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Comment by jtriangle | Hacker News Reader

If you want to do slow star-trek style landings, you need star-trek level tech. Namely, propulsion tech that doesn't exist.

That doesn't mean that it's impossible, just means that it'd require things that don't exist yet.

Worth mentioning that, additionally, reentry heating isn't a huge problem, and you're not going to create new propulsion tech to counter it, you're just going to make better heat tiles. What you need new propulsion tech for is doing expanse type stuff, where you can accelerate for months at 1G so you essentially have artificial gravity and can get places extremely fast. If you're into sci-fi, the show/books "The Expanse" goes into what that looks like in practice fairly well.

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A positive way of framing it is that atmospheric recently is free. If the Earth didn’t have an atmosphere it would take just as big a velocity change to land as it does to get into orbit and getting to be orbit would be as hard as an interplanetary flight. It's worse than it sounds because the rocket equation has a logarithm in it...

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

double the Δv means you square the mass ratio. The space shuttle had a mass ratio of about 16, a mass ratio of 256 would be absolutely insane.

You get this velocity change at the cost of dealing with the heat and all but a tiny fraction of that heat ends up immediately in the atmosphere.

No atmosphere would be much easier.

There's enough energy in a Tesla battery to for the Tesla to reach escape velocity. If you could simply drive at max acceleration (and the car didn't fall apart, and the tires continued to have grip, and a million other reasons why this is impossible) eventually you'd reach escape velocity and still have some percentage left.

In a more realistic sense, a long railgun type system would be very practical in a no-atmosphere environment, and then not being subject to the tyranny of the rocket equation, you could launch whatever you wanted. Enough fuel to decelerate is no problem.

Seems challenging to get decelerated by a rail gun, coil gun or such at the destination. You get one chance to get caught by it otherwise you crash and die.

Use a giant spring! When you land, aim for the spring, and charge it for the next launch!

> There's enough energy in a Tesla battery to for the Tesla to reach escape velocity.

No, it's not even remotely close. A Model S weighs around 2000kg and has a battery of 100 kWh. That's √((100 kWh)/(1/2*2000kg)) = 600m/s of delta-v. Escape velocity for Earth is 11.2km/s, almost a factor 20 more.

My math was off!

This nerd sniped me a LOT, I’m wondering if it’s possible for a chemical battery to reach orbital velocity (not escape).

An idealized Tesla would just be its battery (500kg) perfectly dumping energy to mechanical forward speed. Cutting 3/4 of the weight gets you closer to the delta-v you need, but youre still off by a factor of 5. Though orbital velocity, and leaving from the equator and gaining that speed, means you only need to get up to ~7.2 km/s. Still only a third of the way there.

Maybe you could split your battery into chunks, and expel them once they’re expended?

An airless Earth would be extremely hard to land on. You could easily get to quite low altitude, but orbital velocity at sea level is almost 8km/2... faster than at "orbital altitude".

Once you hit an orbit intersecting the ground, you have to scrub all your speed in whatever that amount of time is, which is gonna be short. It's basically an orbital launch in reverse.

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