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Comment by 1053r | Hacker News Reader

The number one thing you worry about with a launch vehicle is not ISP, it's the tyranny of the rocket equation. We have tech with VERY high ISP (ion drives, for example), but we can't use them until our vehicles hit space because of the extremely low maximum thrust.

Meanwhile the high thrust options we have are all very heavy, which means we have to carry more fuel, which means we need a bigger rocket, which means have to carry more fuel, and so on. The sum of this infinite series is finite, but it is still large.

If this tech lowers the weight of the first stage, it might actually RAISE the ISP of the rocket overall, even if it lowers the ISP of the engine itself.

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>The number one thing you worry about with a launch vehicle is not ISP, it's the tyranny of the rocket equation.

The rocket equation doesn't account for thrust. It's terms are mass and ISP (or exhaust velocity).

>If this tech lowers the weight of the first stage, it might actually RAISE the ISP of the rocket overall, even if it lowers the ISP of the engine itself.

ISP is depends only on exhaust velocity. Changing the mass of the rocket cannot effect it.

> The rocket equation doesn't account for thrust. It's terms are mass and ISP (or exhaust velocity).

Only sort of true. If you naively apply the rocket equation like you're imagining, you would predict a nonzero final velocity for a hypothetical rocket that has enormous ISP but thrust less than its weight. This is true if the rocket is starting out in orbit, but it's totally wrong when you're on the ground. Starting from the ground, you also have to fight gravity, so you care more about (thrust - weight) / mass.

Sure, but "the rocket equation" is 100% talking about the classic ideal rocket equation, which I just discovered is also called the Tsiolkovsky rocket equation.

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

If you want to be more specific, e.g. include gravity then you can't say "the rocket equation".

    > If you want to be more specific, e.g. include gravity 
    > then you can't say "the rocket equation".

You may not be able to say "the rocket equation" in that context, but NASA does. [1]

[1] https://www.nasa.gov/mission_pages/station/expeditions/exped...

Despite Don mentioning gravity along with deltaV he is obviously attempting to describe the classic rocket equation without scaring people away with the math.

It is absolutely true that 'the rocket equation' doesn't account for thrust. It also doesn't account for gravity gradients, aerodynamic drag, relativity, solar wind, the cosmological constant...

It's very accurate when those things contribute little, and totally inadequate for modeling spaceflight if any are significant.

There are more complex versions that account for those things, but 'the rocket equation' is always understood to mean Tsiolkovsky's equation.

The delta-v also depends on the mass fraction you can achieve. Removing the tanks might mean a much higher mass fraction, so you have the same delta-v with much lower Isp.

I don't think you would even necessarily need stages if you don't have any empty fuel tanks to discard. The engines might not be as optimized for operating both in and out of atmosphere, but that seems like a fair tradeoff.

Part of the reason for staging is the tanks but part of it is the weight of the rocket engines themselves. The original Atlas had a single tank that started off with two large and one small engines attached to it. When it had been lofted enough the two large engines dropped off and the small engine finished pushing it into orbit.

Yeah, tanks are not THAT heavy. But you are forced to use multiple engine types due to the bell nozzle.

If we used aerospikes, then it would be a different matter.

> The sum of this infinite series is finite, but it is still large.

Out of curiosity, if g was much greater, could it be infinite? What order of magnitude would g need to be for that to happen?

The ability to launch from the surface is proof of it's finiteness.

As a corralary it should be finite as long as you're not inside a blackhole.

It should be asymptotic up to that point. Mathematically infinite at the horizon and then completely unbounded inside it (in the sense that it converges to infinity versus not converging at all ( see a "flat" universe versus hyperbolic)).

Yes; if the acceleration from gravity exceeds the rocket's thrust-to-mass ratio, the rocket cannot make any upward progress against gravity. The best engine listed on the Wikipedia article was the Merlin 1D with 180.1 gravities, so g ≥ 1767 meters per square second would suffice to keep it on the ground, less if you account for the fuel tanks and such.

You can always make a mass driver system (which is not limited by the rocket equation), but given how insanely hard it is at our own gravity, it would be more insanely difficult at a few times great g.

So there's definitely a quite low maximum gravity allowing practical space access.

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