Questions about Ion Engines

[Zatnikitelman]

Got 2 questions about Ion engines:
1. If an Ion engine was fitted to something like the Orion, could this significantly affect the travel time to the moon? I did some rough calculations at school today and came up with about 292 m/s DV from an Ion engine. I don't think I used Orion's current projected mass, so it could be different, but still.

2. How would I go about converting the thrust force from an Ion engine (or any engine for that matter) into G forces? I need this in order to use AGMFD because I think it uses G forces instead of engine thrust or m/s/s
I think someone on Dan's site said use F=ma, but where would I put what?

Also (this is more about AGMFD, but didn't want to clutter forum with new topic), would it be better to do a standard TLI using a more conventional rocket and then switch on AGMFD for a speedup burn using the Ions, or is it better to try somehow use the Ions from the get-go.

Thanks,
Zat

 

[unussapiens]

In regards to converting forces (In Newtons, I would assume) to Gs the F=ma formula is exactly what you need, you just need to know how to use it.

F=ma
a=F/m

and since 1G = ~9.81m/s/s, divide a by 9.81 to get acceleration in Gs

So our final forumla is:

G=(F/m)/9.81

Please correct me if I'm wrong, but I'm pretty sure it's correct.

 

[n0mad23]

You beat me!

I was just checking my docs against memory, and there was the answer already posted.

With regards to your second question, I think it'd be better to do a standard rocket TLI and then switch to Ion propulsion.

 

[n0mad23]

Zatnikitelman -

You might find this useful:

Engine     Propellant Required Power  Specific Impulse     Thrust
                         (kW)                  (s)             (mN)
NSTAR          Xenon       2.3                    3300               92
NEXT           Xenon       10.5                  3900               364
NEXIS          Xenon       20.5                 6000-7500        400
HiPEP           Xenon       25-50             6000-9000          460-670
RIT 22          Xenon        5                  3000-6000     50 - 200
Hall effect   Bismuth       25                  3000                1130
Hall effect   Bismuth       140                 8000                 2500
Hall effect   Xenon         25                  3250                950
Hall effect   Xenon         75                  2900                2900
FEEP     Liquid Caesium     6x10-5-0.06     6000-10000     0.001-1


Engine     Propellant     Required Power     Specific Impulse Thrust
                                   (kW)             (s)           (mN)    
MPDT     Hydrogen             1500                    4900        26300
MPDT     Hydrogen             3750                  3500          88500
MPDT     Hydrogen             7500                  6000          60000
LiLFA     Lithium Vapor         500                  4077           12000

 

[Urwumpe]

You can say generally: A electric engine requires about twice as much DV for a transfer, as a chemical engine, because of the different trajectory. But this is more than offset by the better fuel economy.

AGMFD should have big problems with realistic electrical engines, as their thrust level is on the scale of milliNewton. The acceleration is thus a very tiny fraction of 1G and not enough for countering gravity in anyway.

Instead you just burn prograde or retrograde all the time and slowly spiral upwards.

VASIMR could be better as it produces more thrust in low gear, but it still has no acceleration comparable to chemical rocket engines.

 

[Zatnikitelman]

'Stead of creating a whole new thread, I'm going to ask my question here:
When a fuel to oxidizer ratio is given on wikipedia or astronautix of something like 5:1, then what is that ratio of? Is it weight, volume, moles, magic units?
The article on the RL-10A-3A is a good example. http://astronautix.com/engines/rl10a3a.htm

 

[n0mad23]

It's mass ratio.

 

[ijuin]

Ion engines are not worthwhile for manned lunar missions, since building up enough velocity to reach the Moon would take several times as long as the preferred free-return trajectory that high-thrust engines would allow. However, for missions to Mars or beyond, they would be worthwhile.

 

[Urwumpe]

More precisely it is mass ratio of oxidizer mass to fuel mass. Oxygen weights twelve times more as hydrogen, for combustion you combine 2 hydrogen atoms with one oxygen atom - a mass ration of 6.0.

Now, it is better to have a lower average molecule mass in the exhaust, so it is common to inject more hydrogen then needed for combustion (ideal is 4.2 as ratio for maximum ISP), with the result that the excess hydrogen just gets heated. This lowers also the chamber temperature, so it pretty good to do it.

But you cannot always do that, when you also have to power pumps.

 

[Lunar Pilot]

Definitely

Yep, Ion engines, while they look all classy on Star Trek, are just not cut out for minuscule flights like traveling to the moon, besides, they only work when there's negligible gravitational pull on the spacecraft.

 

[flaugher]

You might want to read these pages if you haven't already.

http://nmp.nasa.gov/ds1/tech/ionpropfaq.html

http://www.space.com/scienceastronomy/solarsystem/deepspace_propulsion_000816.html

http://www.sciencedaily.com/releases/2009/04/090406132821.htm

http://dawn.jpl.nasa.gov/mission/ion_prop.asp


I hate to disagree with my esteemed collegues here on the forum BUT ...

I think ion technology WILL be the propulsion system of the future, considering how much ground we've covered lately and projecting into the future. The thing is, we need TWO technologies to come along; ion engines (or something like them) and battery and/or supercapacitors.

NASA by itself will probably never be able to push these techs very far very fast, however the satellite community WANTS ion propulsion badly for positioning and re-positioning expensive hardware. Now we're talking investor money, not just theory.

Batteries and supercapacitors likewise are on the move upwards. Look at the push from laptops, cell phones, and i-pods. What if we get serous about electric cars? Are we going to be happy with golf cart batteries? I think NOT.

At some point I think these two technologies will merge to give us the capability of actually maintaining 9.8 m/s/s over a period of days or weeks. That's all we need to get to Mars is under four days in a ship with a 1-G environment. Is that not prize enough to keep pushing forward (pardon the pun).

I've been playing a lot with this laterly using Perseus in earth orbit after separation and relying solely on AGMFD (well, flying by my pants a little for course corrections after midpoint).

Remember, also, that most ion and beam engines are SCALABLE. If you wanna move more weight, just add more engines (and a bigger battery).

 

[RisingFury]

I hate to disagree with my esteemed collegues here on the forum BUT ...

I think ion technology WILL be the propulsion system of the future, considering how much ground we've covered lately and projecting into the future. The thing is, we need TWO technologies to come along; ion engines (or something like them) and battery and/or supercapacitors.

NASA by itself will probably never be able to push these techs very far very fast, however the satellite community WANTS ion propulsion badly for positioning and re-positioning expensive hardware. Now we're talking investor money, not just theory.

Batteries and supercapacitors likewise are on the move upwards. Look at the push from laptops, cell phones, and i-pods. What if we get serous about electric cars? Are we going to be happy with golf cart batteries? I think NOT.

At some point I think these two technologies will merge to give us the capability of actually maintaining 9.8 m/s/s over a period of days or weeks. That's all we need to get to Mars is under four days in a ship with a 1-G environment. Is that not prize enough to keep pushing forward (pardon the pun).

I've been playing a lot with this laterly using Perseus in earth orbit after separation and relying solely on AGMFD (well, flying by my pants a little for course corrections after midpoint).

Remember, also, that most ion and beam engines are SCALABLE. If you wanna move more weight, just add more engines (and a bigger battery).

Batteries are hardly the choice of power for the ion engines. Some of them require several kW's of power over a period of months. That's usually supplied by solar panels. Batteries wouldn't last long enough.

That's one of the reasons why ion propulsion is somewhat limited to inner solar system...

 

[Andy44]

For travel beyond Mars nuclear power is needed to power ion jets.

And unfortunately, ion jet power is not scalable. There is a theoretical limit to all current electric proplulsion systems. A single jet can only put out so much thrust. You can add more jets in parallel, but they have very low thrust to mass ratios, and even worse you need more electric power, which means bigger solar panels or a bigger nuclear plant.