How Elon Musk could recover rockets with balloons — just not the party kind

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On Sunday, SpaceX CEO Elon Musk warned his followers on Twitter that he had an idea that "would seem crazy:" his company could use a "giant balloon" to retrieve the top of its Falcon 9 rocket from orbit.
How could a party balloon help SpaceX bring back a rocket that is traveling thousands of miles per hour through space? Well, Musk has made strange SpaceX decrees before, which later have come true: the company launched its Tesla into deep space, after all.
this balloon concept has a history
And this balloon concept has a history: for decades, NASA and other researchers have studied how to use inflatable structures to slow down spacecraft leaving orbit. Balloons are a lightweight tool that can change the shape and density of a spacecraft quickly, altering the way that vehicle falls to Earth. A balloon can act as a great space brake and provide protection against the enormous amount of heat that a spacecraft experiences when immersed in the atmosphere. "It's like when a Soyuz capsule comes home," Jonathan McDowell, a Harvard astrophysicist and space flight expert, tells The Verge. "You need a heat shield."
Balloons have their own challenges. They can be difficult to stabilize when they fall through the sky, and must be made of extra rigid and durable materials to survive. So, it's not exactly a party balloon, but one made to withstand incredibly high temperatures and impetuous winds. But this approach could allow SpaceX to recover the last large piece of its Falcon 9 rocket intact.

An artistic representation of an inflatable heat shield that NASA was testing Image: NASA

SpaceX has already mastered how to recover most of its Falcon 9 rocket after launch. The first stage of the vehicle, or the 14-story body of the rocket that contains most of the engines and fuel, can return to a landing pad or drone floating boat after takeoff. Once it begins to fall back to Earth, the stage restarts its engines a couple of times to control its descent. This reduces the speed of the rocket enough so that it can land upright on a flat surface.
But the second stage of Falcon 9, the upper part of the rocket that carries the charge in orbit, is much harder to recover. This part of the rocket travels much faster than the bottom of the vehicle. The first stage of Falcon 9 can reach up to 5,000 miles per hour before it separates and returns to Earth. But the second stage remains in space during the entire flight; to leave a satellite in low Earth orbit, for example, the second stage must reach up to 17,000 miles per hour.
every time a second stage falls on our planet, it comes quickly
So, every time a second stage falls on our planet, it enters quickly and is hot. The faster you go down, the more heat you experience. The upper stage can be as much as 27 times hotter than the first stage, according to Dave Akin, an aerospace engineer at the University of Maryland. (And if you look at the engines of a Falcon 9 landed first stage, they burn quite a bit). The extreme temperature experienced by the second stage causes parts of the rocket to melt, and finally everything breaks.

The first stage of the Falcon 9 lands of SpaceX using its enginesImage: SpaceX

Musk is looking for a way to reduce the speed of the rocket without the vehicle getting too hot. Or at least that's what his tweet told Quinn Kupec, a second-year aerospace student at the University of Maryland, who is working on high-altitude balloons and heat shield technology. Kupec saw the balloon tweet and understood what Musk needed. "I thought:" An inflatable structure? That kind of sound looks like a thermal expandable shield, "Kupec tells The Verge, referring to the technology he's testing. He responded to Musk on Twitter. Musk replied: "Yes, exactly! It would be great to hear your thoughts, let's try some approaches."
It is possible that SpaceX can deploy an inflatable balloon at the base of the upper stage, according to Kupec, providing a large shield between the rocket and the atmosphere. This would make the stage larger at the bottom, and that could help extend all the intense heat over a larger area. Therefore, the heat would be less concentrated.
"If I can enter with more area, if I get bigger and flatter, I will slow down faster and faster."
But the most important thing a balloon can do is change how the second stage is affected by air resistance. Objects that are large but light for their size have more difficulty in overcoming air resistance, so they fall much more slowly. You can see this when you drop a piece of paper and a bowling ball from the same height. The document falls at a leisurely pace because it is less dense and gets caught in the air more easily.
The balloon, deployed in the lower part of the upper stage, could turn the rocket into more of a piece of paper than a bowling ball. That means that the stage will begin to decelerate much more quickly when it reaches the upper atmosphere, so there are lower temperatures surrounding your spacecraft. "If I can enter with more area, if I get bigger and flatter, I will slow down faster and faster," says Akin, who is also Kupec's teacher, to The Verge. "And that will be less heat when it goes down into the denser atmosphere."

Now, a balloon is not totally necessary to do the trick. What the rocket needs is some kind of lightweight material that can be deployed quickly to change the shape of a spaceship. Balloons are good for that because you only need thin material and air to fill them quickly. But a light umbrella upside down could also provide a similar effect, something that Kupec and his team have tried.
And it's not just the University of Maryland that has investigated this either. NASA has also tested inflatable heat shields with programs such as the Hypersonic Inflatable Aerodynamic Decelerator, or HIAD, and the Inflatable Reentry Vehicle Experiment, or IRVE. And in 2000, Russia tried to return the top stage of a rocket from orbit using an inflatable heat shield. Nobody knows if it worked; the vehicle was never found.

And then land in a bouncy house- Elon Musk (@elonmusk) April 16, 2018

But inflatable thermal shields can be difficult to handle when falling from great heights. At lower, subsonic speeds, the vehicle may become unstable. "He wants to turn around," says Kupec. However, that is not what the engineers want. One way to fix this could be to deploy a balloon that completely englobe the vehicle, turning it into a large beach ball.
It can also be difficult for SpaceX to catch the inflated rocket on the descent path. The vehicle could use its engine to land, as does the first stage. But the second stage engine is designed to work in space, not inside the atmosphere, so it may not be effective for that. Musk did indicate that SpaceX could try to reach the upper stage, with an "inflatable castle". However, the company has been struggling to catch objects: SpaceX has tried to catch part of the nose cone of Falcon 9, using a boat with a giant net. Those efforts have not yet succeeded, and catching a big inflated ball can be even more difficult than that. "The question is how controllable it is on the way down," says McDowell.
It will be interesting to see what happens to SpaceX (and what exactly Musk means with a bouncy castle). As Musk said the company would try different options, we can see multiple types of balloon-like structures in the Falcon 9. It just depends on what can be installed on the rocket at the lowest cost. "I'm sure he's looking for a simple and lightweight system," says Akin. "Everything you carry to orbit is a burden that you do not get paid to carry."


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