This week, I was at Lockheed Martin Space Systems in Littleton, Colorado for the Critical Design Review of our Touch-and-Go Sample Acquisition Mechanism (TAGSAM) and the Sample Return Capsule (SRC). The review went very well and Lockheed Martin has made great progress in the design of these units, as well as the detailed planning for manufacturing, assembing, and testing of the devices that will fly to Bennu and deliver the samples safely back to Earth.
The review reminded me of one the best experiences I have had to date as OSIRIS-REx Principal Investigator. In October 2012, members of the OSIRIS-REx team worked with the NASA Reduced Gravity Office (RGO) to test TAGSAM under microgravity conditions. We gathered at the Ellington Field RGO hanger on Monday October 15, 2012 to unpack and prepare equipment and conduct the Test Readiness Review. In the afternoon, the test engineers prepared and loaded the test equipment onto the aircraft. The rest of us went through reduced gravity training at the JSC Neutral Buoyancy Facility. The training mostly dealt with what to do if you have to vomit – hence the colloquial name of our ride.
The NASA Lyndon B. Johnson Space Center in Houston, Texas operates the NASA Reduced Gravity Program. It provided a unique “weightless” or “zero-g” environment of space flight for test and training purposes. This environment was created using the C-9 aircraft, a two-engine turbofan aircraft that is a military version of the McDonnell Douglas DC-9 used for many years by commercial airlines. The aircraft flies a parabolic trajectory to achieve the reduced gravity condition. Imagine the feeling you get at the top of a hill on a rollercoaster. That experience is a mini version of the sensation achieved at the top of a reduced gravity parabola.
NASA acquired the C-9 aircraft on August 9, 2003 for $35 million. The aircraft was commissioned for service on January 15, 1970 and was operated by KLM Airlines for over 19 years. The U.S. Navy and U.S. Air Force used the C-9 aircraft in support of passenger transportation, medical evacuation, and special missions from July 1989 through August 2003. Starting in 2003, the primary mission of the NASA C-9 was to provide a platform for NASA and government microgravity researchers to perform research in a reduced-gravity environment. The aircraft was also used for Heavy Aircraft Training for astronaut pilots and to support the movement of the shuttle from landing sites in California and New Mexico back to Kennedy Space Center. Our C-9 crew was composed of a pilot, copilot, flight engineer, two reduced gravity test directors, a flight surgeon, videographer, and a photographer.
We used the C-9 to perform “Test-As-You-Fly” testing of TAGSAM. Our baseline science requirement is to collect 60 grams of regolith from the surface of Bennu. TAGSAM is designed to collect 150 grams of sample, providing margin over the science requirement and providing enough mass to allow the team to measure the amount of sample collected in space. In order to demonstrate that we meet these requirements, we needed to test the sampling system in an environment representative of Bennu’s micro-gravity, which is 100,000 times lower than the acceleration due to gravity at the surface of the Earth. Since most of our testing is done on Earth, it is important to understand how the total sample collected changes as the gravitational acceleration decreases. In addition, we need to assess other effects in reduced gravity, such as the amount of regolith that will backscatter towards the spacecraft.
Based on lessons learned from our previous reduced-gravity tests, we had clear test requirements for this round of flight. First and foremost, we needed the aircraft to maintain a positive acceleration throughout the entire flight. In particular, we wanted to avoid even the slightest negative accelerations. Negative accelerations result in the regolith moving up into the TAGSAM head on its own and invalidate the test results. In addition, we required minimal lateral accelerations, so that the regolith bed remained stationary during testing. Finally, we needed sufficient time in reduced gravity to conduct each test. We expect TAGSAM to remain in contact with Bennu for five seconds, so each test required at least five seconds of reduced gravity conditions.
We also have well-defined test conditions for TAGSAM bulk sample acquisition. To acquire the bulk sample of regolith, the TAGSAM sampler head releases a jet of nitrogen gas that creates a positive pressure area and “fluidizes” the regolith. The nitrogen gas and entrained regolith flows through TAGSAM, and the regolith is captured inside the sample collection chamber. This device is capable of ingesting up to two kilograms of material with grain sizes from dust up to two centimeters. This gas-stimulation, regolith-fluidization technique was chosen for OSIRIS-REx because it is capable of acquiring large amounts of material during a short-duration, five-second contact with the asteroid surface, minimizes moving parts, functions without motors during sampling, and keeps the sample pristine.
Early in the project, we defined regolith simulants for TAGSAM verification testing. Based on the highest resolutions of the surface of asteroid Itokawa acquired by the Hayabusa spacecraft, we defined an “Itokawa 7c” regolith mix. This material is composed of crushed basalt and includes grains less than two centimeters. In addition, we have created a “Tagish Lake 7c” simulant, which uses the same size range of particles but has density and compressibility characteristics similar to the Tagish Lake meteorite, one of the best spectral matches for the surface of Bennu. We test TAGSAM under two conditions with these simulants: flat on the surface and with an obstruction under one edge that creates a 5-centimeter gap. In addition, back in 2012 we were still optimizing the design of TAGSAM and used these flight to evaluate collection performance for different orifice sizes for the nitrogen gas as well as collection performance for contact times as short as one second.
We were able to fly on the C-9 from Tuesday through Thursday, with two flights per day on Wednesday and Thursday. In total we completed five flights, with five tests each, for a total of twenty-five tests. On each flight we completed fifteen reduced gravity parabolas, each lasting approximately twenty seconds. For each TAGSAM test, the pilot entered two test parabolas to ensure that our acceleration requirements were met, and then TAGSAM was fired on the third parabola.
The tests were a great success. We did not experience negative accelerations during any of the parabolas – the pilots “nailed it”. The flight profile for each test resulted in accelerations of 0.05 ± 0.02 g, meeting our requirements. The integrity of the regolith bed was maintained on each flight, meaning that no upward or lateral motion of the gravel occurred. This stable environment allowed us to achieve all of our test objectives. Most importantly, in every test, TAGSAM collected more than the required 150 grams, with several test collecting more than one kilogram of sample. The stability of the regolith bed also enabled us to separate variables of the test conditions, including the accelerations of particles due to gas fire that are distinct from accelerations due to aircraft motion. All in all we collected excellent acceleration data, gas-pressure data, high‐speed video, and still photography for each test. We owe a huge thanks to the RGO office and the pilots for meeting our unique requirements, and for their incredible support during the week.
In addition to bulk sample collection, I took advantage of this unique environment to perform some reduced-gravity contact-pad experiments. In addition to the gas-injection bulk-sample collection strategy, TAGSAM also includes twenty-four “contact-pad” samplers. These pads are spaced evenly around the TAGSAM base plate and are designed to pick up small grains, less than one-millimeter across. These grains are valuable samples in their own right and also provide us with a back-up collection technique, in case something goes wrong with the gas system. Back in 2012, we were still deciding on the material for the contact pads. On these flights, I tested two different pad types, one had an indium pad with a flat surface and one had an indium pad with several one-millimeter holes. Indium is a very soft metal that deforms easily, making it ideal for capturing grains as they become embedded through contact with the asteroid surface.
The contact pad testing demonstrated that indium is an excellent material for collecting loose grains from the surface of Bennu. Unfortunately, it is too efficient and has a high probability of collecting grains up to six millimeters in size or larger. Grains this large pose a risk to mission success because they can interfere with the stowage of the TAGSAM head in the SRC. After evaluating over two-dozen candidate contact pads, we decided to use the loop side of stainless steel Velcro®. The loops act like little tweezers and are quite efficient at grabbing grains smaller than one millimeter. Because of the design, the Velcro® does not pick up any grains three millimeters or larger, negating the risk of picking up an obstructing grain.
Our original plan included another round of reduced-gravity testing using the final flight design for TAGSAM in 2014. Unfortunately, NASA has decided to decommission the C-9 aircraft. Instead, the agency is committed to a commercial approach for future reduced-gravity experiments. The plan is to temporarily relocate the C-9 to El Paso then send it on to Moffett Field at Ames Research Center for final decommissioning. As of this writing, no other NASA-owned planes will take on this capability and the remaining NASA pilots will be reassigned to other NASA planes.
We are looking for alternative reduced gravity opportunities. None of the current options appear to meet our stringent acceleration and lateral velocity requirements. The current commercial providers to NASA focus on human space flight and educational experiments and are not required to meet our environmental conditions. We are still defining the TAGSAM capabilities test plan. Hopefully, other reduced gravity options will become available and we will be able to “Test As We Fly” prior to attempting our first sampling attempt at Bennu.