Thread by @DrPhiltill

One thing that people probably forget when building launch pads is that there is gas pressure pushing up from under the pad. Dirt has air pressure in it. If rocket exhaust finds a crack, it pressurizes the dirt under the launch pad far more. This can lift concrete slabs. /1

2/ If a slab starts to lift, it creates a bigger crack, and the gas that hits its edge comes to a full stop, converting its kinetic energy to super high pressure. This pressure is right at the crack so it drives even more gas to the space below the slab, lifting it even more.

3/ Every disruption of the gas flow also creates high temperature. Concrete gets eaten away by high temperature. The sand grains and gravel thermally expand in random directions creating micro cracks that grow, so material fractures and sluffs off the surface at some rate.

4/ As concrete is eaten away it creates more paths for the gas to get through and under the concrete, and more disruption of the flow converting more kinetic energy into heat and high pressure, accelerating the process. This can run away in an uncontrolled pad failure.

5/ We studied these processes during the Morpheus lander flight tests at KSC. After every flight we examined the concrete and took data.

6/ The GMRO Lab at Swamp Works built the hazard field. We spent some long days in the Florida sun hauling concrete rubble by hand to build up the simulated lunar boulders. Fun times 😅

7/ The simulated lunar soil was actually crushed rock from the NASA KSC Crawlerway. The Crawler pulverized the river rock that makes up the crawlerway and these “crawlerway fines” as we called them have to be periodically removed and replaced with fresh rock.

8/ The Crawlerway fines don’t much look like lunar soil, except in a certain wavelength. The Morpheus lander used a laser system to map the terrain. The lasers were 1.57 micron wavelength and the Crawlerway fines reflected that wavelength exactly the same as lunar soil.

9/ We measured that at the Swamp Works, and after proving we had a material that was (A) abundantly available and (B) matched lunar soil in this way, we selected it for building the hazard field.

10/ In one of the early meetings, I told the Morpheus team that they do NOT want to land their lander on the Crawlerway fines here on Earth. If you land on regolith on the Moon, it is a lot safer than landing on regolith on Earth. Moon plume shown below:

11/ Because on the Moon in vacuum the gas spreads way out and does not dig a hole on centerline, whereas in Earth’s atmosphere is is focused like a “post hole digger” that can create a geyser of dirt and rocks shooting right back up at your rocket.

12/ So I recommended that we “hide” concrete slabs just under the surface of the Crawlerway fines everywhere we want to land Morpheus. That way the plume will blow off the fines making dust and ejecta horizontally like a lunar landing but without a geyser shooting the rocket.

13/ Here is a super cool Morpheus flight video. Watch how the laser system scans the Hazard Field. It finds the safest landing zone and flies to it for landing. We hid the concrete pads under the two safest locations so it would always find them.

14/ During every landing we collected videographic data on the plume effects — some of which was included in that video, and after the vehicle was safed we went to the landing pad to measure and document the damage to the concrete slab.

15/ On the topic of Morpheus, in that video (13th tweet) notice how the plume during *launch* shoots out on only one side. It wasn’t that way for the earliest flights, but we had an accident that required us to modify the launch operation.

16/ On launch, the vehicle slowly turned upside down then drove itself into the ground and exploded. This was a failure of the Inertial Measurement Unit, probably because a connector shook loose during the heavy acoustic vibrations from launch. Flat pads are bad that way.

17/ So among other improvements we made modifications to the launch pad to reduce the plume acoustics. I was PI of a sub-project to design and build a portable flame trench to duct the acoustic energy away. Here I was inspecting it.

18/ We made it from steel, designed so you could cut a hole in the concrete & drop it in. That’s why the plume during launch shoots out only one side, but in the landings the plume and the ejecta blow out in all directions.

19/ We had other cases where we had to study launch pad failures. On STS-124 the rocket exhaust stripped away thousands of bricks from the side of the flame trench, shattering them and spewing them over a couple kilometers. Fortunately the pad was designed to duct them away.

20/ But we were not sure if the Orbiter may have been struck. We had to find out if it was safe for the astronauts to land. We started doing plume simulations to see where the fragments would blow. We needed to know the sizes of the fragments to use in those simulations.

21/ @Ryan_N_Watkins was my intern in the GMRO Lab (not quite yet the Swamp Works). I asked her to set up an “archeological dig” site at the launch pad and measure the size and mass of every fragment in her site. This is Ryan taking the data with our collaborator John Lane.

22/22 Launch & landing pads are touchy. Any little thing that goes wrong can cause a zipper effect that createsa giant problem. That’s because you’re trying to safely dispose of enough super high energy gas to shoot a rocket into the sky. I hope this history made it interesting🙂