I may need 29 megawatts just to lift 180 kgs with ion thruster...

[gggnano]

TL;DR Summary

Few resources online show the exact calculations for ion thruster's power

So after reading a great paper from Utah university (or so) they include the following equation which should apply at least to some type of electric thrusters:

P(power) = T(thrust in N)*S(specific impulse in secs)*G/(2*Efficiency)
x=(1800*1000*9.8)/(2*0.3)
x = 29,000,000 watts needed to lift a human using ion thruster on earth?

1800 = force in newtons, I am using 180kg because this amount kilograms may be an adult human + an ultra light 3d printed aircraft;
1000 = Specific impulse, this one is tricky...normally they say Isp is higher but 1000 is high yet it varies from 100 to even 6000?? for ion thrusters
9.8 = not too punctual, the Gravitational constant on Earth
2*0.3 = just some constants used to measure the efficiency, apparently you can change the 0.3 number to 0.9 assuming higher efficiency.

I did some calculations according to which using chemical energy one may get the heat from say burning sugar or dodecane or ethylene etc
and assuming 30% efficiency use heat engine -> mechanical energy and the 95% to electricity but even so the thruster will fly a couple of seconds only? Actually my calculations were for 100% efficient heat engine and 100kg or less total fuel most of which is sugar or oxygen in the case of dodecane.

Another problem is that unless the specific impulse is insanely high then a lot of gas will be lost with a thrust such as 1000 N or more. For example: 1900 N=9.8*200*x so the "x" here is the mass flow rate or in the example: 0.96 kg lost per second. For 1000 Isp it's 0.19kg and just 0.019 for 10,000 Isp, so it's linear simple calculation.

So it looks like the only way to power ion craft on earth will be using Helium 3 fusion? Which is impossible at the moment, if ever.

 

[phinds]

ion thrusters are not meant for use on Earth, only in space.

 

[FactChecker]

ion thrusters are not meant for use on Earth, only in space.

Exactly. And over very long distances and times. Where minimizing the fuel mass required is an overwhelming criterion.

 

[gggnano]

^ I know, and it's a shame I mean the reason I was hoping for them to be usable on earth was:

1. Quiet, unlike anything that flies on earth and uses spinning rotors (helicopters, boeing, chesna, f16), ok excluding gliders and zepellins, but come on...
2. Safe, unlike spinning rotors + kerosene or hydrogen zepellins;
3. Cheap, apparently a small diesel aircraft can burn 200 litres diesel per 10,000 km so like $400 ($2/litre diesel price?) or less which isn't a lot (still one new laptop though!) but it will be nice if it was say $50 or less and I bet those 200 litres cannot fit in single tank (I just googled it, I admit) some airplanes can last for 10,000 km without refueling but not the smallest ones like 4 seaters or less, military jets can't do it either.
4. Fast: while supersonic military jets are fast enough on earth I was hoping for 4-5000km/h.
5. Flies in space too, universal: Ideally I want something that flies in both atmosphere and outer space/cosmos - clearly this rules out any concept that pushes air (helicopters, drones, any airplane created after the 1920s or so...) which leaves only rockets, pushed-by-energy devices and the Sci-Fi impossible Casimir and wormholes.

I am now looking at the VASIMR again yet once I read they were struggling to get high thrust for many years so I'm not sure if it's worth my time...

 

[berkeman]

so I'm not sure if it's worth my time...

An epiphany!

 

[russ_watters]

^ I know, and it's a shame I mean the reason I was hoping for them to be usable on earth was:

3 of the 5 are not features of ion propulsion, and one is only half.

 

[Rive]

^ I know, and it's a shame I mean the reason I was hoping for them to be usable on earth was:

- no, with the required power they would be anything but quiet or safe: think about a GIANT cutting torch or plasma cutter, but pumped up and on steroids...
- with that kind of input requirement they would be no cheaper than the good old combustion
- and since speed depends on thrust, they won't be fast.

Putting aside the energy requirement it does not feel entirely impossible to construct (well: to imagine) an atmospheric ion thruster (maybe some additional air intake to have some real thrust and some kind of defocused output to make it reasonably safe: and with wings it would be able to give reasonable lift...), but that's entirely Sci-Fi for now.

Sad fact is: even if we are working on alternatives, atmospheric flight is just perfect match for combustion.

Ps.: regarding safety and Sci-Fi: just google up 'Kzinti lesson'

 

[Astronuc]

TL;DR Summary: Few resources online show the exact calculations for ion thruster's power

1800 = force in newtons, I am using 180kg because this amount kilograms may be an adult human + an ultra light 3d printed aircraft;

don't forget payload, vehicle/structure, power/energy system and FUEL!

High Isp means high power and low mass flowrate (= low thrust). Ion or plasma thrusters are designed for low gravity and low atmospheric pressure.

The idea of high Isp is increased efficiency of the propellant.

Getting something from earth's surface, depending on size of payload and vehicle, could involve a magnetically-assisted (railgun) launch system. Using magnetic force would save on propellant and structure to store propellant.

 

[Flyboy]

Putting aside the energy requirement it does not feel entirely impossible to construct (well: to imagine) an atmospheric ion thruster (maybe some additional air intake to have some real thrust and some kind of defocused output to make it reasonably safe: and with wings it would be able to give reasonable lift...), but that's entirely Sci-Fi for now.

I think there was some discussion of using an ion engine for satellites skimming right at the absolute fringes of the atmosphere, but instead of bringing a tank of propellant that runs down over time, you have an intake that scoops up the rarified atmosphere and feeds it to the engine. i don't know how practical it would be, especially if it picks up any oxygen. Might turn your ion exhaust a little engine rich, if you catch my drift.