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Comment by Reason077 | Hacker News Reader

> ”It's being sold in two modes. One with cold gas (no combustion) … and one with real combustion rocket engines”

No, both versions use cold gas thrusters. There are no combustion rocket engines involved here.

The difference between the “thrust” and “rocket” versions is the number of thrusters: “rocket” has additional thrusters that enable it to fly. The “thrust” version just has them at the back for acceleration.

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Can't help but recall the JATO Rocket Car

https://en.wikipedia.org/wiki/JATO_Rocket_Car

The famous JATO Rocket Car story was one of the original Darwin Awards winners. I remember reading it out loud to my buddies and us all rolling withlaughter when it was published in News of the Weird. pre-Internet.

One of the best episodes of Mythbusters when they recreated this.

The first or second time?

I think this was the FIRST episode.

Rocket version also has an additional 400 liter COPV which takes up the passenger seat. Thrust version has a 200 liter COPV behind the seats.

Interesting. Air is approx 1 kg/m^3, so that tank is 0.4 kg at 1 bar, and a typical COPV can go to say 300 bar, so we're talking 120 kg.

For a 1000 kg car and with 200 m/s nozzle exit velocity, to hover the car you need (with m_0 g = m_a v/t) about 50 kg of air per second, so we're talking about roughly 2.5 seconds of flight (from those 400 litres).

I believe the COPV was capable of 10k psi [1] or ~690 bar. The 400L is in addition to the 200L tank giving 600L total, which means roughly 300 kg of nitrogen. At that pressure the Z factor is around 1.5, so it’s closer to 500 kg/m³ than the 690 the 1 kg/m³ approximation gives. With the car at just under 1700 kg [2] you need about 83 kg/s to hover at 200 m/s, so you get roughly 3.5 seconds. That’s before tank mass and pressure falloff though.

[1] https://youtu.be/6D5MlPZQFOw?si=5x7x96SqdW3Z7OzJ&t=769 [2] https://youtu.be/22rvpUecLts?si=pq6EuQnDWdH9tgBf&t=264

Damn, maybe it doesn’t matter and a flying car has many other things to worry but I’d be shitless scared to sit next to anything that has 690bar of hydrogen (or even air).

You need to use 150 bar on average because the pressure will drop while it is being used.

You're right, there's a factor 1/2 missing. Should've studied my integrals.

It look to me you also forgot to account for the weight of the propellant still in the tank.

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