Electromagnetic fluid acceleration as effective rocket propulsion?

[Parkerrhees]

Let me preface this by saying I have no background in physics or any of the above other than hours and hours of reading.

Could someone explain why (if a method was developed) we couldn't use EFA as a thrust system for a rocket? My reasoning is instead of using fuel to fight gravity and push through atmosphere why not use the matter in between the ground and space as the fuel itself? Creating a posative charge in front of the craft and negative behind would in theory cause a "low pressure" and "high pressure" situation around the craft essentially forcing it through the air. The question is do we have the technology to create a strong enough flow to produce enough force to actually move an object in this manner?

 

[mfb]

Where would you get the energy and power from?
Chemical rockets have a really good power density. They can run in conditions that would melt most other things. The energy goes into the exhaust, so it doesn't melt the rocket.

Creating a posative charge in front of the craft and negative behind would in theory cause a "low pressure" and "high pressure" situation around the craft essentially forcing it through the air.

No. You would attract negative charges but repel positive charges at the front, and vice versa at the back. Air doesn't have a net charge. You would need to separate charges first, then accelerate at least one of them (better the positive ions, as they are heavier), then recombine them. All that needs way more energy than rockets have available.

 

[Parkerrhees]

That was my questing, sorry for not being clear, is there a way to Ionize the air molecules in front and attract them to the rear (to put it simply).

Edit: As far as energy, the required amount needed to do this(if possible) isn't something I've got the knowledge to figure out. Are we talking like 100kv or a lot more/less?

 

[Parkerrhees]

Chemical rockets have a really good power density. They can run in conditions that would melt most other things. The energy goes into the exhaust, so it doesn't melt the rocket

also forgot to mention this idea came after reading an article about elon musk wanting "reusable" ships

 

[klotza]

Not exactly leaving the atmosphere, but this is something that was done for the first time in small aircraft just a few years ago http://news.mit.edu/2018/first-ionic-wind-plane-no-moving-parts-1121

I imagine there are some "engineering" problems associated with just pointing the thing in the upward direction. Translation doesn't cause lift when going up, but it does when going forwards with wings. Then you get to the top of the atmosphere and I imagine there are some other issues as the density falls off. It's in "not physically impossible, but difficult" territory.

If you're looking for the term to google, it seems to be "electrohydrodynamic thrust."

 

[Parkerrhees]

Not exactly leaving the atmosphere, but this is something that was done for the first time in small aircraft just a few years ago http://news.mit.edu/2018/first-ionic-wind-plane-no-moving-parts-1121

I imagine there are some "engineering" problems associated with just pointing the thing in the upward direction. Translation doesn't cause lift when going up, but it does when going forwards with wings. Then you get to the top of the atmosphere and I imagine there are some other issues as the density falls off. It's in "not physically impossible, but difficult" territory.

If you're looking for the term to google, it seems to be "electrohydrodynamic thrust."

By that model one could hope for a "fuel less" aircraft powered by a small reactor in the future.

 

[mfb]

To be in orbit spacecraft need ~8 km/s, that corresponds to a specific energy of 32 MJ/kg.
Hydrogen/oxygen is among the most energy efficient reactions you can get. It releases ~13 MJ/kg. Batteries can reach around 1 MJ/kg.

Even if you have a 100% efficient way to increase the kinetic energy of your spacecraft you need nearly three times its orbital mass as chemical fuel, and forget batteries (unless used for fuel pumps). If you want to do anything electromagnetic you need to convert the chemical energy to electric energy. Power plants reach ~1/3 efficiency (in some cases 1/2), but they have cooling water for free and don't need a high power density (the power plants are big) - a rocket will reach much less. So now you are at optimistic 2 MJ/kg electricity. And you still didn't accelerate any air. Once you go fast the efficiency of that will likely be bad, too. This would need an awful amount of staging to get anywhere, if it reaches orbit at all, and the rocket looks really expensive as well.

 

[Parkerrhees]

Answered perfectly! Thank you makes sense on why it wouldn't work. I'll keep thinking lol

 

[Parkerrhees]

https://royalsocietypublishing.org/doi/10.1098/rspa.2014.0912

Gives a few numbers on the amount of thrust that's theorized

 

[mfb]

Their numbers are barely enough for small aircraft under some optimistic assumptions.
A Cessna 162 Skycatcher, a small aircraft, flies with a 75 kW motor.
A Merlin 1D engine has a power of 1.1 GW (thrust*exhaust speed), a factor 15,000 more (rising to 1.4 GW in vacuum). Falcon 9 has 9 of them in the first stage. Even the much smaller Rutherford engine (9 of them powering Electron) has 72 MW.