Dual Propulsion Systems?

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pathfinder_01

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I was reading a bit about electric propulsion systems. I.e. (ion, plasma, ect….) I was wondering what would be the advantages/disadvantages of having a spacecraft with a low thrust but fuel-efficient electric system and a high thrust chemical one. From what I have read it seems that with some systems., esp. the ion one the spacecraft would have to begin decelerating halfway to the destination if you wish to go into orbit. <br /><br />Could a spacecraft use an electric system to break out of earth orbit and cruise to a destination then switch to a chemical one to brake into orbit? <br /><br />Would compromising fuel efficiency lead to a quicker travel time or would the extra mass of the chemical system cause a longer time by having to lug so much mass? <br />
 
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mrmorris

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<font color="yellow">"Could a spacecraft use an electric system to break out of earth orbit and cruise to a destination then switch to a chemical one to brake into orbit? "</font><br /><br />You have it backwards if nothing else. You'd use the chemical booster first -- to eliminate the propellant mass -- then electric for braking (or further acceleration and then braking). It would be pointless to spend energy accelerating the chemicals all the way to an interplanetary target just to be able to brake quickly at the end. You might as well have your quick velocity boost at the beginning (where it will be with you through the long voyage to planet 'x'). In any event -- decelerating at midpoint is only required if you don't use a propellantless braking technology at capture... like aerobraking.
 
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pathfinder_01

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Ah, not sure, My thinking was more along the lines of being able to travel at the fastest possible speed for the longest period of time since in order to get into orbit you probably will have to slow down. Aerocapture is an interesting idea, although I wonder how that impacts about the design of a spacecraft.<br /><br />Nah my thought was more along the lines turn the electric system on and let the craft slowly accelerate to cruise speed then slam the brakes to get into orbit. I was looking for the fastest trip not the most efficient one. <br />
 
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mrmorris

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<font color="yellow">"then slam the brakes to get into orbit. I was looking for the fastest trip not the most efficient one. "</font><br /><br />Yes -- I know. However -- you're not following the logic. The vast majority of the mass of a chemical booster is the propellant -- which goes away within a matter of minutes after you light it off. The vast majority of the mass of an electric propulsion system is the powerplant/engine/etc. -- which stays with the spacecraft forever. <br /><br />So -- given a spacecraft of 2000kg main mass, 500kg of chemical propellant/oxidizer, and 50 kg of propellant for the electric drive -- the missions would be:<br /><br />A -- Use chemical boosters as brake<br /> -- accelerate 2550 kg of mass via electric propulsion to planet 'X'. At planet, kick off chemical thrusters to brake system.<br /><br />B -- Use chemical boosters as to accelerate<br /> -- In earth orbit, kick off chemical thrusters to accelerate system. Use electric propulsion to decelerate 2050 kg of mass via electric propulsion to planet 'X'. <br /><br />Mass is everything. Shifting 500kg of mass all the way from Earth to planet 'x' will make the trip take longer because the craft will have a lower acceleration rate.
 
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scottb50

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The biggest consideration is whether ion engines would add enough to justify their weight on such a relatively short trip. More propellant mass for braking and return would be available by eliminating the mass of an ion engine. <br /><br />The best solution I see is using returnable boosters and vehicle engines to send the vehicle on its way and to slow at the last possible point to enter orbit. The engines would again be used to boost back to Earth orbit and slow into orbit. <br /><br />The heavier the vehicle gets the more boosters you add, having them return to LEO for re-use allows a fairly cheap and simple transit system. Putting the engines and fuel tanks into orbit, as upper stages, and refueling them in orbit for re-use, only makes sense. <div class="Discussion_UserSignature"> </div>
 
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mrmorris

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<font color="yellow">"The biggest consideration is whether ion engines would add enough to justify their weight on such a relatively short trip. "</font><br /><br />Exactly how short of a trip is it to planet 'x'?<br /><br />Are you reading a destination in this thread that I'm missing?
 
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scottb50

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Actually I was thinking of the Moon and Mars. From there it's a long time to anywhere. I doubt we will go much beyond Mars, with manned missions for a long time. <div class="Discussion_UserSignature"> </div>
 
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rogers_buck

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Actually, I like the prospects for using ion engines for the parts of a mission that don't contain humans. Borrowing a page from the Scaled Composites lunar bid, send jerry cans full of gas, ice chests full of beer and frozen pizzas, and other mission provisions or even landers on their way ages in advance to the humans using ion drive elements. Send the humans on the fast track with nuclear propulsion or chemical rockets.<br /><br />Picture a string of supplies moving towards mars at exquisitely selected accelerations so that the manned CEV exactly matches their velocity and position at a precise point in time. Doesn't matter that it took the ion driven components months to get to that point and the rocket men mere weeks, they could meet up along the way and the supplies could be loaded up.<br /><br />Let aerobraking slow down the fully configured mission and let the ion empties swing round mars and work their way back home for refilling.
 
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