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Electric Propulsion Systems and reentry

Posted by: Mikhail - Mon Oct 31, 2005 10:12 pm
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Electric Propulsion Systems and reentry 
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Post Electric Propulsion Systems and reentry   Posted on: Mon Oct 31, 2005 10:12 pm
If I'm not mistaken, current operable electric propustion systems are very weak in relation to chemical rockets. I was wondering if anyone more knowledgable than me (high school senior) would think that those two systems could be combined to create a sort of hybrid-less powerful than chemical rockets but more punch than current electrical systems, maybe useful for slowing down a craft on reentry?


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Post    Posted on: Tue Nov 01, 2005 12:40 am
Here's the deal: electric propulsion systems (ion is the only one that's been flown that I know of -- I'll wait for publius, SawSS1, or Peter to come correct me) are insanely efficient -- try around 30,000ISP -- but the actual thrust generated is in the micronewton range. It's just pathetic, which is why when you see the "artist's conceptions" of all those big Mars-bound ion-powered spaceships, they have a teensy-weensy tiny little crew cabin that situated right in front of these unbelievably huge engines that look like they belong on an Imperial Star Destroyer. Ion engines are great for long, slow burns, but they're practically worthless if you want to get across the stree at some point before you're due to collect Social Security. Chemical rockets, on the other hand, are pretty lucky to get an ISP of 300, but can produce millions upon millions of pounds of thrust. And, on top of that, they're pretty cheap, as thrust-generating devices go.

Now, people are starting to come up with a few more powerful electric engines, but it's tough going -- they're creating a whole new field of engineering as they go along. Look on this page for links to old forum threads related to this topic: http://spacefellowship.com/Forum/viewtopic.php?t=1306

Good luck, and welcome to the boards!

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Post    Posted on: Tue Nov 01, 2005 9:38 am
Hello Mikhail, nice to have a new voice around here.

NASA's Deep Space 1 had an ion drive that produced 85mN of force and required about 2kW of electrical power for full thrust. Brilliant for a long steady push over a couple of months to get up speed but totally useless for launching off a planet's surface (probably not powerful enough to even escape the gravitational pull of one of Mars's moons).

As for combining ion and chemical propulsion, they are totally different systems. You could argue of course that they have been combined already as Deep Space 1 and SMART-1 were launched on chemical rockets. :)

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Post    Posted on: Tue Nov 01, 2005 6:15 pm
The VASIMR engine can provide higher thrust, but not as high as chemical. Here is a link:
http://spaceflight.nasa.gov/shuttle/sup ... asimr.html
Google “VASIMR” to get more links.
Here is a link Andy posted in another thread about many kinds of electric rocket engines.
http://www.waynesthisandthat.com/ep2.htm
Curiously it does not include VASIMR, possibly because VASIMR has never flown. Many of the ones in that link have flown.


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Post    Posted on: Wed Nov 02, 2005 6:26 pm
Chemical first, then Nuclear thermal with a LOX afterburner like Stan Borowski at Lewis wants. Large ion craft can wait for the TAU type payloads.
(Thousand Astronomical Unit mission).

Here is how a true intersteller flight can proceed:

A Heavy Lift launches an Nuclear Thermal Kuiper Belt explorer.

A compact ion drive payload separates and continues out into the void as the NTR slows and investigates Pluto, Sedna, etc., using a secondary electric drive as well.

The smaller craft only has an ion drive and will pass Voyager in but a few years.


Last edited by publiusr on Fri Nov 04, 2005 6:05 pm, edited 1 time in total.



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Post    Posted on: Thu Nov 03, 2005 4:09 am
If the French'd just get their fusion reactor up and running, then we'd have our perfect rocket. Forget fission, man, fusion's the way to go.

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Post    Posted on: Thu Nov 03, 2005 8:04 pm
How about using Saturn's moon Tethys as a fuel depot.
http://saturn.jpl.nasa.gov/science/moon ... ?pageID=15
It is almost pure water and has low gravity (1/60 Earth's). A fission reactor could split the water into LOX and LH2. I bet it is comparatively easy to liquefy hydrogen if you start at the temperatures common that far from the Sun, and the oxygen might liquefy all by itself if you just left it to cool, maybe behind a shade. Or throw the LOX away and just use the LH2 in your nuclear thermal rocket. If you have a working fusion reactor the LH2 could fuel that too.
All you have to do is get to Saturn first. And build all sorts of new hardware of course.
Gee, this is a bit off topic, isn't it. :oops:


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Post    Posted on: Thu Nov 03, 2005 9:31 pm
I'm Sorry, for i could not get to reply very quickly. I appreciate the links you provided, and realized that I misunderstood the anatomy of most electric propulsion systems, thinking their process of creating thrust could be combined with combustion in chemical rockets.


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Post    Posted on: Fri Nov 04, 2005 4:45 am
You're fine: that's the point of being here, to learn. Trust me, I've learned far more from talking to the people on this webboard than from my classes back at my old community college. Oh, for anyone who's actually interested: I get into real AE classes this spring! Thermo/compressible fluids, Sophomore Design Competition, Statics (hey, everything equals zero)! Yippee!

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Post There could be something in this   Posted on: Thu Dec 22, 2005 1:09 pm
Hey wait a minute,

A point that has been missed here is use of the word 'reentry'! this implies atmosphere, possibly ionised, to manipulate.

I don't think electric propulsion is inherently weak, it's just that the whole point of ion drives is to shoot out reaction mass as fast as possible so the craft is going as fast as possible before the reaction mass runs out. Momentum is preserved, but much more energy goes into the ion that is expelled, because energy is proportional to velocity squared, if I remember my physics. So anything based on low-mass high-velocity propellent will be power hungry.

On the other hand if you have a lot of reaction mass, eg you are an electric train and your reaction mass is the earth, you could get really good thrust if you wanted too. (for some reason passengers just don't seem to want to be subjected to 5g acceleration in order to reach their max velocity of 300kph a few seconds earlier ;) )

Another point is why cant' we just fly into space with a standard airplane? Its not as simple as 'because there is no air'. there is always some air, why can't you just go faster to interact with more of it for lift? Im guessing friction even though reduced in thin atmospheres grows faster than the lift required to fly higher into yet thinner atmosphere. If you could invent a frictionless plane, perhaps using some electromagnetic method of pushing on very thin air, perhaps you could just glide into space.

Another way of creating very low friction is an airspike to make a vaccum for the craft to travel through. perhaps there could be some form of magnetic manipulation of the air ionised by the spike as it passes around the craft for thrust?

These suggestions are just wacky speculation, but I stand by my assertion that electric propulsion is not inherently weak. To be pedantic, chemical reactions are just another example of the electromagnetic force anyway ;)


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Post    Posted on: Fri Dec 23, 2005 7:47 pm
Hello BEM. You make some good points about our unfounded assumptions regarding re-entry. However, before you go there, you might stop off here.

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Post    Posted on: Sat Dec 24, 2005 10:11 am
Thanks for that link. Thats really pretty cool.


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Post    Posted on: Sat Jan 28, 2006 4:09 pm
Anyone heard anymore about this announcement that NASA was supposed to make last Wednesday on a new Ion thruster prototype?

http://www.nasa.gov/centers/glenn/news/ ... uster.html

Seems a bit strange that this was supposed to happen in the same week that ESA announced their new ion engine. I've had a look on NASA's Glenn site but there is nothing mentioned there at all.

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Post    Posted on: Sat Feb 04, 2006 9:16 am
Just in case somebody missed the article, it seems that NASA is transferring VASIMR development to a commercial company. Houston-based Ad Astra Rocket Co has taken over developing the new engine and will be lead by former astronaut Franklin Chang-Diaz.

http://www.spaceflightnow.com/news/n060 ... smarocket/

For anyone not familiar with the VASIMR concept here is a NASA short article. The engine runs on hydrogen and if used on the ISS could be fuelled from the waste hydrogen currently dumped overboard and powered from its solar panels removing the need for visiting spacecraft to constantly boost its altitude.

http://www.nasa.gov/vision/space/travel ... lsion.html

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Post    Posted on: Thu Feb 09, 2006 8:40 pm
That is actually a good candidate for space privatization. They are not trying to make a vehicle--and the engine itself will most likely be for the payload of an HLLV-launched nuclear Mars ship.

Since there is no need for launch pads, Range control, etc--all work can be done largely in-house. This is where the alt.spacers could really shine, and it would be in that setting that a DC-X or Rutan type know-how (no red-tape) is best served.

The problem is that the Tumlinson-types are not interested in such matters and want to re-invent the wheel--wasting their limited resources upon launch infrastructure we already have via the cold war.

It is rather like me building a toll road right next to a free interstate--and wondering why nobody invests.

Much better to sell cameras and cell phone towers to line the interstate with.


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