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Comment by vessenes | Hacker News Reader

In the video they claim the air is supersonic at exit, and point out the mach diamonds -- and I think they said 60 kilonewtons of thrust max, which is really a lot. So I think the volume of air would decrease roughly 60% from your estimate? I'm not a physicist.

I'm imagining this in two phases: phase one - you drive around, and excess energy goes to compressing air. Phase two - you use that compressed air which takes very little additional power, so you get to double up on force. I guess likely a 'running mode' where it's using some power to keep suction on.

I'm curious how much time you get at full blast vs 'just' enough to keep the car pinned to a wall, e.g. 1 vs 2 g.

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So let’s double the speed - 1.2 Ma. You need half the mass now - 25kg/sec. If you want to have 10 seconds worth of hover - that’s 250kg of gas. Plus the weight of the storage tank. Plus - this is the amounts for hover. Not for any extra force on top of that.

It still doesn’t make much sense to me. And that’s before considering what happens when you get rocket stream hitting a tarmac road. Never mind public roads, this sounds like a major hazard anywhere. Plus the quantity of compressed gas son board (if that’s how it works)… not a pretty crash. At these numbers, imagine 10MJ released all of a sudden - apparently that’s about 2.5kg of TNT.

Surely I’m missing something… as I say, I can’t make sense of it.

Your math is pretty good, Tesla actually gave the storage tank capacity as 250kg

ETA: this is a horrifying amount of energy to be sitting on top of.

I guess on the plus side SpaceX has been trucking much worse around in and out of orbit verrry reliably for years now in tough conditions.

1% failure rate for space rockets might count as very reliable, but if 1% of all cars explode then we are in for a ride.

F9 reliability is 99.58%. The current version is at 99.84% and the last 2 years over more than 250 launches is at 100%.

> F9 reliability is 99.58%.

Ok. 0.5% of all cars exploding still doesn't sound reassuring.

> the last 2 years over more than 250 launches is at 100%.

Sure, if we tune the statistics such that we only look at those that didn't fail, then nothing failed. I love magic tricks!

Right, so 0.42% and 0.16% failure rate, respectively.

In the US alone, people make some 600m trips a day, so that "reliability" would result in about 2.5m to 1m failures a day.

The Falcon 9 has a much lower failure rate than that.

Can't wait to see the crash test video.

what's there to miss? if these ever make it to market their only purpose will be conspicuous consumption for vice-signalling ding dongs

The "pin to a wall" part uses fans, not the cold gas thrusters. It can probably stay there until the battery runs out. The gas thrusters are only needed if you want to go 0-60 in .8s (with the Thrust version), or if you want to fly (with the Rocket version). Probably those only last a few seconds before the air tanks need to recharge.

Mach diamonds in a compressed air jet aren’t surprising. You can get them from a canned compressed air duster: https://hackaday.com/2024/02/18/canned-air-is-unexpectedly-s...

> phase one - you drive around, and excess energy goes to compressing air.

It's an electric car - unless you've got an incredibly long charging cable, there is no "excess energy" when you're driving around.

There could be if you started in Denver.

The question of when to compress the air in the tank is an interesting one. Ideally you’d use the rockets for acceleration, breaking and cornering; which is all the time.

So probably the compressor will run all the time.

You’ll have to do a lot of modelling to work out the best time to use the rockets, my instinct is that fast cornering is the best use of the rockets. Maybe with a boost button to get you off the line fast at the beginning.

From the marketing materials it looks like straight line acceleration is being prioritised tho, so you charge the tank during braking and cornering.

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