Skydiving and Air Pistons: It’s Just Math
Addressing the challenge of designing a net for the ultimate skydive wasn’t a light task for this engineer.
You may have seen that, back on July 31st, a daredevil skydiver named Luke Aikins performed a world record skydive by jumping from a plane at 25,000 feet into a net, wearing no parachute or wing suit or any safety device whatsoever. I was there to see the event live, and let me say it was nothing short of spectacular.
Back in April my phone rang, and it was a friend and colleague named John Cruikshank, a local civil engineer, asking if he could meet with me in my office. Of course I said yes, and I asked him what was up? He said that he couldn’t tell me yet.
A few days later, John came in and asked me to sign an NDA (Non-Disclosure Agreement), which I did without hesitation since he is a dear friend. He then proceeded to tell me about the plans for this jump. He got involved because the event co-creator, Chris Talley, was his friend since the 1st grade. Chris asked John to help him with the design of the net, also known as the trap.
As a civil engineer, John designed the concept of the trap being supported at four corners by four cranes. How tall would the cranes have to be? The design criteria were based on Luke’s weight, about 200 pounds, and the fact that his body can sustain 4g’s (four times the acceleration of gravity). Do the math and you can calculate that he needed about 120-feet to decelerate from 125 mph free fall without doing bodily harm. Add some extra height for the slack of the net and a little wiggle room below and John decided that 200-foot tall cranes would be required.
What John needed help with, and the reason he came to me, was the design of a dampening mechanism for the net. The trap had to absorb the deceleration of Luke’s mass without bouncing like a bungee cord. We focused on the idea of air dampening pistons, and they would have to be large. Each of the four pistons would have to absorb a force of 200#:
F = ma = m(4g)
F = 200#/32.2 x 4(32.2) = 800#
800#/4 = 200# per piston
Further, as with any construction project, the lead time would have to meet the project deadline of July 31st, plus allowing for a month of testing time, they would have to be available in about two months.
I asked John where they were planning on doing the jump and he said their first choice was the Orange County Great Park, the former El Toro Marine Base here in So Cal, which is being redeveloped into a massive multipurpose park facility – and it also happens to be one of my projects. With the realization of this coincidence, I put John in touch with KPRS, the general contractor at the Great Park.
The team met with KPRS on site at the Great Park. The site was perfect – it even had a runway left over from the marine base that could be used for the jump plane take-off and landing. KPRS met with the developer (the Irvine Company) and sadly after much debate they balked at the use of their site for the event – it was too risky.
Other sites were batted around, like the parking lot of Angel’s Stadium, which would have set the jump for shortly after game four of the Red Sox/Angels series, but that too did not work out. Eventually a site was selected on a private ranch up in Simi Valley called Big Sky Movie Ranch. This is where Little House on the Prairie was filmed, plus Gunsmoke and about 270 other well-known movie titles.
While all this was going on I helped John finesse the air pistons. Initially we thought the dampeners would have to absorb all 120-feet of movement, but when the project team selected the net, it was realized that the net itself would dampen about 100 feet of the deceleration, so the dampeners would only need about 15-20 feet of travel. We fairly easily sourced four 15-foot dampeners so the project was off and running, so to speak.
As you can see in the YouTube image of the net (a video I suggest you watch if you haven’t seen it), the net itself forms a fairly sharp cone at the point of impact. This geometry meant that Luke had to land on his back – something he had never done in over 18,000 jumps. So the team hired a performer from Cirque du Soleil to teach Luke how to land in a net, and how to roll from front to back just before impact. This conical net shape is what simplified the design of the dampening pistons.
In early July, the four 200-foot cranes were mobilized and practice jumps began. The first realization was that the weight of the 100-foot x 100-foot net (about 800 lbs) was greater than the static force of the dampening pistons, so a system had to be put into place to hold the net and release it on impact. Several automatic means of doing this were explored, but all proved to be too complicated. The team settled on a manual release device, meaning there was a team member in the control room of the jump who had the responsibility to release the net a split second before impact. This guy had a lot of responsibility since Luke’s life was in his hands.
Before Luke could jump into the net, it had to be tested. A 200-pound weight was dropped from a helicopter and it tore a hole through the net. The weight was too dense. Instead a 200-pound fireman’s dummy was dropped into a new net and this worked quite well. An accelerometer indicated that the force had not exceeded 4g’s, so the net was ready for a live test.
Luke practiced jumping into the net from a helicopter at higher and higher heights. This allowed him to learn how to roll and hit the net with limited risk. None of these tests involved a parachute because the parachute on his back could have injured him on the net. Then, on July 31st, he did the world record jump from 25,000 feet. He had three fellow sky divers to assist him with oxygen and video. They pulled their chutes at 5,000 feet. When he approached the net he came in at a slight angle and, for a split second when he came into view, it appeared he was going to miss it. But he didn’t, although he was at a quarter point closer to one edge which was scary. And thankfully the release and dampening system worked perfectly.
As I watched this amazing event, what struck me was the sense of being totally committed. I don’t think I have ever been so totally committed to anything in my life like that, ever, as Luke was when he left that plane without a chute. But how amazing would it be to live our much more ordinary lives with that level of commitment? Whether it’s work or family or friends or fitness or fun – to try to emulate that “no turning back” sense of commitment is an amazing idea just to contemplate, let alone accomplish.
Timothy Allinson, P.E., LEED AP, is vice president of Engineering at Murray Co., Mechanical Contractors, in Long Beach, California. He holds a BSME from Tufts University and an MBA from New York University. He is a professional engineer licensed in both mechanical and fire protection engineering in various states, and is a LEED accredited professional. Allinson is a past-president of ASPE, both the New York and Orange County chapters. He can be reached at firstname.lastname@example.org.