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How Hot Does the Ground Get When a Rocket Lands?
How a lander's engine plume heats the ground and the structure around it, how we measured temperature and flow in a plume heating test, and the network cables that burned through.
In films the lunar module settles down gently. Underneath it, the engine is firing a jet of fast, hot gas straight at the ground.
While I was at Purdue, I worked on a test campaign with Exos Aerospace. We used a rocket engine to simulate a plume hitting the ground as a lander touches down, and collected data for assessing the thermal environment around a lunar lander leg.
When the plume hits the ground
The exhaust leaves the engine very fast. When it hits the ground head-on, the gas at the centre has to stop within a very short distance, and that point is called the stagnation point. The gas’s kinetic energy turns into pressure and heat there, much as a bicycle pump warms up while you use it, so the area around the stagnation point sees the highest pressure and the strongest heating.
After hitting the ground, the exhaust can only spread outwards along the surface. It sweeps past the lander’s feet, and some of it bounces back up to heat the underside of the vehicle. The legs are the structure closest to the plume and have to survive the few seconds of touchdown, so the temperatures around them need their own study.
The Moon adds complications. With no atmosphere the plume spreads wider, and fine lunar dust gets blown around. Apollo 12 landed near the earlier Surveyor 3 probe, and parts the astronauts removed from Surveyor and brought home were pitted by that dust.
Measuring temperature and flow
We measured temperature two ways. The test articles were coated with temperature-sensitive paint, which changes colour with temperature and shows where a whole surface is hottest. We also placed temperature sensors, which cover only a few points but read precisely, so they can check the paint. Paint alone is not precise enough and sensors alone can miss the hottest spot, which is why we used both.
Flow was measured with Pitot tubes. A Pitot tube is a thin tube with its opening facing into the flow. It measures the pressure of the gas brought to a stop, and the flow speed can be worked out from that pressure. My part was a row of them, called a Pitot rake, and its data acquisition system.
The cables that burned through
The data travelled from the test stand to the data acquisition station over more than 300 ft (about 90 m) of Ethernet cable. The cables and equipment were wrapped in aerogel insulation blankets, and some cables still burned through during testing. Our fix was simple: after each test we wrapped the remaining cables in another layer of aerogel blanket so the data could still get back during the next tests.
It made me realise that the cable carrying the data out sits as close to the plume as the test articles do, and its protection deserves the same standard.
The test window
Engine firing windows are short. People, the site and the engine are booked in advance, and a problem cannot be fixed by starting over. Before going on site I wrote the test readiness plan, a risk matrix and contingency plans, listing what could go wrong, who would handle it and how.
On site we still hit three faults in hardware communication and sensor calibration. After tracking down and fixing each one, we finished all the planned data collection within the window.
More photos from the test site are on the project page.