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ainventor
2003-Dec-27, 01:42 AM
When reentering the earths atmosphere, the speed doesn't have to be extremely fast. The idea is to get slowed down first. My question is, "Why not use an Ion engine to slow the craft down first before reentry? How long would it take for an Ion engine to slow a craft down to a dead stop, from a speed of 17500 mph?

Prime
2003-Dec-27, 04:02 AM
Ion Propulsion for the 21st. century, delayed by oil greed, the scourge of the first part of the next 100 years.

http://community-2.webtv.net/WF11/HappyHolidays/

Faulkner
2003-Dec-27, 05:30 AM
I followed this link suspecting just some more crackpot UFO-freak rubbish that tends to creep into this forum - (sometimes can be fun to read, tho&#39;&#33;) - but lo, it&#39;s a Popular Mechanics article from the &#39;60s... very interesting, too&#33; It seems to me there were loads of cool ideas back in the &#39;40s/&#39;50s/&#39;60s...were they ever followed up?? Or perhaps the military hijacked all these patents??? Hmmm... <_<

Faulkner
2003-Dec-27, 05:37 AM
I don&#39;t think an ion engine will do the job of slowing down a space capsule for reentry. The thrust is tiny...only useful outside of gravitational influence...the good thing about it, tho&#39;, is that it is a continual thrust/acceleration, so the velocity builds up exponentially.

What are the spec&#39;s for the ion engine? What is the maximum velocity an ion-powered probe could achieve? If powered by a nuclear battery, surely it must come close to lightspeed "c"? Just might take a while to build up that speed...

Hydrok
2003-Dec-27, 04:18 PM
well i think you hit the problem that no one has solved yet, sure you can get that speed, and it would take a while, to get that speed, but it will take you equal time to slow yourself that it took to accelerate yourself, and the latest i heard that is long past alph centauri

Faulkner
2003-Dec-28, 06:57 AM
So, Hydrok-M35, it takes at least TWO light-years (by current technological standards) to accelerate to light speeed?

What if you make the spaceship massless (by surrounding it in a BEC (ie. mercury) metal), then you could just levitate outward via magnetics&#33;

Surround yourself in an ATOM&#33;

GOURDHEAD
2003-Dec-28, 03:17 PM
Reference: http://members.aol.com/malcolmbmcneill/Gen...n_Concepts.html (http://members.aol.com/malcolmbmcneill/General_System_Design_Concepts.html)

This reference contains beam power/velocity/energy/propellant/trip duration/etc., profiles for trips (one way) to the Alpha Centauri (Proxima) system. I am unschooled in the design of ion engines and hope ones (or a set) can be built that are tolerant of whatever materials are available in interstellar space. Since hydrogen is the most prevalent, its mass was used in building the profiles. Elements with larger masses shift the volt/ampere values to favor larger voltages and smaller currents.

An important feature of the system is that the ion propellant force must be about twice the sail force which is controlled by varying the kilograms of propellant per second (q) and the average speed of the particles being expelled (Ve). This ratio is required because, when accelerating, the two forces are additive, but when stopping, they are in opposition. The sail force is everpresent because if the beam is turned off, both forces are lost and the ship is both out of control and out of power needed for crew survival. Designs using fusion or matter/anti-matter processes would be free of this constraint; however, I believe we can master beam collimation and direction control long before we master either fusion or matter/anti-matter processes, and the sun will be outputting this energy whether we use it for propulsion or not.

Profiles containing trip times of 10 years or less are probably too optomistic; 40 year trip times are probably achievable with todays technology. A robotic craft, which doesn&#39;t require the stopping feature, could get to the Alpha Centauri system much sooner--maybe less than 10 years--then follow a path to stars beyond this system. :rolleyes:

Matthew
2003-Dec-30, 03:07 AM
An ion engine provides very little thrust and wouldn&#39;t be much use to just turn on and hope you slow down on reentry.

Anyway, if you slowed the spacecraft down too much, wouldn&#39;t you be gioing to slow for re-entry, and either be knowcked away from the atmosphere, or burn up totally as the spacecraft is pulled in at too slow speeds so the descent takes too long.

GOURDHEAD
2003-Dec-30, 12:17 PM
It depends on the engine. The ones currently in use are not very powerful. My view is that they can be made as powerful as is needed to use the power source efficiently provided that propellant material is available. Propellant mass availability is the big hurdle.

GOURDHEAD
2003-Dec-30, 12:31 PM
Another point. The ship that travels between the stars will not be the one that lands. The interstellar vehicles will go into orbit around an object in the destination system and shuttles (carried on board) will actually land on any desirable moon or planet. The shuttles will be powered with ion engines.

ainventor
2004-Jan-03, 12:06 AM
It&#39;s amazing how everyone who answered in this topic didn&#39;t even read the question, or even think about the question before answering. Ion engines don&#39;t use solid or liquid fuels. You turn yourself around and fire an Ion Engine. You simply slow down first before reentry. If you slow down first before reentry, you don&#39;t have to worry about burning up when you do reenter. Unmanned spacecraft, such as Beagle 2, wouldn&#39;t have had to enter Mar&#39;s atmosphere at the speed it did. It&#39;s more than likely laying in a million pieces on the surface. So what if it takes 2 or 3 weeks, or even a couple months to slow down before reentry, at least you can enter at a safe speed. They have already used Ion Engines successfully in space, so we know they work. My question was why don&#39;t we use Ion engines to slow down first? :unsure:

GOURDHEAD
2004-Jan-04, 04:04 AM
Ion engines can and will be used to slow down to whatever the optimal speed is. To slow down from 17500 to 0 depends on the mass of the ship and the propulsion force supplied by the ion engines. For the cases I have considered for interstellar travel using ion engines powered by energy supplied from a collimated beam, where the propellant rate of usage and the average velocity of the propellant particles can be selected to maintain the propellant force much larger than the sail force, the stopping time (propulsion less sail) and distance is larger than the acceleration time (propulsion plus sail) and distance. Since a number of tradeoffs can be made, the stopping times can vary over an appreciable range. The cases I have considered so far indicate that the larger is the ratio of propulsion to sail force, the nearer to equal are the acceleration and deceleration times.

Matthew
2004-Jan-05, 03:37 AM
One reason ion engines haven&#39;t been used is that they take a long time to work. They only fire a small amount of energy out, which give the craft a little bit of propulsion, but it takes a while for a craft to speed up with an ion engine. So you would need to begin slowing down for months with an ion engine.

Halloween
2004-Jan-05, 04:26 AM
I read that the ion engines currently in use have about as much thrust as a peice of printer paper being pulled down by Earth&#39;s gravity. That is, not much at all. However over time (a signifigant period of time) it tend to buils to great velcities.

So, unless one is willing to wait months/years to deorbit, it wouldn&#39;t be a good option.

Planetwatcher
2004-Jan-05, 05:41 AM
It would be impractial to use an Ion engine to slow a space craft down for re-entry.
They just don&#39;t have the power, and can&#39;t be made to have that kind of power.

Ion engines would do best to serve as they are designed. Consistant propulsion, and or very slow acceleration for long space journeys.
Unlike anything else, an ion engine can operate continually for over a hundred years, and that is basicly it&#39;s only advantage.
A ship with ion drive would need other means of propulsion as well.
Such as conventual rockets, gravity assitsts, nuclear, solar sails, or other methods of propulsion.

The best senero would be to use alternitive propulsion to achieve it&#39;s maximum speed potental, then use ion drive to slowly build up speed for the rest of the trip.

As for re-entry, rockets, and gravity will likely serve best for many years to come.

damienpaul
2004-Jan-06, 03:59 AM
or a space platform with very long elevators

GOURDHEAD
2004-Jan-06, 03:14 PM
:o Let&#39;s not sell ion engines short.

The decided advantage they have over chemical combustion rockets is very high propellant expulsion velocities, Ve. My view is that linear particle accelerators <span style='color:orange'>can be </span> designed to deliver tens of kilograms per second of ions at near (0.1 to 0.5 c) light speed (huge specific impulse). This is much larger than the propellant velocities achieved by the best chemical combustion rockets (probably liquid oxygen/liquid hydrogen). If q is used to designate the mass of propellant expelled per second, then the power (energy per second) applied to propel the vehicle is 0.5 q * Ve^2 per second. Since the mass density in interplanetary and interstellar space is extremely low (small q), the larger Ve can be, the more propulsion force is generated. The relativistic mass increase of q as Ve approaches light speed is an added advantage.

I am not at all familiar with the details of either linear particle accelerator or ion engine design, but I am optimistic that since we know how to accelerate particles, we can with a little practice (experimentation) use electrostatically charged grids to accelerate the ions of whichever elements are present in interstellar space (we don&#39;t want to carry much of this propellant mass with us if we don&#39;t have to) with minimum turbulence in the propellant stream. This process, when it is made to work efficiently at power levels near 10^18 watts, grid erosion notwithstanding, can propel a 1.2*10^10 kilogram ship at near one g acceleration to maximum velocities > 0.5c within 3 years and to reach and orbit stellar systems 5 light years away in 20 to 40 years.

If some of you know that my assumptions are way off base, let me know soon with convincing logic. Save me from myself&#33;&#33; :ph34r:

damienpaul
2004-Jan-06, 03:15 PM
i understand you fully, then again, i would not entirely trust my judgement

Manchurian Taikonaut
2004-Mar-20, 06:09 AM
I&#39;m not sure it would work, just like the space elevator idea our technology is a little behind, Ion engines are improving very well and have great efficency the Ion drive is a new science and we have much to learn, it is very true that we need new ideas for space craft designs, this types of ideas will also help Space become more accessable and improve safety.

Mettalica1
2004-Mar-31, 08:23 PM
It wouldnt take that long but what if the engine was to burn up do to the heat and pressure of the earths atmosphere :ph34r:

Planetwatcher
2004-Apr-01, 01:58 AM
It&#39;s not so likely the engine itself would burn up. At least not before the rest of the space craft did. :P

jitte
2004-Aug-02, 04:48 PM
The Hayabusa probe for the round trip Japanese Areospace Exploration Agency MUSES-C mission to land on and take a sample from asteroid 1998SF36 uses an ion engine.

"The M-V launch vehicle will put MUSES-C into a transfer orbit, but MUSES-C employs a high-performance electric propulsion engine which is continuously activated for the flight to and from the asteroid. Instead of using a chemical reaction, such as the combustion of hydrogen with oxygen, MUSES-C&#39;s engines will ionise atoms of the rare gas Xenon and then, with a strong electric field, expel these ions at high speed. The propellant Xenon is first ionized by the microwave, followed by acceleration with high voltage to generate the thrust. Ion engines, as well as many other types of electric propulsion, have a very high efficiency when compared to chemical propulsion methods.

The drawback is that the levels of thrust produced by these electric systems tend to be several orders of magnitude lower than those given by chemical methods. This means that to achieve the same overall change in momentum the engine must operate for longer and must therefore be more reliable than its chemical counterparts. Overall, for extended missions such as an asteroid rendezvous, an electric propulsion system can be a very efficient propulsion method and more interplanetary missions are starting to consider it in their proposals."

*snip*

"The spacecraft will then fire its engine to cruise back to Earth. The sample return capsule will be separated from the spacecraft at a distance of about 400,000km from Earth and re-enter the Earth&#39;s atmosphere in June 2007. When it survives a re-entry phase, the capsule will deploy a parachute to soft land on the ground."


You could even send your name to be left on the asteroid.

http://www.uberkomplex.com/prince.html

Dan Luna
2004-Aug-04, 04:50 PM
Also SMART-1:

http://www.esa.int/export/esaSC/120371_index_0_m.html