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In his Edge.org Annual Question response, Progress in Rocketry, George Dyson exults in the success of Blue Origin and Space X in landing their rockets after pushing them past the Kármán line, and then goes on to predict that this will help pave the way to rockets with separate substances as propellant and as fuel:

There is no reason the source of reaction mass (propellant) has to be the same as the source of energy (fuel). Burning a near-explosive mix of chemicals makes the process inherently dangerous and places a hard limit on specific impulse (ISP), a measure of how much acceleration can be derived from a given amount of propellant/fuel.

and

All the advances in autonomous control, combustion engineering, and computational fluid dynamics that allowed these two rockets to make a controlled descent, after only a handful of attempts, are exactly what will be needed to develop a new generation of launch vehicles that leave chemical combustion behind to ascend on a pulsed energy beam.

Leaving aside whether, and to what extent, the Blue Origin and Space X results get us in that direction, are rockets like that even contemplated as a realistic possibility in the accessible future? Kerbal beamed power aside, is there any way for a rocket to climb from the surface to Earth orbit without physically carrying the source of energy that will get it there? Or what is Dyson talking about?

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separating fuel and propellant, or not carrying the energy source? Please clarify. – Hohmannfan Jan 9 at 1:12
    
Both, really. Two characters. – E.P. Jan 9 at 1:13
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Add "LASER launcher". Reaction mass is heated and expelled with energy source by ground based LASERs (or MASERs). Used in MANY SciFi stories plus there is some real-world research thereon. Large LASERS and tight beams and adequate beam steering are needed. Lack of clouds helps. – Russell McMahon Jan 9 at 6:50
    
I think Dyson got the idea from the proposed nuclear Orion spacecraft (he has written a pretty good book about that) which would have been fueled by nuclear bombs and propelled by whatever material was turned into plasma by the detonation of the same. Nuclear bombs are not an option at the moment, so he's looking for another source of energy. – Eike Pierstorff Jan 9 at 12:18

To beam the power to the rocket, especially when needing high acceleration, is not what I will expect to show up in the near future. However, separating the energy source from the propellant is the idea behind the nuclear thermal rocket. Here, energy comes from a reactor, and you can choose more freely in higher performing propellants. Because a NTR is limited by the temperature at which the reactor starts to melt, you want an exhaust-gas with low molecular mass, most commonly hydrogen. Working prototypes are built. A separate energy source is also the idea behind an ion thruster, where high efficiency is achieved by accelerating the propellant (often xenon) using electrical energy. That is the technology used by the ongoing Dawn mission.

ion thruster

But this is also actually a possibility with chemical rockets. In his book !Ignition, John D. Clarke tells the story about how mercury was seriously considered as a part of the fuel in order increase the density impulse of a missile, trading ISP for delta-v. That is achieved because you can then fit more propellant into the same tanks. Delta-v is then calculated by:

$$\Delta v=ln\left(\frac{m_0+V*\rho}{m_0}\right)v_e$$

where $m_0$ is the dry mass of the rocket, $V$ is tank volume, $\rho$ is propellant density and $v_e$ is exhaust velocity.

As for a solar sail, it carries neither the energy source nor any propellant. Note that you can also point a laser on the sail to increase the thrust.

Heating propellant with an external laser may also be a feasible concept, because you can then have the same efficiency as a NTR, but not having to carry a heavy reactor around. Several concept for this can be found here. The temperature of the exhaust gas is also then no longer limited the melting point of the fuel elements in a nuclear reactor.

A chemical rocket is actually just a special case, where the fuel and the propellant happens to be the the same thing. In a jet engine, for example, the fuel brought along is half the energy source, and the oxidizer is collected from the air. The air passing through the engine is also the propellant used.

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Kevin Parkin has been the lead researcher into Microwave Thermal Thrusters, which was his PhD thesis at CalTech. He continues to work on the technology. From an interview with NextBigFuture:

We are scheduled to take delivery of the gyrotron in April 2012. We hope to have a subscale demonstration by 2018, and to prove suborbital capability within a decade. Within two decades, we should have true orbital, single-stage-to-orbit capacity. Of course, if we had higher funding, we could reduce those timeframes substantially.

The gyrotrons produce the microwaves that hit a heat exchanger on the rocket, which heats liquid hydrogen, which propels the rocket. It's been a while since I looked at his paper, but if I recall correctly, the heat exchanger is the biggest problem. It has to withstand very high temperatures and efficiently pass heat to the propellant.

enter image description here

Parkin was for a time Deputy Director of Ames Research Center, so he is highly regarded in the field. In 2012, the Millimeter-Wave Thermal Launch System program at Ames was created jointly by NASA and DARPA, based on Parkin's work.

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At first this looks impractical. I'd say the biggest problem wasn't the exchanger, but rather focusing microwaves onto small target so far away (btw, hence the need for a large exchanger.) However when you see he's talking about launches, you see that there's potential to increase specific impulse (though it would have to be for small rockets launched often, as the power is enormous.) It might have applications in station keeping for low earth orbit satellites to extend their life. But I'd like to see a practical microwave weapon before I'm convinced of this as a spacecraft power source. – Level River St Jan 9 at 10:59
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@steveverrill - this is a microwave weapon. NASA is just a conduit, and this stuff simply gets civvie funding but is dual-use. – Deer Hunter Jan 10 at 20:58
    
@DeerHunter haha, that explains why NASA fund it, O_O. Seriously though, 800W at close range is enough to injure a man if you get it all on target. Show me a weapon drawing 1kW power (I'll give you an allowance of 80% efficiency) that can mildly bother a 2m tall man at 1km distance by heating him to 60C, and then let's talk about propelling a 20m long spacecraft at 20km distance by heating it to 2000C. – Level River St Jan 10 at 21:10
    
@steveverrill - you know there are radars, right? And that's not even the most powerful one. – Deer Hunter Jan 10 at 21:14
    
@DeerHunter sure, you can detect me with your radar, but can you burn me with it at 1km distance? Is your beam focused enough? It's more of a challenge to do it with 1kW than 1MW. But I bet even with 1MW you have difficulty building something that can burn me at 1km distance. At those efficiencies, how much of the country's electricity supply will we need to get our little communications satellite off the ground? – Level River St Jan 10 at 21:32

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