100s of kN thrust ion thrusters - possible?

[Skye]

Simple, really. Are Ion þrusters with hundreds of kNs of þrust possible?

[tbellman]

Simple, really. Are Ion þrusters with hundreds of kNs of þrust possible?

(Something in your environment is converting the two letter combination "th" into the icelandic letter thorn (þ).)

In principle, yes.  Just take twenty-thousand VASIMR thrusters at 5 newton each, and mount them together...

In practice, difficult and unlikely to be useful.

The big issue is that the thrust-to-mass ratio for ion thrusters are extremely poor.  For example, Wikipedia lists a mass of 620 kg for the VASIMR engine; 20000 of them would mass 12400 tonnes...  You very quickly get into diminishing returns; as you add more and more thrusters, a larger and larger portion of the spacecraft mass is just the mass of the thrusters, and they will spend the vast majority of their thrust accelerating themselves instead of the payload.

Another issue is that electric propulsion is very inefficient in terms of energy.  And that is because they have very high exhaust velocity.  The kinetic energy, or rather the kinetic power (watts, while energy is measured in joules), in the exhaust, goes as P = ṁ*v_e²/2 = F*v_e/2, where ṁ (pronounced "em-dot") is the mass flow (kg/s), v_e is the exhaust velocity, and F is the thrust.  For 100 kN with a I_sp=5000s, you have almost 2.5 GW just in kinetic power in the exhaust, and to that you have to add all inefficiencies in the engine, including the power needed to ionize the propellant.  So you may need a rather large nuclear reactor for that.

[Skye]

No, no, I’m þ-ing myself. It’s just a nice alternative to ‘th’ and I’ve become mildly obsessed wiþ it since yesterday, (I even use it in schoolwork for some reason!)

Also I just figured out how to colour text!



Ahh, I see. I suspected it would not work, for one reason or another. I got þe idea from back in t’day, when I played spaceflight simulator (as opposed to Juno (formerly SimpleRockets 2)), and I used part editing to have a bunch of huge ion þrusters on þe bottom of a craft (þey weighed þe same as tiny ones, only the þrust & size changed because SFS is fairly janky), and it could do single stage from þe surface of þe Earþ all þe way to Jupiter orbit wiþ no refuelling (+ tons of orbital manœuvring, HJO to LJO to Jupiter-synchronous orbit, þen back to HJO) and þen back to þe launch pad using propulsive landing wiþ some excess propellant left over (like, 4% or something?). All with a payload of, what, 40 tonnes? It was insanely dumb. Was just wondering if anything similar would be possible in real life. Thanks 

[InterestedEngineer]

Another issue is that electric propulsion is very inefficient in terms of energy.  And that is because they have very high exhaust velocity.  The kinetic energy, or rather the kinetic power (watts, while energy is measured in joules), in the exhaust, goes as P = ṁ*v_e²/2 = F*v_e/2, where ṁ (pronounced "em-dot") is the mass flow (kg/s), v_e is the exhaust velocity, and F is the thrust.  For 100 kN with a I_sp=5000s, you have almost 2.5 GW just in kinetic power in the exhaust, and to that you have to add all inefficiencies in the engine, including the power needed to ionize the propellant.  So you may need a rather large nuclear reactor for that.

For reference, Raptor2 is about 7.5GW power, of which 5GW is kinetic energy and 2.5GW is thermal energy. All while mass < 2t.

It's FAR easier to extract GW of power from chemistry than it is to do it with electricity.  The equivalent 7.5GW plant would be in the 100t+ ton range, not including get rid of the ~2.5GW of waste heat.  (chemical rockets toss all the waste heat out the back with the propellant.  There's not enough propellant mass to do that for electric rockets)

it's interesting how max 2/3 thermal efficiency crops up everywhere.

It's why electric propulsion will never make sense outside of station keeping or multi-year mission probes.

[Robotbeat]

Starlink uses it for orbit raising. Boeing’s electric GSO satellites use it for GSO orbit insertion, too.

[InterestedEngineer]

Starlink uses it for orbit raising. Boeing’s electric GSO satellites use it for GSO orbit insertion, too.

I include that in station keeping.   Anytime you have plenty of time to get there with < 20t of dry mass low power ion thrusters make sense.

[sdsds]

Electric propulsion pretty much implies long duration propulsive maneuvers. AEPS will take ~1 year of thrusting to get PPE+HALO to somewhere even close to the Moon. Maybe that makes sense at scale for maintaining the orbit of a Mars cycler?