Again in 2007, I talked with Rob Manning, engineer extraordinaire on the Jet Propulsion Laboratory, and he advised me one thing surprising. Regardless that he had efficiently led the entry, descent, and touchdown (EDL) groups for 3 Mars rover missions, he stated the prospect of touchdown a human mission on the Pink Planet could be unimaginable.
However now, after almost 20 years of labor and analysis — in addition to extra profitable Mars rover landings — Manning says the outlook has vastly improved.
“We’ve made large progress since 2007,” Manning advised me once we chatted a number of weeks in the past in 2024. “It’s attention-grabbing how its developed, however the basic challenges we had in 2007 haven’t gone away, they’ve simply morphed.”
The issues come up from the mix of Mars’ ultra-thin environment—which is over 100 occasions thinner than Earth’s — and the ultra-large measurement of spacecraft wanted for human missions, possible between 20 – 100 metric tons.
“Many individuals instantly conclude that touchdown people on Mars needs to be straightforward,” Manning stated again in 2007, “since we’ve landed efficiently on the Moon and we routinely land human-carrying automobiles from area to Earth. And since Mars falls between the Earth and the Moon in measurement and within the quantity of environment, then the center floor of Mars needs to be straightforward.”
However Mars’ environment gives challenges not discovered on Earth or the Moon. A big, heavy spacecraft streaking by Mars’ skinny, risky environment solely has only a few minutes to sluggish from incoming interplanetary speeds (for instance, the Perseverance rover was touring 12,100 mph [19,500 kph] when it reached Mars) to below Mach 1, after which shortly transition to a lander to sluggish to have the ability to contact down gently.
In 2007, the prevailing notion amongst EDL engineers was that there’s too little environment to land like we do on Earth, however there may be really an excessive amount of environment on Mars to land heavy automobiles like we do on the Moon by utilizing propulsive expertise alone.
“We name it the Supersonic Transition Downside,” stated Manning, once more in 2007. “Distinctive to Mars, there’s a velocity-altitude hole beneath Mach 5. The hole is between the supply functionality of huge entry methods at Mars and the potential of super-and sub-sonic decelerator applied sciences to get beneath the velocity of sound.”
The biggest payload to land on Mars up to now is the Perseverance rover, which has a mass of about 1 metric ton. Efficiently touchdown Perseverance and its predecessor Curiosity required an advanced, Rube Goldberg-like sequence of maneuvers and units such because the Sky Crane. Bigger, human-rated automobiles will likely be coming in even sooner and heavier, making them extremely troublesome to decelerate.
“So, how do you decelerate to subsonic speeds,” Manning stated now in 2024 because the chief engineer at JPL, “to get to speeds the place historically we all know find out how to hearth our engines to allow landing? We thought larger parachutes or supersonic decelerators like LOFTID (Low-Earth Orbit Flight Take a look at of an Inflatable Decelerator) examined by NASA) would enable us to perhaps decelerate higher, however there have been nonetheless points with each these units.”
“However there was one trick we didn’t know something about it,” Manning continued. “How about utilizing your propulsion system and firing the engines backwards —retro propulsion — if you are flying at supersonic speeds to shed velocity? Again in 2007, we didn’t know the reply to that. We didn’t even suppose it was potential.”
Why not? What may go fallacious?
“While you hearth engines backwards as you’re transferring by an environment, there’s a shock entrance that varieties and it might be transferring round,” Manning defined, “so it may come alongside and whack the car and trigger it to go unstable or trigger harm. You’re additionally flying proper into the plume of the rocket engine exhaust, so there may very well be additional friction and heating potentialities on the car.”
All of that is very laborious to mannequin and there was nearly no expertise doing it, as in 2007, nobody had ever used propulsive expertise alone to sluggish after which land a spacecraft again on Earth. That is principally as a result of our planet’s lovely, luxuriously thick environment slows a spacecraft down simply, particularly with a parachute or inventive flying because the area shuttle did.
“Individuals did research it a bit, and we got here to the conclusion it might be nice to strive it and discover out whether or not we may hearth engines backwards and see what occurs,” Manning mused, including that there wasn’t any additional funding laying round to launch a rocket simply to observe it come down once more to see what occurred.
However then, SpaceX began doing checks in try to land their Falcon 9’s first stage booster again on Earth to re-use them.
“SpaceX stated they have been going to strive it,” Manning stated, “And to try this they wanted to sluggish the booster down within the supersonic section whereas in Earth’s higher environment. So, there’s a portion of the flight the place they hearth their engines backwards at supersonic speeds by a rarified environment which may be very a lot what’s like at Mars.”
As you may think about, this was extremely intriguing to EDL engineers fascinated by future Mars missions.
After a number of years of trial, error, and failures, on September 29, 2013, SpaceX carried out the primary supersonic retropropulsion (SRP) maneuver to decelerate the reentry of the primary stage of their Falcon 9 rocket. Whereas it in the end hit the ocean and was destroyed, the SRP really labored to decelerate the booster.
NASA requested if their EDL engineers may watch and research SpaceX’s knowledge, and SpaceX readily agreed. Starting in 2014, NASA and SpaceX fashioned a three-year public-private partnership centered on SRP knowledge evaluation referred to as the NASA Propulsive Descent Know-how (PDT) undertaking. The F9 boosters have been outfitted with particular devices to gather knowledge particularly on parts of the entry burn which fell inside the vary of Mach numbers and dynamic pressures anticipated at Mars. Moreover, there have been visible and infrared imagery campaigns, flight reconstruction, and fluid dynamics evaluation – all of which helped each NASA and SpaceX.
To everybody’s shock and delight, it labored. On December 21, 2015, an F9 first stage returned and efficiently landed on Touchdown Zone 1 at Cape Canaveral, the first-ever orbital class rocket touchdown. This was a recreation altering demonstration of SRP, which superior the data and examined the expertise of utilizing SRP on Mars.
“Based mostly on the analyses accomplished, the remaining SRP problem is characterised as certainly one of prudent flight methods engineering depending on maturation of particular Mars flight methods, not expertise development,” wrote an EDL crew, detailing the outcomes of the PDT undertaking in a paper. Briefly, SpaceX’s success meant it wouldn’t require any fancy new expertise or breaking the legal guidelines of physics to land massive payloads on Mars.
“It seems, we discovered some new physics,” Manning stated. They discovered that the shock entrance ‘bubble’ created across the car by firing the engines in some way insulates the spacecraft from any buffeting, in addition to from among the heating.
EDL engineers now consider that SRP is the one Mars entry, descent and touchdown expertise that’s intrinsically scalable throughout a variety and measurement of missions to shed sufficient velocity throughout atmospheric flight to allow secure landings. Alongside aerobraking, this is among the main technique of touchdown heavy tools, habitats and even people on Mars.
However nonetheless, quite a few points stay unsolved in the case of touchdown a human mission on Mars. Manning talked about there are a number of unknowns, together with how a giant ship similar to SpaceX’s Starship could be steered and flown by Mars’ environment; can fins be used hypersonically or will the plasma thermal surroundings soften them? The quantity of particles kicked up by massive engines on human-sized ship may very well be deadly, particularly for the engines you’d prefer to reuse for returning to orbit or to Earth, so how do you defend the engines and the ship? Mars may be fairly windy, so what occurs for those who encounter wind shears or a mud storm throughout touchdown? What sort of touchdown legs will work for a big ship on Mars’ rocky floor? Then there are logistics issues similar to how will all of the infrastructure get established? How will ships be refueled to return dwelling?
“That is all going to take quite a lot of time, extra time than individuals notice,” Manning stated. “One of many downsides of going to Mars is that it’s laborious to do trial and error until you’re very affected person. The following time you may strive once more is 26 months later due to the timing of the launch home windows between our two planets. Holy buckets, what a ache that’s going to be! However I feel we’re going to be taught lots at any time when we will strive it for the primary time.”
And at the very least the supersonic retropropulsion query has been answered.
“We’re principally doing what Buck Rogers advised us to do again within the Thirties: hearth your engines backwards when you’re going actually quick.”
2007 article: The Mars Touchdown Strategy: Getting Giant Payloads to the Floor of the Pink Planet
