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When I was in primary school, I spent countless hours designing spaceships that probably wouldn’t work. It’s only now that I’ve stumbled across some of the old drawings and actually thought if it could be possible.spaceship drawing

My drawings included VASIMR VX-200 Plasma engines as main and sometimes only propulsion. Would these generate enough thrust to do a harrier style vertical takeoff?

If so how many of them would I need?

(For reference the length is about 300m long and weighs about 700Tons)

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  • $\begingroup$ If there are any details I have missed or my question is unclear please tell me. $\endgroup$ – Tim Li Dec 3 '19 at 10:40
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    $\begingroup$ I would like to say that I only drew spaceships in primary school, as you did, but the truth is I still draw spaceships when I'm supposed to be paying attention to something else. $\endgroup$ – Wayne Conrad Dec 3 '19 at 17:55
  • $\begingroup$ Haha lol. I spend too much time daydreaming about spaceflight $\endgroup$ – Tim Li Dec 3 '19 at 20:07
  • $\begingroup$ Shameless self promotion: I asked and answered a question on how you take off from an airless body most efficiently with vasimir engines. space.stackexchange.com/questions/27842/… This solution is only feasible for tiny asteroids, due to the aforementioned thrust problems. $\endgroup$ – Ingolifs Dec 3 '19 at 23:59
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The absolutely most basic idea? Maybe, but a VASIMR engine capable of planetary takeoff or landing is not a VASIMR engine.

VASIMR engines are a type of thermal rocket -- they work by heating gas and expanding it out of a nozzle (much like chemical and nuclear thermal engines, but NOT like gridded ion thrusters). Diffuse gas is ionized into plasma and contained in a magnetic field, heated by radio frequency energy to a very high temperature, and then allowed to expand out of a magnetic nozzle. This requires a large amount of power, but it allows a pretty good ISP, a better thrust than gridded ion thrusters, and the ability to adjust the ISP by adjusting the temperature the plasma is heated to (and therefore the energy required per unit thrust).

There are two problems:

  1. As mentioned by the other answer, you need a LOT of energy to produce significant thrust with any electrical thruster whatsoever -- you'd better have a panpoly of incredibly lightweight, efficient, high-power nuclear reactors.

  2. The principles by which the VASIMR engine work are pretty dependent on the plasma being pretty diffuse -- i.e. the sort of thing that works in vacuum or near-vacuum, but not in an atmosphere. Also, it means that you just don't get very much thrust per engine even if you have plenty of power, and therefore the engine weighs more than it can lift (even before you get the nuclear reactor aboard!)

All is not lost. There is a lower-level form of electric rocket that uses a somewhat similar principle to the VASIMR engine, but rather than pre-ionizing plasma and containing it in magnetic fields This is the Arcjet, which is basically built like an ordinary chemical rocket, but instead of a combustion chamber it has a chamber where an electric arc (like that produced by an arc welder) heats gaseous propellant at high pressure, which then is expanded out of a normal bell nozzle. Arcjets can produce higher thrust, but you still will need an exceptionally powerful reactor to hover or take off with them.

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  • $\begingroup$ The powerful reactor is easy. At least in my mind. Thanks for your help. I think that is exactly what I need. $\endgroup$ – Tim Li Dec 6 '19 at 10:29
  • $\begingroup$ @TimLi Only if you have futuristic ultra-tech. Even if you can make a very powerful, lightweight reactor, you also need to dissipate waste heat. $\endgroup$ – ikrase Dec 6 '19 at 10:39
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The VASMIR 200 is listed as having a thrust of 5.4 newtons, and you need 9.8 newtons to lift 1kg against earth's gravity. So 700 tonnes is going to need more than a million engines and be consuming more than 254 GW of electricity.

So even if the engines are weightless this is not lifting off from earth without co-opting the power generation of a sizable country.

Electric propulsion also tends to lose most of the efficiency when working against an atmosphere, though with a quarter of a TW in play it might start to work like like a poorly performing nuclear thermal rocket by heating the atmosphere.

Electric propulsion generally does not work well in an atmosphere or at high thrusts, so is generally reserved for missions where maneuvers can be completed over very long periods (weeks to months).

If you want to explore this type of vehicle Kerbal Space program with some of the near future mods can be a useful tool to to see what becomes possible and what remains impossible.

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  • $\begingroup$ What's the odds though of creating a vehicle that could achieve, say, starting from in space and not using these kind of engines to launch, 100 km/s speed with acceleration times in the range of 1-5 Ms (up to ~58 days), and deceleration at destination? $\endgroup$ – The_Sympathizer Dec 4 '19 at 3:52
  • $\begingroup$ @The_Sympathizer the thrust of a VX200 is too low to achieve that sort of performance with any plausible ship mass. The Isp is in the right sort of ballpark at least, but without both a significantly more powerful (but similarly efficient) rocket design and some kind of suitable nuclear reactor, you'll be out of luck. May as well be asking for a fusion rocket ;-) $\endgroup$ – Starfish Prime Dec 4 '19 at 10:14
  • $\begingroup$ @Starfish Prime : So what is the ideal use case of these rockets then, exactly? $\endgroup$ – The_Sympathizer Dec 4 '19 at 11:53
  • $\begingroup$ @The_Sympathizer beats me! the might be more useful one day, once the inconvenient power source issue has been solved, but who knows if they'll still be relevant by then. $\endgroup$ – Starfish Prime Dec 4 '19 at 12:01
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    $\begingroup$ @The_sympathizer they are potentially an incremental step up from ion drives due the trajectory flexibility from allowing periods of lower efficiency/higher thrust, say to operate around an asteroid (reduce gravity loses) or using the Oberth effect. The claimed peak ISP is still lower than a pure ion drive so not a contender of maximum theoretical D/V in a given vehicle mass, just where trajectories get more complicated. $\endgroup$ – GremlinWranger Dec 4 '19 at 13:24

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