Minus One to Space Gun | Nato Saichek

**Jonathan Brebner** (0:00)
We're gonna make a giant 15 kilometer long space gun, and we're gonna shoot stuff into space. To most people, that just sounds like, no, you're not. That's insane, right?

**Nato Saichek** (0:09)
Look, well, I did the math, like I've got the physics. The team that designed the SR-71 wasn't an army, and it wasn't billions of dollars. They had a broad enough group of engineers with diverse talents and willingness to work really hard. And, you know, magic happened. First thing you need for conviction is the idea that it might not be impossible.

**Jonathan Brebner** (0:31)
My name is Jonathan Wallen, it's South Park Commons. I am privileged to be joined today by Nato Saichek, who is the co-founder and CTO of a company called Longshot Space, where they are building a space gun?

**Nato Saichek** (0:46)
A space gun.

**Jonathan Brebner** (0:47)
Is that the preferred term?

**Nato Saichek** (0:48)
We're trying to lean away from the gun terminology.

**Jonathan Brebner** (0:51)
Space cannon?

**Nato Saichek** (0:53)
An accelerator?

**Jonathan Brebner** (0:55)
It's not a punchy space gun.

**Nato Saichek** (0:57)
It's the world's largest potato cannon.

**Jonathan Brebner** (0:59)
Okay, the world's largest potato cannon. So to kind of start things off here, let me set the stage. Say it's sort of sometime early in the next decade, some remote part of the world.
There is a, what, roughly three meter wide and 10 or 15 kilometer long potato cannon that has a several hundred pound or kilo projectile in the chamber.

**Nato Saichek** (1:28)
Yep.

**Jonathan Brebner** (1:29)
Someone's hand is floating over a giant red button. Does the red button say, do you say launch or fire?

**Nato Saichek** (1:37)
We usually say fire.

**Jonathan Brebner** (1:38)
Oh, yeah, it's a space gun.

**Nato Saichek** (1:40)
Yeah.

**Jonathan Brebner** (1:42)
Someone hits that button. What happens?

**Nato Saichek** (1:45)
So we do have a red button right now, even for the small tests. Like it was important. It's an important characteristic. The first thing that happens is so the projectile, you've got to basically, the projectile fills the circumference of the barrel. And along the length of the barrel are a series of pressure vessels that are fully charged with high pressure hydrogen.
Somewhere in the neighborhood of, again, for a space launch system, it might be a thousand PSI, something like that. And when you push the button, a burst disc gets ruptured on the first two pressure vessels. And those release high pressure hydrogen into the barrel behind the projectile. It starts moving. And then as it moves down the barrel, periodically down the length of the system, more pressure vessels are sort of distributed down the length of the barrel. And as the projectile passes each set of pressure vessels, more burst discs pop, more gas floods in, and accelerates the projectile down the barrel. And that gets you up to a certain speed.
That's a pretty traditional, that's a potato cannon. Like that's a multi-injection potato cannon. But you're just pushing a projectile down a barrel with gas. From a physics perspective, you can only, you know, if you think of a gas as like a collection of particles, those particles are moving at some speed. And that speed is governed basically by the composition and the temperature of that gas. So if it's hydrogen, it's sort of a proxy for that, the motion of those particles that the speed of sound in the fluid. So hydrogen has a speed of sound of 1,270 meters per second. That's room temperature hydrogen. What that means is that a shock wave in hydrogen travels through hydrogen at that speed. And so if you had a volume full of hydrogen and you moved one of the walls, the hydrogen is going to want to fill that void. And some of the molecules can travel a little bit faster than that speed of sound of the fluid, but less and less of them the faster you're going. So when you start moving away from this volume of hydrogen, the pressure that the projectile feels drops off pretty rapidly once you're faster than that speed of sound of the fluid. So you can get up to the neighborhood of twice the speed of sound just by pushing the projectile from behind. After that, you're down in the neighborhood of like, you know, if you had a thousand psi behind the projectile and the projectile is traveling at two and a half kilometers per second, you're feeling something like less than a hundred psi. You're feeling less than 10% of the actual pressure in the gas.
So what that means is you're, you know, all that energy that you put into the gas to pressurize it is basically being wasted at that point. And two and a half kilometers per second is nowhere near the speed you need to go to space.