The Mars Rover Sky Crane, SpaceX Self-Landing Booster, and the Apollo Lunar Lander are just a few examples of dramatic, radically unconventional thinking, leading to astounding successes.

 

You have undoubtedly been awed by the stunning and fully successful landing of the Mars Perseverance Rover on February 18, 2021. It used a radical, hovering “Sky Crane” platform to lower it to the surface. This was the second use of this audacious approach to landing a heavy craft on the Martian surface and using the many design lessons and experiences of its predecessor Mars Curiosity Rover, which landed in 2012.

Prior to Curiosity’s landing, several spacecraft had landed successfully on Mars (and an almost equal number had failed) (Figure 1). Now, NASA and its teams were planning to land a much larger unit dubbed the Curiosity Rover, weighing 899 kilograms (1,982 pounds). In the years leading up to its November 26, 2011 launch, engineers tried to work out a way to do a soft landing with a rover having this much mass. Previous landers used a combination of a parachute and small retro-rockets, but due to the thin Martian atmosphere with just 1% the density of Earth’s, even a huge chute would be inadequate to slow the heavy craft down.

Among the other ideas was to have the lander first enter into a Mars orbit, then begin a slower descent from there. However, this brought a new set of severe technical problems, including the large amount of autonomous operating capability needed, which would burden the lander (complexity, weight, electrical power) for the complicated maneuver; monitoring and control between Earth and Mars is not real-time, as there’s a typical one-way link lag of 20 minutes due to the distance.

Eventually, a team looking at alternatives suggested a crazy idea: why not have what they soon dubbed a “sky crane” (Figure 2). This large platform would carry the Curiosity Rover and have a parachute, of course, but would not land. Instead, it would act as a landing platform for the rover. It would slow down using the parachute and retro-rockets, hover at a distance of about 25 feet (7.5 meters) above the Mars surface, lower the Rover via cables and cut it free when the Rover touched ground. After that, the Sky Crane would propel itself sideways and away from the rover, so it does not crash right on top of the precious load it has just deposited.

At first, this idea was dismissed as totally impractical and highly risky, especially given an entirely autonomous operating mode. But as alternatives were explored and then rejected for various reasons, it soon becomes the only option. Of course, just saying it’s the only option still didn’t make it viable, as there were many technical questions and uncertainties in making this audacious idea happen – along with many risks. Keep in mind that many tests which would normally be done to increase confidence and verify performance can only be approximated on Earth since gravity on Mars is about 38% of its value on Earth.

But we know how it ended, with a perfect deployment and soft landing. The full fascinating story is told in the engaging book “The Right Kind of Crazy: A True Story of Teamwork, Leadership, and High-Stakes Innovation” by Adam Steltzner (who led the Entry, Descent, and Landing team of the Curiosity project) with co-author William Patrick.

The above examples are just a few of the many instances of audacious engineering. The next two parts of this article look at the Apollo lunar-lander concept development formally called the Lunar Orbit Rendezvous (LOR) mission mode, driven almost entirely by John Houbolt.

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External References

1. Murray Cox & Catherine Cox Bly, “Apollo: The Race to the Moon”
2. “SpaceX Lands All 3 Falcon Heavy Boosters for the First Time” (video)
3. “How SpaceX Lands Rockets with Astonishing Accuracy” (video)  (around 5:45 for schematic)
4. SpaceX, “Falcon Heavy”
5. NASA, “The Rendezvous That Was Almost Missed: Lunar Orbit Rendezvous and the Apollo Program”
6. NASA, “Enchanted Rendezvous: The Lunar-Orbit Rendezvous Concept”
7. Space Stack Exchange, “Text of John Houbolt’s letter proposing Lunar Orbit Rendezvous for Apollo?”
8. MIT, Buzz Aldrin, “Line-Of-Sight Guidance Techniques For Manned Orbital Rendezvous“
9. Buzz Aldrin, “Orbital Rendezvous“
10. NASA, “John C. Houbolt, Unsung Hero of the Apollo Program, Dies at Age 95”
11. Adam Steltzner and William Patrick “The Right Kind of Crazy: A True Story of Teamwork, Leadership, and High-Stakes Innovation”