge was how to test a parachute this large. Would the larger chute exhibit the same characteristics as its smaller predecessors? Would the material hold up with such a large surface area?

There were three main questions to answer: will the chute deploy correctly at speed as it falls through the atmosphere, will it inflate properly in the thin Martian atmosphere, and will it provide the expected and necessary drag and stability to deliver its payload safely to the surface.

A parachute’s deployment is fairly easy to test on Earth, the key is using a wind tunnel. The parachute is packed into its canister. Wind speed in the tunnel is increased to mimic the air speed the payload will encounter during descent. The mortar fires, releasing the parachute into the airstream. (Right, a parachute on the way to full inflation during a wind tunnel test.)

Engineers watch how the parachute unfurls and how long the it takes to inflate; any delay or awkward movement is usually indicative of a structural problem or an unbalanced design. The bulk of problems come from air catching the edge of the parachute, problems such as “squidding” occur when the edges of the chute are forced into the canopy and it fails to inflate at all. An uneven unfurling can cause the chute to turn inside out, inflating but not providing the drag needed to slow the payload.

Wind tunnel tests are also a good way to test a parachute’s strength. If the chute rips, a stronger or different material might be in order. This is also a way for engineers to get a sense of the design’s stability. Any oscillations can be cancelled by increasing the thickness of the band. If it deploys and remains solid in the wind tunnel without oscillating, the design is ready to fly.

Wind tunnel tests have the obvious drawback of putting the parachute through its paces at sea level where the atmosphere is thicker and there is more air to inflate the chute. The thin Martian atmosphere is better approximated through drop test – dropping the chute from altitude through thinner upper atmosphere is a good indication of how the chute will react on Mars in terms of inflation, strength, and stability. A parachute, either full or part-scale, is attached to a weight that mimics the relative weight of its payload. The configuration is towed to altitude and dropped while engineers on the ground watch and observe. (Left, a drop test as seen from the perspective of the payload.)

But MSL’s parachute deployment is more complicated than can be easily tested in a wind tunnel alone or a simple drop test. The mission has the parachute inflate at 1,000 miles per hour (447 metres per second). In the Martian atmosphere this translates to Mach 2.0. The parachute will have to deploy and inflate at supersonic speeds. This increases the loads and stress put on the canopy and also presents a less stable environment. Prior testing showed that parachute deployment at speeds in excess of Mach 1.4 present unstable conditions.

So while drop tests approximate the thin atmosphere, there are limits; the top speed at deployment and inflation is the terminal velocity of the payload, certainly a subsonic speed. Testing inflation in a thin atmosphere at supersonic speeds is a harder nut to crack, but NASA has done it in the past with high altitude rocket-assisted drop tests. These tests offer the closest Earthly approximation of conditions of an EDL on Mars.

In the late 1960s and early 1970s when NASA was enjoying the financial freedom associated with the Apollo program, there existed three programs designed exclusively to test parachutes: the Planetary Entry Parachute Porgram (PEPP), the Supersonic Planetary Entry Detector Program (SPED), and the Supersonic High Altitude Parachute Experiments (SHAPE).

Between these three programs, one series of tests put various parachute designs through sixteen high altitude supersonic tests. Eight tests used a DGB parachute. One test in 1968 used a 65-foot (19.8 metre) chute (video). The method was simple. A packed parachute attached to its payload was towed to an altitude close to 25 miles (40 kilometers) by balloon. After separating from the balloon, rockets accelerated the simulated spacecraft to supersonic speeds and the parachute system was deployed. All this happened at altitude to simulate Martian atmosphere. Speeds reached were equivalent to those anticipated during entry.

The tests were successful and proved the feasibility of deploying a parachute at supersonic speed in a thin atmosphere. It would inflate and could produce enough drag to deliver its cargo safely to the surface.

But repeating these tests for MSL wasn’t an option. Early in the program, NASA determined it would be prohibitively expensive to test the parachute via high altitude rocket-assisted drops. So, to determine MSL’s parachute’s likely flight characteristics, NASA relied heavily on the data gathered over 40 years previously.

NASA took its reliance on prior experience further. Rather than design a new parachute, MSL’s chute is a scaled up version of the design used with the Viking landers. This allowed engineers also use the data gathered and experience gained form previous landings on the planet. (Pictured, a full-scale wind tunnel test. A fully inflated, solid chute is a flight worty chute.)

Scaling up a previous design, however, is more complicated than simply maintaining the ratio between parachute and payload. A larger parachute requires a larger pack and a more powerful mortar to deploy the system. Without drop tests, firing this larger and heavier parachute was limited to wind tunnel tests.

Qualifying the Viking-designed parachute for MSL required certain upgrades. The bridle that connects the parachute to its payload – the line that will sever when the Sky Crane’s retros take over – was strengthened. A new material was used for the tension lines. The stronger material and size of the chute requires a series of cords to keep the whole things stowed during its trip to Mars.  The casing for the parachute had to be totally reconfigured to accommodate the added volume these changes brought.

In addition to the full-scale tests, NASA ran a series of smaller scale wind tunnel tests to verify the materials used for the MSL parachute. A 4 percent scale model was used – 2.6 foot (0.7 metre) diameter. The mini parachute was successfully deployed at speed upwards of Mach 2.5 and displayed no structural or inflation problems.

Still, as with all interplanetary missions and spaceflight in general, there are situations engineers and program managers can’t predict and potential problems they can’t simulate. The Viking parachutes and 1960s tests have served as a guide, but the full MSL parachute hasn’t and can’t be tested on Earth. At least, not without expensive high altitude rocket-assisted drops. (Left, an artist’s concept of Spirit and Curiosity on Mars. The size difference is evident.)

As with all systems sent into space, MSL’s parachute is designed to work perfectly. It should therefore come as no surprise if and when the MSL parachute deploys and inflates perfectly, delivering Curiosity to the surface in perfect health. Still, a failure of the parachute would be catastrophic. There’s nothing to do but wait and see what happens. The system will go through its only full test on Mars. Here’s hoping it’s a success.

Suggested Reading/Selected Sources

1. David W. Way et al. “Mars Science Laboratory: Entry, DEscent, and Landing System Performance” NASA Langley Research Centre. 2006.

2. Douglas S. Adams. “Lessons Learned and Flight Experience from Planetary Parachute Deployment”. JPL, California Institute of Technology.

3. Robert Mitcheltree et al. “Mars Science Laboratory Entry Descent and Landing System Verification and Validation Program. JPL, California Institute of Technology.

4. Reinhold A Lemke et al. “Design Report 65 Foot Diameter D-G-B Parachute Planetary Entry Parachute Program”. NASA. 1967.

5. Footage of two MSL wind tunnel deployment tests.

6. High Definition footage of a wind tunnel parachute test for the Mars Exploration Rovers Spirit and Opportunity.

Trackbacks

Leave a Reply