A new Mars landing approach: How to land large payloads on Mars

Back in 2007, I spoke with Rob Manning, a distinguished engineer at the Jet Propulsion Laboratory. Then he said something shocking to me. Despite successfully leading entry, descent and landing (EDL) teams on three Mars rover missions, he said the prospect of landing a manned mission on Mars may be impossible.

But after nearly 20 years of study and research, and the successful landing of a rover on Mars, Manning says the outlook has improved significantly.

“We’ve made tremendous progress since 2007,” Manning told me a few weeks ago when we spoke to 2024. “It’s interesting to see how it’s evolving, but the fundamental challenges we had in 2007 haven’t gone away, they’ve just changed form.” ”

The problem arises from the combination of Mars’ extremely thin atmosphere (more than 100 times thinner than Earth’s) and the very large spacecraft required for a manned mission, perhaps weighing between 20 and 100 tons.

“Many people are quick to conclude that landing humans on Mars will be easy,” Manning said in 2007. With the size and amount of atmosphere of the Earth and the Moon, it should be easy to find a halfway point between Mars. ”

But Mars’ atmosphere poses challenges not seen on Earth or the Moon. Large, heavy spacecraft navigating through Mars’ thin, unstable atmosphere will have to travel at interplanetary entry speeds (for example, the Perseverance rover was traveling at 12,100 mph (19,500 km/h) when it arrived on Mars). It takes just a few minutes to slow down from below Mach 1. It then quickly transitions to the lander and allows it to land slowly.

Back in 2007, it was common knowledge among EDL engineers that Mars had too little of an atmosphere to land on Earth like we do on Earth, but in reality, Mars has too much atmosphere and could only be done using propulsion technology. It was not possible to land large vehicles such as the moon.

“We call this the supersonic transition problem,” Manning said again in 2007. “There is a speed and altitude gap below Mach 5 that is unique to Mars. That gap is between the delivery capabilities of Mars’ large entry systems and Mars’ capabilities. Supersonic and subsonic deceleration technologies below the speed of sound. .”

The largest payload ever to land on Mars was the Perseverance rover, which weighed about 1 ton. The successful landings of Perseverance and its predecessor, Curiosity, required a complex series of Rube Goldberg-like maneuvers and equipment such as sky cranes. Larger vehicles that humans value will be faster, heavier, and incredibly difficult to slow down.

“So how do you decelerate to subsonic speeds? Traditionally, we know how to ignite the engines to enable a landing,” said Manning, now JPL’s chief engineer in 2024. “To reach that speed, we considered either a larger parachute or a supersonic decelerator.” NASA-tested devices such as LOFTID (Low Earth Orbit Flight Test for Inflatable Decelerators) may be able to slow things down better, but both devices still had problems. ”

“But there was one trick we didn’t know anything about,” Manning continued. “What about using the propulsion system to fire the engines backwards (retropropulsion) while flying at supersonic speeds to reduce speed? Back in 2007, we didn’t know the answer. I never thought that was possible.”

Why not? What could go wrong?

“When you fire an engine backwards while it’s moving through the atmosphere, you create a shock front that moves around,” Manning said. “So that shock front comes and hits the vehicle, causing instability and damage. There is a possibility.” You’ll also be flying into the rocket engine’s exhaust plume, creating extra friction and potentially heating up the craft. ”

All of this is very difficult to model, and I had no experience doing it in practice. Back in 2007, no one had ever used propulsion technology alone to slow a spacecraft down to land on Earth. This is primarily because our planet’s beautiful and luxuriously thick atmosphere tends to slow down spacecraft, especially for creative flights like parachutes and the space shuttle.

“People have researched it a little bit, and we came to the conclusion that it would be great to try it and shoot a fire truck backwards and see what happens,” Manning said. muses, adding that he has no additional funds lying around to find out what happened, and launches the rocket only to see it fall again.

But then SpaceX began testing the Falcon 9’s first stage booster in order to land it on Earth and reuse it.

Manning said, “SpaceX said they were going to try that, and to do that they needed to slow down the booster in the supersonic phase in Earth’s upper atmosphere. So for part of the flight, The engine will be injected backwards.” It travels through the thin atmosphere at supersonic speeds, just like Mars. ”

As you can imagine, this was of great interest to EDL engineers thinking about future Mars missions.

After years of trial, error, and failure, on September 29, 2013, SpaceX performed the first supersonic retropropulsion (SRP) maneuver to slow the reentry of the first stage of its Falcon 9 rocket. Although it ultimately crashed into the ocean and was destroyed, the SRP actually worked to slow down the booster.

NASA asked if EDL engineers could monitor and study SpaceX’s data, and SpaceX quickly agreed. Starting in 2014, NASA and SpaceX entered into a three-year public-private partnership focused on SRP data analysis called the NASA Propulsion Descent Technology (PDT) project. The F9 booster was equipped with special equipment specifically to collect data on the portion of the entry burn that falls within the range of Mach numbers and dynamic pressures expected on Mars. Additionally, visual and thermal imaging campaigns, flight reenactments, and fluid dynamics analyzes were conducted, all of which benefited both NASA and SpaceX.

To everyone’s surprise and delight, it worked. On December 21, 2015, the F9’s first stage returned and successfully touched down in Landing Zone 1 at Cape Canaveral, marking the first ever orbital-class rocket landing. This was an innovative demonstration of SRP that advanced knowledge and tested the technology for using SRP on Mars.

“Based on the completed analysis, the remaining SRP challenge is characterized as one of careful flight systems engineering, dependent on the maturation of the specific Mars flight system rather than technological advances,” the EDL team said, The paper detailed the results of the PDT project. So SpaceX’s success means landing big payloads on Mars doesn’t require flashy new technology or breaking the laws of physics.

“It turns out we learned some new physics,” Manning said. They discovered that the shock front “bubble” created around the spacecraft by the engine ignition somehow insulated the spacecraft from any impact and some heating.

EDL engineers currently believe that the SRP is the only Mars entry, descent and I believe it is a landing technique. Along with aerobrakes, this is one of the primary means of landing heavy equipment, habitats, and even humans on Mars.

However, many questions remain unanswered when it comes to landing a manned mission on Mars. Manning said there are several unknowns, including how a large ship like SpaceX’s Starship would be maneuvered and flown through the Martian atmosphere. Can the fins be used at hypersonic speeds, or will the thermal environment of the plasma melt the fins? The amount of debris kicked up by the large engines of a human-sized ship would be difficult, especially for orbit or return to Earth. This can be fatal for engines you want to reuse. So how do you protect your engine and your boat?

Mars can be very windy, so what happens if you encounter wind shear or a sandstorm while landing? What kind of landing legs would work for a large ship to touch down on Mars’ rocky surface? Then there are the logistical issues, such as how to establish all the infrastructure. How will the ship refuel to return home?

“This is going to take a lot of time. It’s going to take a lot longer than people think,” Manning said. “One of the downsides to going to Mars is that unless you have a lot of patience, it’s hard to do trial and error. The next time you can try again is due to the timing of the launch window between the two planets. It’ll be 26 months later. Bucket, what a pain! But I think you’ll learn a lot if you can try it for the first time.

And at least questions about supersonic retropropulsion have been answered.

“We’re basically doing what Buck Rogers taught us in the 1930s, which is to fire the engine backwards when you’re going very fast. .”