Re: Last point

[Keith Watt <kilo@xxxxxxxxxxxxxxxxx>]

Hello all - 

I've hesitated to jump in here, but I think there is a misconception that needs correcting.  I get the impression that some people on this list feel that ion drives are very fast, high-propulsion drives.  Nothing could be further from the truth.  What is wonderful about ion drives is that they are extremely -efficient- drive, having Isp's in the 2000-10000 secs range, but the thrust is at best on the order of 1 Newton (see Humble, Henry, and Larson, _Space Propulsion Analysis and Design_ for data -- I highly recommend this book if you have an engineering background).  For comparison, just the maneuvering thrusters on the spacecraft I work with, 2001 Mars Odyssey, are 22 Newton thrusters.  Ion drives produce a tiny, tiny, amount of force.  You may have read that the exhaust velocities can reach 10,000 m/s, and that's certainly true.  But remember that momentum is velocity * mass.  The mass of these particles being ejected is beyond tiny, so the actual force produced is very, very small.  You need to think of ion drives as being in the same category as solar sails, because that's what their orbits look like -- non-Keplerian spirals.

So why use ion engines at all, you might ask?   Because, as I said, their high Isp makes them phenomenally fuel efficient.  That means that you can accelerate almost continuously.  It takes years to get up to a decent velocity, but you can reach velocities chemical rockets can't, simply because they will run out of fuel first.  You would never want to use an ion drive, for example, to escape Earth orbit or to travel to the Moon.  It would take nearly a year to get there.  Even Mars is a bit too close to really see a benefit, though you might could make a case for it.  Basically, if you want to go a very long distance, electric (ion) propulsion is your best bet; if you want high accelerations, you'll never beat a chemical rocket.  Also, if you are using ion propulsion, you'd better be extremely accurate in your piloting, because changing course is very, very difficult once you've built up some velocity over a couple of years...

If you have any questions about it, I'll do my best to answer them.  I am currently on the Mars Odyssey THEMIS Camera team, but before that I was a spacecraft design and mission analysis engineer at NASA/Marshall Space Flight Center.

Regards,
Kilo
-----------------------------
Keith Watt, M.A., M.S.
2001 Mars Odyssey THEMIS Camera Team
K.Watt@asu.edu


On Saturday, December 21, 2002, at 02:35 AM, Dark DAVE wrote:

> Ever heard of Deep Space 1? that's a retired ion drive satelite. It's the 2nd of it's kind and is now has been manualy shut down in space. It also took photos of a comet, and then an asteroid.
>
> A plasma drive prototype already exist, i think it belongs to JPL, not sure, but it is working on the ground quite well... it's only a matter of time before they build one in a space craft. A plasma drive works almost the same as an ion drive, but it uses heat to propel the ionized propelant.
>
> do a search on google.com for Deep Space 1.
>
> David
>
>
>
> >From: "Technician" 
>
> >Reply-To: orbiter@firedrake.org
>
> >To: 
>
> >Subject: Re: Last point
>
> >Date: Fri, 20 Dec 2002 13:23:20 -0600
>
> >
>
> >Well, what I meant by not being realistic is that, as far as we know, there
>
> >is no working ion drive spacecraft prototype in NASA's inventory.
>
> >:)
>
> >-----Original Message-----
>
> >From: DarkDAVE 
>
> >To: orbiter@firedrake.org 
>
> >Date: Thursday, December 19, 2002 11:03 PM
>
> >Subject: Re: Last point
>
> >
>
> >
>
> > >Sounds interesting! I'll have to read up on the centurion manual. So you
>
> >say
>
> > >it's not realistic? I am only interested in realistic models, even if it's
>
> > >hypothetical it can still be realistic if the manual indicate where all
>
> >that
>
> > >thrust and fuel come from and how it's produced in real life using real
>
> > >physics maths and models.
>
> > >
>
> > >I fully intend to make my own nuclear powered plasma drive ship my self
>
> > >(plasna druve us an ion drive that heats up the ions). And ill back up all
>
> > >the performance specs with a technical manual to indicate how it can work
>
> >in
>
> > >real life. I might have to use some of NASA's current designs for going to
>
> > >Mars.
>
> > >
>
> > >Just out of interest has anybody modeled the X-33 or the X-43A? The X-33
>
> > >program failed, but the X-43A as i recall is a scramjet/rocket hybird ship.
>
> > >
>
> > >Not sure if i can answer your question tho, ill leave it to some one else
>
> >to
>
> > >do so.
>
> > >
>
> > >David Ong
>
> > >
>
> > >
>
> > >> The Centurion 2069 ship I imported into Orbiter is set up for 'ion drive'
>
> > >> engines. Capable of velocities well over 250Kps. Atmospheric flight is
>
> > >> obtained by conventional airfoil wings, control surfaces and rudders.
>
> > >> However, I have to mount much smaller engines for slower flight speeds,
>
> > >> since the primary engines put out way too much thrust to control landings
>
> > >> very well, even in Earth's gravity and atmosphere, extremely difficult to
>
> > >> control when landing on the Moon or other low gravity/atmosphere
>
> > >> planet/moon.
>
> > >>
>
> > >> In this configuration, going to Brighton Beach Moon base from Cape
>
> > >Canaveral
>
> > >> takes about an hour or so at 120Kps. Slowing down is not a problem, OI is
>
> > >> not a problem. Next test is to fly from Brighton Beach to Olympus, Mars,
>
> > >at
>
> > >> a test speed of 200Kps.
>
> > >>
>
> > >> Ok, so it isnt following real world scenarios. But, all things advance.
>
> > >NASA
>
> > >> has re-opened the space nuclear power research and soon we could see
>
> > >either
>
> > >> the shuttles or shuttle replacements powered by nuclear reactors, which
>
> >in
>
> > >> turn provides plenty of juice for HV ion drive engines.
>
> > >>
>
> > >> I do have one inquery. Is there a way to have such a high velocity ship
>
> > >have
>
> > >> retro thrusters that automatically adjust their density output based on
>
> > >> velocity, and adjust themselves as the velocity changes? Can this be
>
> > >applied
>
> > >> to the retro, pitch, bank and yaw thrusters as well? Will the engine in
>
> > >the
>
> > >> sim run such engines in this manner?
>
> > >>
>
> > >> RFB
>
> > >
>
> > >
>
> > >--
>
> > >Orbiter discussion list:
>
> > >- mail orbiter-request@firedrake.org with subject "unsubscribe" to leave
>
> > > or ask the listmaster (@firedrake.org)
>
> > >- archives at http://lists.firedrake.org/orbiter/
>
> > >
>
> >
>
> >
>
> >--
>
> >Orbiter discussion list:
>
> >- mail orbiter-request@firedrake.org with subject "unsubscribe" to leave
>
> > or ask the listmaster (@firedrake.org)
>
> >- archives at http://lists.firedrake.org/orbiter/
 
>
<image.tiff>
> Add photos to your messages with MSN 8. Get 3 months FREE*.