Purdue students joined with students from the University of Illinois on a 3U CubeSat, Student Aerothermal Spectrometer of Illinois and Indiana (SASSI2).

The S.S. Roger Chaffee, Northrop Grumman’s recently renamed NG-11 Cygnus spacecraft, is scheduled to launch atop an Antares rocket April 17 to deliver supplies and scientific experiments to the International Space Station.

Part of the scientific payload on the resupply mission was built by students from Chaffee’s alma mater, Purdue.

A team of undergraduate students in Purdue’s School of Aeronautics and Astronautics developed an advanced pressure sensor to show the density of the atmosphere at reentry for a 3U CubeSat, Student Aerothermal Spectrometer of Illinois and Indiana (SASSI2). SASSI2 was developed with students from the University of Illinois through NASA’s Undergraduate Spacecraft Instrument Program (USIP).

Purdue’s sensor platform will take advantage of the natural reentry experienced by all CubeSats. The sensor platform will measure bulk flow properties as well as ambient conditions. Once combined with chemical species information from onboard spectrometers, this data will enable scientists and engineers to determine the chemical reaction rates needed to validate their models.

Alina Alexeenko, a professor in AAE, is the principal investigator for the Purdue science instruments. David Spencer, an associate professor in AAE, is co-investigator. Zach Putnam, an assistant professor at Illinois, is the PI for SASSI2, and Deborah Levin, a professor, is Co-PI.

“For Purdue team, it’s especially meaningful that the S.S. Roger Chaffee will lift up the SASSI2 instrument to space. It’s the first instrument for an orbital spacecraft that was built by AAE students,” Alexeenko says.

The SASSI2 project was implemented through a multi-semester AAE450/490 CubeSat design, build, and fly course that included more than 40 students, starting in Summer 2016 through last spring.

McClain Goggin has been involved with the project for about five semesters, the first two as project manager while he was an undergrad at Purdue and the rest as graduate mentor. He presented papers at two conferences in 2017, one at the AIAA/USU Conference on Small Satellites and also at the International Astronautical Congress. He helped develop mission requirements, concept generation and selection and some prototyping before Purdue delivered its payload to Illinois over winter break last year. After that, Purdue worked on ground testing as well as helping prepare for post-processing of data recorded during the experiment.

“We’re trying to get a better understanding of the aerothermal environment, at low altitudes for a spacecraft, really high altitudes when you’re looking at the atmosphere,” Goggin says. “What that will do, if we have more confidence in those models, then we can actually reduce the amount of thermal protection that we put on spacecraft on reentry bodies, which will reduce the mass and costs. It can also help you if you aren’t trying to reenter but you want to burn up in the atmosphere. It can help you model where that’s going to happen and how.”

Purdue’s sensor platform includes three pressure sensors on the front plate of the satellite as well as a sensor that will measure temperature and heat flux. Designing the pressure sensors presented a considerable challenge: The incredible heat generated by gases once the satellite hits the atmosphere could potentially burn up the sensors. So students had to design, essentially, a frame to not only protect the sensors but also cool down the gas. They did that by using a diffusion chamber.

“If you bounce around the air molecules a whole bunch, you smack them off a couple different things, they’re going to use a lot of energy, a lot of speed. So if you bounce them off of something, they’ll cool down, they’ll lose all that energy. So that’s exactly what we did,” says AAE senior Andrew Binder, who was project manager last spring after being assistant project manager for a semester. “We built the chambers for the pressure sensors so that the gas would quite literally, on its way in, hit the front side of it, then goes into this angled tube, and it bounces all the way until it hits this big open area where it’s cool. It’s lost a lot of its energy. Then, you can measure the pressure. It’s not going to burn up the sensor. It’s really tough to get that to work, but we did. That’s ultimately why the payload works is because we got the gas to cool down.”

That wasn’t the end of the challenge, though. Because the idea isn’t to measure the pressure of cooled-down gas — it’s to measure the pressure from the atmosphere. So students had to figure out the relationship between the cooled-down gas inside the sensor to what’s happening in the atmosphere. That was accomplished through simulations, test setups, research, and experimentation.

Now, it’ll be tested in space.

The CubeSat is in a spring-loaded P-pod aboard the vehicle and will be deployed in a slightly elliptical Low Earth Orbit. The CubeSat will detumble using onboard magnetic torque rods and face the sensors along the velocity vector. When orbit dips below 200 kilometers (about 124 miles), the CubeSat will begin taking pressure and heat flux data. When this altitude corresponds to a night pass, UV spectral readings also will be taken.

As the CubeSat continues to deorbit due to drag, increased measurement frequencies will be accompanied by an increased number of transmissions to the Globalstar network, which will communicate with Illinois mission control.

Mission lifetime is about 10 days, and then the satellite will burn up.

The research could have applications not only on small satellites but also larger spacecraft by helping predict what types of chemical reactions are happening at what rates. If successful, this experiment will provide the first-of-a-kind spectra in flight since the Fire II experiment in 1965, according to Illinois.

Binder and Leonardo Facchini, an AAE student who is the current project manager, will attend the launch with Alexeenko and an Illinois team in Wallops Island, Va. It’ll be Binder’s first launch, and he also had the unique opportunity of seeing the rocket the satellite will be launched on when he worked for Orbital ATK last summer.

“It’ll be an experience,” Binder says of the launch. “SASSI is one of the first Purdue satellites to go up in space and perform a mission. I don’t believe it’s the first, but it’s one of very few that have come from our university, so going to that is almost a historic event for our school.”