Futuristic propulsion of spacecraft

[GTOM]

I read about ion thrusters. If they had enough energy, is it possible, that the descendants of the current engines fires the ions with relativistic speeds, and a small yacht could go from LEO to Moon and back, with small amount of propellant, in hours?

 

[Antiphon]

They already fire them with relativistic speeds. So I think the answer to your question is no.

 

[GTOM]

i have found this one.

https://www.physicsforums.com/showthread.php?t=108685

210 km/s is far from relativistic.

 

[mfb]

LEO to Moon and back is not limited by fuel even for current ion drives. The limit is the maximal thrust (limited by the design, and to a lesser extent the power), which is quite low for current engines.

Relativistic speeds would require large accelerators structures (>10m) and probably a small fission reactor for the necessary power.

 

[GTOM]

Bigger than 10m? That fits to my image of small yacht or ship.

Although i don't know, what should be the minimum size of the fission reactor.

 

[Ryan_m_b]

To get to the Moon and back to Earth in just under a day would require a constant acceleration of 0.1G (with two periods of acceleration/deceleration). That's quite a high level of thrust, this is beyond any current ion drives. You may have come across the proposal for VASIMR which is in the early stages of development and is intended to be operational at higher thrust but even that has (IIRC) a proposed specific impulse of ~3000 seconds. You'd have to make your ship 30 parts fuel for every 1 part ship for this venture and that's ignoring the mass of the fuel itself, the strain this would put on the engine and the fact that you'd need a hefty power source to run the whole thing.

 

[mfb]

Well, high-tech cavities reach ~30MeV/m for singly charged particles. If you accelerate protons (which is a bad idea for current ion drives, but probably unimportant for relativistic speeds), this would need ~10m for some significant fraction of c. This does not account for the ion source, the issue that the cavities need pulses and not constant beams, and other problems. In addition, the fission reactor needs space, mass, a cooling cycle and large radiators.

Let's see: Assume a ship with a mass of 10 tons (probably too low, but whatever), about 10.8km/s (~10h to the moon) and ~1 hour acceleration/deceleration at the moon. There, I neglect its gravity and details of orbits - which is a good approximation at the moon (but not at earth). This requires an acceleration of 3m/s and therefore a thrust of 30kN.

With E=300MeV per proton, the momentum is ~800MeV/c, which requires 7*10^22 protons per second and the nice power of ~3300 GW, which is a bit more than the total output of all power plants on Earth ;).

However, with non-relativistic speeds, the thrust to power ratio is better. There, 2P=Fv. With 1GW, v~100km/s and the journey would require some tons (~2 per direction) of reaction mass. With 100MW and 1/10 of the acceleration, you get the same numbers (apart from the time to reach the moon) and the power becomes more realistic. However, packing all this stuff in 10 tons of total ship mass is still unrealistic.

 

[GTOM]

I see.
So a 100 ton ship and a voyage of a week would be more realistic?

 

[mfb]

Longer journey times need a smaller acceleration and therefore a smaller power supply. Alternatively, they can use the same power and less reaction mass.

"Realistic"... well, up to now, no ion drive could generate a thrust of this order of magnitude. So it is science fiction anyway, but at least it has some science in it.

 

[Ryan_m_b]

I've just found an article that fits well with this thread, it outlines a slow (6 month) but relatively cheap Orbital Transfer Vehicle that continually can shuttle payloads to and from the Moon:

Projected Lunar Cargo Capabilities of High-Power VASIMR Propulsion
Tim W. Glover, Franklin R. Chang Díaz et al
Presented at the 30th International Electric Propulsion Conference, Florence, Italy
September 17-20, 2007
http://www.adastrarocket.com/Tim_IEPC07.pdf

A lunar cargo architecture based on solar-powered VASIMR plasma propulsion is considered. Performance in terms of the mass of cargo delivered to the lunar surface is presented as a function of specific impulse. A principal advantage of the VASIMR over other electric propulsion technologies for this application is its use of abundant and inexpensive argon as propellant. While it is generally believed that solar electric propulsion offers significant economic advantages over chemical propulsion to a large-scale lunar exploration program, the cost of solar photovoltaic power will be a critical factor in achieving real cost savings. Solar electric power cost will strongly affect the choice of thruster technology and optimal specific impulse.

 

[GTOM]

Is it possible to solve the mentioned problems of energy density, if the ship could wield an antimatter battery?

 

[mfb]

With antimatter, the stored energy density is enough for all needs in the solar system. The issue is just to store and extract this energy in an efficient way. Oh, and you have to produce the antimatter first, of course.

 

[Astronuc]

Is it possible to solve the mentioned problems of energy density, if the ship could wield an antimatter battery?

With antimatter, the stored energy density is enough for all needs in the solar system. The issue is just to store and extract this energy in an efficient way. Oh, and you have to produce the antimatter first, of course.

Antimatter production is the biggest impediment.

Even with a source of antimatter, one still has to have a chamber in which to allow the anihilation reaction, which then has to transfer that energy to propellant, which is usually hydrogen. The thruster chamber (usually assumed to be a magnetic confinement system) pressure is usually the limiting factor.

The downside is the loss of about half the energy to neutral pions and (e⁺e^-) to gammas.

The current problem with VASIMR is the low thrust and relatively low specific energy.

 

[Ryan_m_b]

Is it possible to solve the mentioned problems of energy density, if the ship could wield an antimatter battery?

An antimatter battery as if we didn't need more dangerous technology.

 

[GTOM]

Well, if we want to rescue a damaged ship for example, it is worth the risk, to get there fast... get near to it fast, but outside the radius of a possible anti matter explosion or whatever.

Basically the best futuristic way would be convert stabil matter into energy, but as far as i know, currently there is no way to do that.

 

[Ryan_m_b]

Well, if we want to rescue a damaged ship for example, it is worth the risk, to get there fast... get near to it fast, but outside the radius of a possible anti matter explosion or whatever.

Basically the best futuristic way would be convert stabil matter into energy, but as far as i know, currently there is no way to do that.

The point is you have to keep antimatter caged up and hope that the storage mechanism never fails. The energy we are potentially talking about here is horrendous, if you manage to bring antimatter production down to reasonable prices then you've proposed a system whereby biosphere destroying devices are available for an unreasonable but possible price. Think of it this way: any vehicle fitted with a few grams of antimatter will release a Hiroshima scale explosion when damaged. A few kilograms and you've got the release of >Tsa Bomba scale explosion when damaged.

The potential harm of what you are proposing more than outweighs it's uses IMO.

 

[FTL]

FTL drive :P

 

[Ryan_m_b]

FTL drive :P

There is no good reason to think this is possible.

 

[qraal]

I read about ion thrusters. If they had enough energy, is it possible, that the descendants of the current engines fires the ions with relativistic speeds, and a small yacht could go from LEO to Moon and back, with small amount of propellant, in hours?

The minimal power (dE/dT) for a voyage is when acceleration - forward or reverse - occupies 2/3 of the total trip time. To the Moon and back, 768,000 km, in six hours ('a few') requires 14.8 m/s² acceleration, which is perhaps a touch uncomfortable for the four hours under thrust.

Total delta-vee - the velocity change - is 213.3 km/s, which is doable by advanced ion drives, but impossible to achieve in just four hours under thrust. Ion drives have strict thrust limits due to arcing from excessive voltage. Plasma drives, particularly VASIMR, have no such voltage limits, but they do have heat-loading issues. Current designs take weeks to reach the Moon, albeit using much less propellant than chemical rockets.

 

[mfb]

The minimal power (dE/dT) for a voyage is when acceleration - forward or reverse - occupies 2/3 of the total trip time. To the Moon and back, 768,000 km, in six hours ('a few') requires 14.8 m/s² acceleration, which is perhaps a touch uncomfortable for the four hours under thrust.

Why would you need a higher (peak?) power when you accelerate 1/2 of the trip and decelerate the other 1/2? In addition, you can reach the moon with 100W and an ion drive. It just needs ages.
I think the 2/3 are the result of some other optimization process.

 

[qraal]

Why would you need a higher (peak?) power when you accelerate 1/2 of the trip and decelerate the other 1/2? In addition, you can reach the moon with 100W and an ion drive. It just needs ages.
I think the 2/3 are the result of some other optimization process.

For a given distance, over a given travel-time, there's a minimum power. Kinetic energy depends on the square of the velocity, but the required peak velocity increases for the greater time spent accelerating. Ignore the complications of mass-ratios, and it can be proven that the minimum power to make the trip is when 1/3 the total time is spent accelerating, 1/3 coasting and 1/3 braking. Power being the first derivative w.r.t time of the energy. Do you want a reference?

 

[mfb]

A reference would be interesting, indeed.

With 1/3, 1/3, 1/3 for the distance d in time T and uniform acceleration, the maximal velocity v can be determined via 2/3T*v=d <=> v=3d/(2T). The required acceleration is then given by a=9d/(2T).
With 1/2, 1/2, the maximal velocity is given by 1/2T*v=d <=> v=2d/T and the required acceleration is a=4d/T, which is smaller.

As long as the mass of the rocket does not change significantly during the trip, power is proportional to (or at least monotonic in) acceleration. If the mass changes, we need additional data about that. In this case, I would expect asymmetric solutions.

 

[qraal]

My reference for the minimum power is Moeckel, W.E. from here...

Comparison of advanced propulsion concepts for deep space exploration

...though you have to extract it from his discussion as he doesn't lay out the algebra involved. In other words, an exercise for the reader.

 

[mfb]

In this case, I'll just assume that his calculation involves some other constraints (like max delta v), otherwise see the calculation in my post for the algebra.

 

[GTOM]

Well this one is optimistic about antimatter.
http://worldofweirdthings.com/2012/05/16/why-wed-want-to-make-some-more-antimatter/

(Well i know it investigating fringe things, but from what i read i don't consider it a pseudo scientific site, although I am not PF mentor, so sorry if i am wrong.)

Is there any theoretical chance, that antimatter can be wrapped in neutral particles somehow, like a proton wraps in the positron?

Also someone had the idea to build giant magnetic sails, and propel them with neutralized particle beams. He thoughts it is better than laser. Can particle beams act like ultra-short wavelength lasers?

 

[Ryan_m_b]

Well this one is optimistic about antimatter.
http://worldofweirdthings.com/2012/05/16/why-wed-want-to-make-some-more-antimatter/

(Well i know it investigating fringe things, but from what i read i don't consider it a pseudo scientific site, although I am not PF mentor, so sorry if i am wrong.)

I took a brief look and couldn't see anything crackpot about it (though I could have easily missed it) but regardless this is some random person's blog. All they are doing is talking about a few articles they have read, how does that add anything to the discussion? Aside from this nothing they say changes the objections I raised above.

Is there any theoretical chance, that antimatter can be wrapped in neutral particles somehow, like a proton wraps in the positron?

I don't think this is true, positrons are not just sitting in the proton. Regardless all you are proposing is a system whereby the ship would then have to carry a particle accelerator to make the antimatter it is about to use (which would consume hideous amounts of energy).

Also someone had the idea to build giant magnetic sails, and propel them with neutralized particle beams. He thoughts it is better than laser. Can particle beams act like ultra-short wavelength lasers?

See here http://en.wikipedia.org/wiki/Magnetic_sail

 

[GTOM]

I read the Tsiolkovsky_rocket_equation. Did i get it right that with chem fuel, in order to reach 1000km/s, you need more fuel than the mass of the Moon?

Is there any possibility we can think, to reach anywhere near it, with just a tiny probe?
Is there any plan to build giant mass drivers on the Moon, the boost interplanetary spacecraft ?

 

[Ryan_m_b]

Why'd don't you show your working so people can either learn if it is correct or help you if it is incorrect? As for mass drivers there are no plans for it no.

 

[mfb]

I read the Tsiolkovsky_rocket_equation. Did i get it right that with chem fuel, in order to reach 1000km/s, you need more fuel than the mass of the Moon?

Right.

Is there any possibility we can think, to reach anywhere near it, with just a tiny probe?

There are better propulsion systems that can reach 1000km/s.

Is there any plan to build giant mass drivers on the Moon, the boost interplanetary spacecraft ?

There are concepts, but that doesn't mean much. There are concepts for nearly everything.

 

[GTOM]

"There are better propulsion systems that can reach 1000km/s."

What do you think, what can achieve that? Fusion torches, antimatter batteries? (Yeah i can't deny the last one is a really unsafe option.)
Or maybe systems, where the ship doesn't have to carry all its power source (like sails with laser assist)?