Are electric rockets possible?

[Rdivine]

Is it possible to make a rocket that only runs on electricity, nothing else, and make it reusable?

Let's imagine a rocket that is split into 2 stages. The first stage might be able to use propellers to get itself to a suitable altitude.

After which, the first stage separates and comes down to land. Now, what would the second stage run on?

Ion Engines? Is it possible to manipulate microwaves/light to generate a force large enough for the rocket to reach orbit?

 

[SargeRho]

Yes, using beamed power, which would only use electricity. The laser or microwave beams would be aimed at a heat exchanger on the spacecraft, which would heat hydrogen propellant. This engine would be 2-3 times as efficient as a chemical rocket, according to Escape Dynamics' test result, which is typical for thermal rockets.

Purely electric thrusters have very low thrust, though, I think something on the order of 1 newton per gigawatt.

Edited January 16, 2016 by SargeRho

[Gaarst]

Photons have momentum. If you shine a big enough laser in one direction, you'll go the other way.

I insist on the "big enough" part though.

[Temstar]

No, it's not possible. You are asking for a reactionless drive which violates Newton's third law of motion. Even beam power or light craft or ion drive all have propellent that they shoot out the back to produce thrust (even if they can do it much more efficiently).

Photon rockets take 300 megawatts of power for one lousy Newton of thrust. For anything other than spacecrafts with undetectable low level of thrust its beam will be a weapon of mass destruction, Death Star style. And even then it's not quite reactionless since that 300 megawatt per newton has to come from somewhere and a good bet is that you're doing some kind of matter annihilation, so even with a photon rocket you will eventually run out of matter.

[AngelLestat]

beamed energy to be used as thermal source does not qualify as electricity, of course rockets needs propellent, so I guess there is no way to go around that.
Or maybe he was asking for the turbo pump.. 
In this case would be an electric turbo pump using beamed energy.
 

[cubinator]

Any light beam strong enough to propel a spacecraft to orbit is also strong enough to plasmify and tear away the entire atmosphere, so I doubt this type of propulsion will be used on launch vehicles anytime soon. However, there has been some talk about the so-called EmDrive created by Roger Shawyer several years ago. It seems to violate conservation of momentum by bouncing microwaves in a resonant cavity and producing thrust without any exhaust, and there is a lot of skepticism on whether or not it actually works. If it is proven to work (IMO this will only happen when someone puts one in space) it would be exactly the thing you are looking for, especially if we create one with a superconducting cavity, which supposedly would create enough thrust to lift a small car with just one engine powered by a typical microwave oven magnetron.

[Albert VDS]

25 minutes ago, AngelLestat said:

beamed energy to be used as thermal source does not qualify as electricity, of course rockets needs propellent, so I guess there is no way to go around that.
Or maybe he was asking for the turbo pump.. 
In this case would be an electric turbo pump using beamed energy.
 

A turbo pump would be able to propel fast enough to get anything into orbit. You need to heat it up, in chemical rockets this is done by combustion.
But this could also be done with a microwave beam and only hydrogen as fuel like what Escape Dynamics developing.
The microwave beam heats up the hydrogen in the spacecraft and propels it out of the back. They are estimating the ISP above 800s.

Edited January 16, 2016 by Albert VDS

[Kaos]

Nothing else than electricity is an unclear definition. If it would use the earth magnetic field to push against it, is it purely electrical? It would say no, as you use the earth then, which is nonelectrical to some degree.

I guess the question is more about a completely refillable ressource in free space in our solar system. Then the mentioned approach of Escape Dynamics would qualify, as hydrogen is part of the solar wind and can collected, in principal at least.

[magnemoe]

It has been ideas of using charged parties from the sun in solar sails instead of photons, benefit is that you don't need an solid sail just an magnetic field however it has to be far larger than the solar sails who are large themselves and will require superconducting materials. 
Probably the closest you can get. 

Note that you could also use an magnetic field around an planet to change orbit. 

[Kaos]

Solar sails use mostly charged particles and not just the photons. The advantage of the magnetic sail is that you do not need to cover the whole area with material but just with magnetic fields. As generating this fields requires complicated electronic systems there is a base weight to this propulsion system which has to be compensated by bigger sails. In principle it is even possible to start with this concept from earth at the magnetic poles, but I do not know of any project that has seriously considered this.

[AngelLestat]

But propulsion using solar wind is 2 orders lower magnitud than solar sails. 
Some said that is possible to create a sail made with magnetic plasma that would reflect photons providing a low density area. 

[fredinno]

10 hours ago, Gaarst said:

Photons have momentum. If you shine a big enough laser in one direction, you'll go the other way.

I insist on the "big enough" part though.

Photons have no mass though, so it would be reactionless.

So no, that would not work. I think.

7 hours ago, magnemoe said:

It has been ideas of using charged parties from the sun in solar sails instead of photons, benefit is that you don't need an solid sail just an magnetic field however it has to be far larger than the solar sails who are large themselves and will require superconducting materials. 
Probably the closest you can get. 

Note that you could also use an magnetic field around an planet to change orbit. 

But technically, it's the protons and electrons in the field causing those changes...

Edited January 16, 2016 by fredinno

[Gaarst]

13 minutes ago, fredinno said:

Photons have no mass though, so it would be reactionless.

So no, that would not work. I think.

Mass is not needed for reaction.

Newton's second and third law of motion do not mention mass anywhere in their original definition.

The equation: F = m*a is a simplification of the 2nd law in the case of constant mass. The actual formula is: F = dp/dt with p the momentum which does not necessarily depend on mass.

The third law uses forces, but not masses so action-reaction is possible with massless objects.
An example of this is the radiation pressure: radiation pressure is a general term for several phenomenons in which the interaction between EM radiation and a surface exerts a pressure on the surface (or a force). One of these is the reflection of photons on the surface: they impact the surface and, when they are reflected, the surface feels a small force (because of conservation of momentum and 3rd law).

Edited January 16, 2016 by Gaarst

[Kaos]

11 minutes ago, fredinno said:

Photons have no mass though, so it would be reactionless.

...

Photons have mass, they just have no rest mass.

[fredinno]

3 minutes ago, Gaarst said:

Mass is not needed for reaction.

Newton's second and third law of motion do not mention mass anywhere in their original definition.

The equation: F = m*a is a simplification of the 2nd law in the case of constant mass. The actual formula is: F = dp/dt with p the momentum which does not necessarily depend on mass.

The third law uses forces, but not masses so action-reaction is possible with massless objects.
An example of this is the radiation pressure: radiation pressure is a general term for several phenomenons in which the interaction between EM radiation and a surface exerts a pressure on the surface (or a force). One of these is the reflection of photons on the surface: they impact the surface and, when they are reflected, the surface feels a small force (because of conservation of momentum and 3rd law).

Oh, my bad.

[Gaarst]

2 minutes ago, Kaos said:

Photons have mass, they just have no rest mass.

Photon are massless, or at least their mass is so small we cannot measure. I would like to hear how you justify this statement.

[Kaos]

Photons carry energy (as you can see for example as photovoltaic works) and E = m c^2, hence Photons have relativistic mass. They have no rest mass, though. Which I already said.

One photon of green light weighs for example about 4 * 10^-36 kg.

[Elthy]

Why would you want to use electric power anyway? Its the type of energy thats the hardest to aquire (we use chemical energy in lots of powerplants to create electrical energy with bad efficency). For a enviromental friendly rocket you could still rely on chemical propelants, e.g. hydrogen created by electrolysis of renewable energy. Thats way easier due to the high energy density of chemical propulsion...

[Gaarst]

Just now, Kaos said:

Photons carry energy (as you can see for example as photovoltaic works) and E = m c^2, hence Photons have relativistic mass. They have no rest mass, though. Which I already said.

One photon of green light weighs for example about 4 * 10^-36 kg.

The E = mc² formula is not the total energy of the particle, but the energy at rest.

For moving particles such as photons (which have no rest state), you have to use the following form: E² = m²c² +p²c⁴, where p is the relativstic momentum of the particle.

This last equation shows that the photons do not need to have mass and that your justification is not valid, especially since photons have non-zero momentum.

Furthermore, the basic definitions of special relativity show that if the photons had a non-zero mass they couldn't travel at the speed of light. This would have heavy implications to laws of electrodynamics which we do not observe.

Finally, the best estimates of a hypothetic mass for photons place an upper-limit at 10^-14 eV/c² which is a lot lighter than the result you gave (by 14 orders of magnitude since 1 eV/c² = ~10^-34 kg).

 

Also, the concept of relativstic mass is outdated and very controversed. It is better to use relativistic momentum to avoid confusion: relativistic momentum can be applied everywhere and does not imply changes to the object itself whereas relativistic mass does.

[Kaos]

11 minutes ago, Gaarst said:

The E = mc² formula is not the total energy of the particle, but the energy at rest.

For moving particles such as photons (which have no rest state), you have to use the following form: E² = m²c² +p²c⁴, where p is the relativstic momentum of the particle.

This last equation shows that the photons do not need to have mass and that your justification is not valid, especially since photons have non-zero momentum.

Furthermore, the basic definitions of special relativity show that if the photons had a non-zero mass they couldn't travel at the speed of light. This would have heavy implications to laws of electrodynamics which we do not observe.

Finally, the best estimates of a hypothetic mass for photons place an upper-limit at 10^-14 eV/c² which is a lot lighter than the result you gave (by 14 orders of magnitude since 1 eV/c² = ~10^-34 kg).

 

Also, the concept of relativstic mass is outdated and very controversed. It is better to use relativistic momentum to avoid confusion: relativistic momentum can be applied everywhere and does not imply changes to the object itself whereas relativistic mass does.

E = mc^2 can be used with the particle at rest for rest energy and rest mass, it can be used for the total energy and relativistic mass. There might be controverse about it I am not aware of, but it is surely not settled that this concept is obsolete. And as it is widely believed that energy conservation holds and one can violate it, if photons in motion have no gravity effect, I would suggest, that this concept has some point. But I do not want to start a discussion about relativity theory here (it is off-topic and not every discussion that can be made has to be made).

The relevant thing I wanted to point out, is that photons carry impulse, which is why I would not call this reactionless.

[wumpus]

20 hours ago, Elthy said:

Why would you want to use electric power anyway? Its the type of energy thats the hardest to aquire (we use chemical energy in lots of powerplants to create electrical energy with bad efficency). For a enviromental friendly rocket you could still rely on chemical propelants, e.g. hydrogen created by electrolysis of renewable energy. Thats way easier due to the high energy density of chemical propulsion...

There are plenty of reasons to use electric power once you are in orbit.  Mainly because solar panels and RTGs keep producing electricity without requiring any [more] mass.  Most of the suggestions for using electricity to send spacecraft into orbit are mainly about not encumbering the spacecraft with the weight of the fuel (and engines).  Electric propulsion means then include blasting the thing into space via laser (/maser choose whatever EM wave is efficient), railguns (all the problems of a cannon, with vastly higher cost), and probably even less sane ideas.

The bit with the laser might even make sense, but I wouldn't be surprised if its harder than a space elevator in practice.

[magnemoe]

22 hours ago, Elthy said:

Why would you want to use electric power anyway? Its the type of energy thats the hardest to aquire (we use chemical energy in lots of powerplants to create electrical energy with bad efficency). For a enviromental friendly rocket you could still rely on chemical propelants, e.g. hydrogen created by electrolysis of renewable energy. Thats way easier due to the high energy density of chemical propulsion...

Most high isp engines use electricity to accelerate the reaction mass: ion or vasmir. Chemical engines is limited to 450 isp, using heat you are limited to 800-1000 and something. 
electrical you could get past 10.000. 
No they are not pure electrical but they need power to run.