Most realistic propelling technologies?

[Armantium]

This is for hard science fiction only.
Current propelling technology is limited to:

• jet engines, which need air flow, so they can only propel vehicles within planetary atmosphere
• rocket engines, which need high quantity of fuel, making them extremely inefficient, unstable, and lacking in maneuverability

So, let's say you have a shuttle-type vehicle like the one shown in the movie Elysium, which can smoothly and efficiently go from an orbiting space station, acting as a short-range spacecraft , to landing anywhere on Earth, acting as a VTOL aircraft.

What kind of realistic propelling technology would make that possible?
Engines which have a mini-nuclear source?
Completely different kind of rocket fuel?

 

[.Scott]

The real problem is the amount of energy that you need to store to go from the Earth's surface to orbit. If you had that amount of energy, then some sort of linear reaction mass accelerator or ion drive would work (assuming engineering is no limit).
You want high energy with low mass. Matter/antimatter comes to mind, but there would be an impossible problem with shielding the craft from the resulting radiation.

 

[Armantium]

Matter/antimatter comes to mind, but there would be an impossible problem with shielding the craft from the resulting radiation.

I was thinking of more conventional means. What kind of realistic explanation is usually put forward for such vehicles as the Elysium shuttle, which can act both as a short-range spacecraft and as a VTOL aircraft?

This is pretty much in every movie and game ever.

 

[.Scott]

Just make up your favorite energy storage device... Like "Xenon trap energy cells". Whatever.
Don't forget to keep them charged.

 

[Armantium]

Just make up your favorite energy storage device... Like "Xenon trap energy cells". Whatever.
Don't forget to keep them charged.

You mean using compressed Xenon gas as the source of thrust material, while some kind of efficient power plant(fusion) acts upon that source to initiate and maintain the thrust?

I thought that Xenon gas was not very reactive.

 

[.Scott]

You mean using compressed Xenon gas as the source of thrust material, while some kind of efficient power plant(fusion) acts upon that source to initiate and maintain the thrust?

I thought that Xenon gas was not very reactive.

No. I was just making something up. Use fusion - and don't worry about what the exact technology would be.

 

[Armantium]

But I do worry.
So, you have a fusion reactor, which can power up jet engines without a problem. But that only works in atmosphere filled with air.
What does it power when in the vacuum of space?

 

[.Scott]

Once you have you energy source, you can use it to power ion drive or reaction mass acceleration.
There are two separate problems: The first is to be able to provide the power without a huge amount of mass. The second is to be able to provide reaction mass without a huge amount of mass. In order to keep the reaction mass low, you need to accelerate it to very high speeds. For example, if you are going into an orbit at 17Kmph from a 3Kmph jet cruise and your reaction mass is only 20% of your total mass, then you need to jettison that mass at well over 70Kmph. There are only so many ways of doing that. One is using some sort of on board gun - magnetic or electric gun, a.k.a. mass driver. Another is with ion drive - but with much greater capacity than existing ones.

So, moving on to the "power without huge amount of mass", you have a few choices. Matter-antimatter can produce the power - but will release deadly gamma rays in all directions. You'll have liftoff, but the shuttle will be sterilized of all life forms. The next is fusion - not provably impossible. It would have to be something that also did not create huge amounts of gamma radiation. Perhaps some yet-to-be-discovered cold fusion method. Finally, there is storing the energy in some sort of electro-chemical, electro-mechanical, whatever. It would be a few orders of magnitude more efficient that what is available in our world. When I suggested "Xenon trap energy cell", I was thinking that one might strip a bunch of electrons off the Xenon as a form of stored energy. In a SciFi, you set up some level of plausibility and then hold to that rule. So I would think some combination of high voltages, chemistry, and the right materials might be best. If nothing else, it leaves you free to develop maintenance and refueling rules to support your story.

 

[Armantium]

So I would think some combination of high voltages, chemistry, and the right materials might be best. If nothing else, it leaves you free to develop maintenance and refueling rules to support your story.

OK, here is my possible solution for the Elysium-type shuttle, falling into the hard science fiction territory:

• a VTOL craft with hybrid engines, which auto-configure internally from airbreathing jets to non-airbreathing jets, as needed. So it can transverse both space and atmosphere.
• while in airbreathing jet mode, the craft can collect hydrogen from air, which is stored(compressed) or used as power source
• the power source(for the airbreathing jet mode flight and everything else) is a plasma reactor using the Neutral Beam Injection method for heating the plasma, this method uses the external source of hydrogen

What do you think?
Feasible in less than hundred years?

 

[.Scott]

I take it that the "Plasma Reactor" is a fusion reactor - so, aside for a standby battery, that will be your only power source.
You don't need to collect hydrogen for fuel because you have a fusion reactor - and the fusion reactor will require very little hydrogen. Of course, if you're thinking that the hydrogen is a chemical energy source, the hydrogen would require an oxidizer to burn.

You may want to collect gas as reaction mass. But it is probably easier to store than up while on the ground or while docked.

 

[Armantium]

Of course, if you're thinking that the hydrogen is a chemical energy source, the hydrogen would require an oxidizer to burn.

Yes, the plasma reactor to power the air-breathing jet engines during atmospheric flight, and the compressed hydrogen as source to power the non-airbreathing jet engines during space flight.
So, you would just remove the hydrogen collection as impractical in the future?

 

[Carrock]

There's another fuel which I read about in a reprint of a 1950's comic - isospin monatomic hydrogen.
It's got the best specific impulse of any chemical fuel, with no worries about radioactivity or toxicity.
It does have a few drawbacks...

I believe it's only been made in microscopic amounts, one atom at a time.
It can only be stored in a container lined with liquid (or maybe frozen) helium as contact with anything else flips the spin of some atoms and initiates a chain reaction conversion to $H_2$.
(It's also supposed to be the only substance which can't be frozen.)

It's the chemical rocket fuel of the future and probably always will be...

Still, there's no theoretical reason it shouldn't be usable.
The likelihood of such a spacecraft blowing up could be a useful plot device.

 

[.Scott]

So, you would just remove the hydrogen collection as impractical in the future?

Since you're using fusion power, it serves no purpose.

 

[russ_watters]

• while in airbreathing jet mode, the craft can collect hydrogen from air, which is stored(compressed) or used as power source

Besides being pointless, there basically isn't any free hydrogen in the air anyway.

Feasible in less than hundred years?

This is science fiction. The core technology is just technobabble. So there is no feasibility timeline. Charitabily, if we just call it a "fusion reactor", it still doesn't get much better because we aren't very close to figuring out how to make a fusion reactor, much less a tiny one. Nor does a "reactor" generate electricity, it just generates heat (always neglected in sci fi). One of the key enemies of tiny power sources is heat. Unless we find a way to harness the heat energy at really high efficiency, the waste heat of such a reactor could heat a small city. Right now, the only feasible method is taking the heat and using it to boil water and spin a turbine. See, so you don't just have to invent the power source, you also have to invent a way to convert the power generated into electricity and do it small.

On the more general question of if a small single stage to orbit craft is ever going to become possible, I doubt it. In addition to the incredibly dense power system, the propulsion system even for a small craft would destroy anything in its path due to the artillery machine gun level kinetic energy of the reaction mass.

 

[DrStupid]

Besides being pointless, there basically isn't any free hydrogen in the air anyway.

Free hydrogen is not required. Water would be sufficient.

we aren't very close to figuring out how to make a fusion reactor

There already are working fusion reactors. I guess you mean a fusion reaction that release more energy than it consumes.

Right now, the only feasible method is taking the heat and using it to boil water and spin a turbine.

A rocket engine can provide thrust without boiling water or tubines. Why shouldn't that work with fusion instead of a chemical reaction as energy source?

In addition to the incredibly dense power system, the propulsion system even for a small craft would destroy anything in its path due to the artillery machine gun level kinetic energy of the reaction mass.

Large chemical rockets destroy everything behing them as well. That was no reason not to usem them. Of course this problem will be even worse with dramatically increased specific momentum and would need to be handled accordingly but is not new. A possible solution would be using air breathing engines for takeoff and landing and the rocket engine in space or at high altitudes over safe territory only.

 

[russ_watters]

Free hydrogen is not required. Water would be sufficient.

Sufficient for what? Please note that in order to get hydrogen from water you need to "unburn" it. That's exactly the opposite of what usually happens in a rocket engine. So after you unburn it (by what process?), what do you do with it?

A rocket engine can provide thrust without boiling water or turbines. Why shouldn't that work with fusion instead of a chemical reaction as energy source?

That's only half an idea - and only the easy half. A rocket engine uses the combustion of hydrogen and oxygen to produce heat and propel its own exhaust products out of the engine. The propulsion of the exhaust products out of the engine as a reaction mass is the entire point of the engine. So when you use a fusion reactor (which does nothing except produce heat), what is the reaction mass and how do you expel it?

Large chemical rockets destroy everything behind them as well. That was no reason not to use them.

Normal large chemical rockets require hardened/protected launch pads. The Elysium/District 9 drop ship landed/took off from a populated area.

A possible solution would be using air breathing engines for takeoff and landing and the rocket engine in space or at high altitudes over safe territory only.

Yes, such a craft would probably require a secondary, gentler, propulsion system for takeoff. What would it be and how heavy would it be? That's not a trivial problem: it isn't easy to use a single propulsion system for VTOL, much less a second.

 

[DrStupid]

So after you unburn it (by what process?), what do you do with it?

unburn: electrolysis or thermal decomposition
use: nuclear fusion

So when you use a fusion reactor (which does nothing except produce heat), what is the reaction mass and how do you expel it?

reaction mass: fusion products and unburned fuel
expel: let it escape through a hole in the containment of the fusion recator and maybe focus the resulting plasma jet to increase the efficiency

Yes, such a craft would probably require a secondary, gentler, propulsion system for takeoff. What would it be and how heavy would it be?

With our current technology the best choice would be a scramjet. With future technology the answer will most probably be different.

 

[russ_watters]

unburn: electrolysis or thermal decomposition

Electrolysis requires electricity...

use: nuclear fusion

Ok...so how large would the capture and extraction system be vs how large a tank of liquid or compressed hydrogen you would need to power the ship for a year? I'm thinking you would need thousands of pounds of equipment to replace a pound of stored hydrogen. That isn't an improvement.

reaction mass: fusion products

Unlikely: the fusion products have extremely small mass. And how are you going to propel them?

and unburned fuel

Huh? What unburned fuel? And how are you propelling it?

expel: let it escape through a hole in the containment of the fusion recator and maybe focus the resulting plasma jet to increase the efficiency

So just use the heat? That's what a normal rocket engine does except with many orders of magnitude more reaction mass. What you are describing is not an improvement.

With our current technology the best choice would be a scramjet.

Scramjets are ramjets for supersonic combustion. Like all ramjets, they don't do anything while sitting on the ground.

What you are describing here is idle speculation so far from reality it is practically technobabble. Please reel it in.

[edit] Meh, nevermind. This is the science fiction section. When someone asks if a technobabble technology is going to be possible in real life, the discussion is pretty much doomed to go off the rails irreparably. Thread locked.