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Comment by samstave | Hacker News Reader

But you still need to give it a supply of noble gas, correct?

So how many AU per unit of noble gas do you reach at terminal velocity of the thing?

Also what are the manufacturing limitations for noble supply - given all the talk about us running out of helium.

Aside: it would be ironic if they had helium balloons at the launch party of some such rocket...

Finally, what is the state of the noble gas when consumed by this engine in a full vacuum of space? Is it destroyed into a state that is unrecoverable?

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> terminal velocity of the thing?

What?

Assume you have a thing: rocket, satellite, probe, etc, and you’re pushing it to mars or whatever, and you have an engine that, based on the comment states in consumer noble gasses as fuel, one would assume you have to provide the noble gas, correct?

So after you falcon9 the thing into orbit, and push it along, and it then uses this engine to go the distance, will it constantly increase or hit a max terminal velocity. In either case, would that not tell you how much fuel to give it; which comes down to “if it is powered by helium and we are supposedly running out of helium, how much helium would it use vs how much helium is available?”

Is that not a sound question?

The second part of the question is “if we give it one ton of fuel, how far can it get?”

Pretty simple MPG question in my mind...

There are a lot of variable not accounted for, like the mass of the ship(and cargo) you are going to attach it to. In theory it can keep accelerating as long as it has fuel, and the maximum speed would be related to the speed of the exhaust. At very high velocity we would have to account for friction with the interstellar medium, space isn't entirely empty just the density is very (very) low.

In the real world you probably have a destination in mind... and maybe want to come back. For one way mission, accelerate till half your fuel is gone, perhaps coast for a bit, then decelerate with the remaining fuel (maybe keep some reserves for maneuvering).

So basically, step two of getting to Mars is to setup interstellar refulling stations.

Once you get to mars, or any destination, you need fuel to get back/other things...

So assuming we have an engine that can deliver us back and forth in a reasonable time, then we need to think about deploying intermediate refueling drones... and then drones to refill them... and then how to manufacture and deliver that fuel to the various nodes...

And if we are running out of helium, on earth, we need to find the most harvestable noble gas we can in the solar system...

What are the atmospheres of the other planets made up of, specifically Jupiter, how could we slurp off its atmosphere to Bush is around the solar system?

Assuming other energy sources don’t pan out

Interstellar refueling stations don't make any real sense because you spend as much fuel rendezvousing with the refueling station as you'd get back.

Ideally, you find fuel at your destination, or you have to take enough with you to get back. Or you make plans to stay...

The parameter you are looking for is delta-V, the total change in velocity that the craft can accelerate to. It relates to inverse specific thrust, which is the parameter discussed earlier, comparing slow heavy exhaust to fast light exhaust.

https://wiki.kerbalspaceprogram.com/wiki/Cheat_Sheet#Delta-v...

You are right. The answer is, it depends. The others just downvoted you because of the ”terminal velocity” thing.

Guess it’s better than reaching terminal volatility on twitter as we have seen how those experiments have been going...

Not everyone is an aerospace investment banker so don’t be a dick.

Trying to learn here, not be belittled.

There were a lot of questions your comment could imply. I couldn’t think of a simpler way to request clarification. Didn’t mean to offend.

On Earth we’re used to power scaling with maximum speed because of drag. In space, there is no drag. The weakest engine can propel something to close to the speed of light given the time and energy.

There is a measure called specific impulse. It asks how long an engine would hover if fuelled with a take-off weight ratio of one in Earth’s gravity (ignoring the mass of structure, tanks, et cetera). I don’t have an answer for this engine, but ion thrusters clock in around 30,000 seconds while most rockets are between 250 and 500.

Terminal velocity relates to the interaction between drag and an object in free fall.

That's not what specific impulse is... Or at least, I've never heard it explained that way. Certainly no chemical thruster has a specific impulse in the thousands. The SSME was the most efficient chemical rocket for a long time, with an Isp of ~450 seconds.

Specific impulse is directly proportional to exhaust velocity, and the units of "seconds" come dividing the velocity (m/s) by the acceleration of gravity (m/s^2) leaving 'seconds'.

Pre-coffee brain fart. Should be 250 to 500. The physical analogy is, within approximation, an accurate way to think about isp.

Yes, ok I understand your confusion to my comment as I had forgotten that terminal velocity applies to atmospheric based objects?

But I haven’t thought about specific impulse much, so I am very naïve (but curious)

So, what, if any, does the mass of the object being pushed by an engine with the 5.4 Newton’s of energy have of the ability of the engine to push it?

If you push a 1-ton thing with 5.4, and a 100-ton thing with 5.4 Newtons will they reach mars at the same time? And what will be the fuel consumption diff?

> ability of the engine to push it

Acceleration. For equal mass a more powerful engine would make the object go faster, faster.

> If you push a 1-ton thing with 5.4, and a 100-ton thing with 5.4 Newtons will they reach mars at the same time?

    Force = Mass * Acceleration
    Acceleration = Force / Mass
    a: 5.4 / 1000 =    0.0054m/ss
    b: 5.4 / 100 000 = 0.000054m/ss

As the space-crow flies, the 100 ton thing would take longer - it's accelerating 100 times slower. You also have to factor in the mass of the fuel, both starting weight and consumption. As the engine depletes the fuel reserves it gets lighter and therefore accelerates faster (increased Jerk[1]). So if the 100 ton engine was a chemical engine the situation might be different.

The following would be the same time: "assuming reactionless drives, if you push a 1-ton thing with 5.4N, and a 100-ton thing with 540N."

[1]: https://en.wikipedia.org/wiki/Jerk_(physics)

Awesome, so the next question is to know the fuel consumption per engine vs the acceleration of the mass... basically their efficiency.

So if we smack 4 engines on 100 tons, assuming the same fuel... how far do we get.

I’d love to get these numbers.

But I noticed you had a stated m/s - so is there a constant of Newtons of thrust to acceleration of 1ton of mass in space?

(Sorry I don’t know how to word that question better)

I dont know why - but I was under some weird impression that mass is less affected in space due to lack of drag.... Stupid thing to think.

I guess that this is why the slingshot method is so important.

> mass is less affected in space due to lack of drag

Your intuition is closer to correct than many might think. It's harder to move massive things than non-massive things in space. This is because of inertia. Inertia may be more fundamental to physics than mass [1], and could be thought of as matter "dragging" against the gluon (and possibly other) fields [2].

[1] https://www.princeton.edu/~pear/pdfs/2001-inertial-mass-quan...

[2] http://web.mit.edu/physics/news/physicsatmit/physicsatmit_03...

> Stupid thing to think.

There's a difference between stupid and unaware. We live in an environment that is vastly different to space. You have to know quite a bit of Newtonian mechanics before it really starts making intuitive sense. Asking questions is how you get there.

Living in the small town of Newton NC, my car broke down and I took it into the exclusive dealership and shop I got it from - the only place I know - but It wouldn’t budge after I got there. Completely motionless.

I asked the guy to fix it and he exclaimed “oh, sorry can’t help you there - all of these cars are dead. We can’t work on any of these! It will tend to just stay!”

Just then, another car of the exact same model and color drove out of the lot, right by us!

I said “what! That’s the same freaking car! Same color and body style and everything!”

And he said, well you can find another person to try to work on it, but that body tends to stay in motion, this one... not so much...

Clearly I don’t know enough Newtonian mechanics!

My apologies,

Appreciate the response.

Do you have to provide it with a fuel load of noble gas that it will consume?

Assume I give 1 ton of noble gas fuel in whatever format that is, how far and fast will it get before the fuel is consumed?

Ah, got it. So when we think of “rocket fuel” we’re conflating two things: energy source and propellant. Kerosene and oxidiser gives you both heat and combustion products for the heat to fling out of your nozzle.

Hall effect thrusters decouple energy and propellant. The noble gas is the propellant. It’s stuff you’re throwing. The energy, however, must come from elsewhere, e.g. solar panels or a nuclear reactor.

There is an aerospace term called specific impulse [1]. It measures engine efficiency. Ion thrusters are about as efficient as the turbofans on a modern jetliner. Those, in turn, are about 12x more efficient than the Space Shuttle’s solid-fuel boosters and like 7x better than cryogenic, i.e. hydrogen-oxygen, fuelled engines.

[1] https://en.m.wikipedia.org/wiki/Specific_impulse

Apologies then for my other reaction.

I was a reactive gas...

probabbly a little less

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