Comment by FabHK
1 day ago
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
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.