Laser-beam propulsion

Philipum · Veteran Member

Laser-beam propulsion

10kBq Jaro · Senior Member

From way back in February 22, 2003 :

Regarding the article by Anthony Tate about the GCNR "Liberty Ship," in "Opening the Next Frontier," at http://www.nuclearspace.com/a_liberty_ship.htm

Mr. Tate says that "we find we need 2,400,000 pounds of reaction mass.... Since hydrogen is the best reaction mass physics allows, and is cheap, plentiful, and we have decades of experience handling it, we will use it."

But hydrogen is about as "perfectly transparent" to the light from the NLB as is the fused silica he suggests be used for the transparent wall (called a low-opacity wall).

The only way for the hydrogen propellant to capture the energy radiated by the NLB is if it is "seeded" with a high-Z material like tungsten ( "high-Z" refers to atoms with high atomic number - Z - which are increasingly more opaque to light than low-Z atoms like hydrogen & helium).

All of a sudden the 2,400,000 pounds of hydrogen reaction mass gets a lot heavier (high-Z materials like tungsten are also some of the densest known metals - they also tend to be toxic) .....and a lot more expensive (tungsten isn't cheap like hydrogen !).

One potential mishap Mr. Tate doesn't mention in his article is if the tungsten-seeded hydrogen propellant for some reason looses its tungsten seeds for just a moment (say due to a glitch in the propellant mixing & supply system) -- then "poof" and the surrounding "Liberty Ship" vaporises, as it becomes the nearest opaque material after the hydrogen.

Mr. Tate also doesn't mention that the fused silica wall of the NLB can only take about 1300°K, requiring internal cooling by an inert low-Z buffer gas (neon or argon), which continuously flows inward, separating the wall from the much hotter fissioning plasma by a buffer zone about an inch thick.

The mixture of fuel and buffer gas needs to be continuously removed, separated and recycled - with a typical residence time in the NLB of about 4 seconds. The recycling machinery must of course also be high-temperature resistant.

It is quite misleading to say that "The silica bulb just has to be transparent enough to let the gigantic power output of the fissioning core flow through."

This little detail is crucial, as is the fact that the heat radiation coming out of the NLB is only coming out of the outer part of the fissioning plasma, not its central region (again, due to the opacity of the high-Z uranium material, pressurized to about 300 atmospheres, in order to have sufficient density and mass for nuclear criticality).

This means that the NLB silica glass bulbs need to have a large surface-to-volume ratio, which leads to fairly thin, long things resembling oversize conventional nuclear fuel rods.

Indeed, the "fissioning core" comprises not a single NLB tube, but a series of them stacked next to each other, much like fuel rods in an ordinary reactor, with neutrons diffusing freely throughout the entire assembly in order to maintain the fission chain-reaction and criticality (ie. steady power output).

Taking into account the wall material and cooling, the highest practicable plasma surface temperature with this type of arrangement has in fact been found to be approximately 5000°K (see reference at bottom).

The center of the NLB may be slightly hotter - perhaps as much as 9500K, achieved in a test of a water-cooled NLB at the United Technologies Research Center - but again, its the outer part of the plasma that determines the temperature of the thermal radiation going out and the temperature of the tungsten-seeded hydrogen propellant.

Its the latter which determines the performance of the rocket, and it is clearly much lower than Mr. Tate's "gas core reactor.... operating at temperatures of 25,000°C."

Consequently, the specific impulse of Mr. Tate's GCNR "Liberty Ship" NLB engine would be much less than claimed - 3060 seconds - more like 2000 seconds being about the highest you could hope for.

Given the size and operating parameters of the NLB - particularly the wall cooling rate - one can expect at most approximately 80 megawatts of thermal power from a single lightbulb.

So for Mr. Tate's GCNR "Liberty Ship" NLB "engine producing 1,200,000 pounds of thrust.... from a thermal output of approximately 80 gigawatts," there would have to be a THOUSAND lightbulbs in each engine. For the seven engines in Mr. Tate's GCNR "Liberty Ship" there would have to be a total of seven thousand lightbulbs !! .... a truly fantastic glass menagerie, making a mockery out of Mr. Tate's claim of a "design... able to achieve a thrust to weight ratio of ten to one, so the engine and all of its safety systems, off-line fuel storage, etc, weighs 120,000 pounds. I think we can build this engine easily for 60 tons."

An optimistic guess for a single NLB assembly might be a couple hundred pounds. For a thousand of them, you're easily talking a quarter-million pounds, without even mentioning the enormous size and structure it would take to keep them all in one spot on lift-off (never mind all of them operating flawlessly - one tube blows and they could all end up in a great big heap of glass !).

No, the whole idea of using NLBs for an orbital launcher is mistaken.

This type of engine has excellent specific impulse (relative to chemical rockets), but a thrust-to-weight ratio only marginally better (OK, big margin) than solid-core NTRs.

Therefore the NLB concept, like other nuclear thermal rockets, was intended - and is only suited - for in-space propulsion, with perhaps half-a-dozen to a dozen NLBs in a single engine, at most.

=========

.....and from back in March 25, 2004:

Re: Neutron Flux for GCNR or Bob Zubrin's Salt Water Rocket

Quote:

Though *Extream* NTR's such as Liberty Ship are an interesting subject to explore I think that perhaps smaller units generating low(er) levels of high ISP thrust at more modest power levels may be more practical.

EG a 5Gw GCNR lightbulb varient may be possible with current materials even if an 80Gw one is not! A thrust of 75,000Lbs at an ISP of 3000 is pertty usefull for a "space" tug or ferry.

--------------------------------------------------------------------------------

A reduction from 80GW to 5GW doesn't make much difference, IMO, in the case of a nuclear lightbulb (NLB) rocket, in terms of practical feasibility.

Given the size and operating parameters of the NLB - particularly the wall cooling rate - one can expect at most approximately 80 megawatts of thermal power from a single lightbulb.

So for the GCNR "Liberty Ship" NLB "engine producing 1,200,000 pounds of thrust.... from a thermal output of approximately 80 gigawatts," there would have to be a THOUSAND lightbulbs in each engine. For the seven engines in the GCNR "Liberty Ship" there would have to be a of total of seven thousand lightbulbs !! .... a truly fantastic glass menagerie.

An optimistic guess for a single lightbulb assembly might be a couple hundred pounds. For a thousand of them, you're easily talking a quarter-million pounds, without even mentioning the enormous size and structure it would take to keep them all in one spot on lift-off (never mind all of them operating flawlessly - one tube blows and they could all end up in a great big heap of glass !).

For a 5GW NLB engine the thousand lightbulbs reduces to 63 -- which is still way more than contemplated in "realistic" designs proposed back when the concept was in vogue (something like a dozen, at most, was thought to be a realistic number).

There are many other technical problems with the way the GCNR "Liberty Ship" NLB engine was characterized. Here are some of the main ones:

It is stated that "we find we need 2,400,000 pounds of reaction mass.... Since hydrogen is the best reaction mass physics allows, and is cheap, plentiful, and we have decades of experience handling it, we will use it."

But hydrogen is about as "perfectly transparent" to the light from the NLB as is the fused silica he suggests be used for the transparent wall (called a low-opacity wall).

The only way for the hydrogen propellant to capture the energy radiated by the NLB is if it is "seeded" with a high-Z material like tungsten ( "high-Z" refers to atoms with high atomic number - Z - which are increasingly more opaque to light than low-Z atoms like hydrogen & helium).

All of a sudden the 2,400,000 pounds of hydrogen reaction mass gets a lot heavier (high-Z materials like tungsten are also some of the densest known metals - they also tend to be toxic) .....and a lot more expensive (tungsten isn't cheap like hydrogen !).

One potential mishap that isn't mentioned is if the tungsten-seeded hydrogen propellant for some reason looses its tungsten seeds for just a moment (say due to a glitch in the propellant mixing & supply system) -- then "poof" and the surrounding "Liberty Ship" vaporises, as it becomes the nearest opaque material after the hydrogen.

Also, it isn't mentioned that the fused silica wall of the NLB can only take about 1300°K, requiring internal cooling by an inert low-Z buffer gas (neon or argon), which continuously flows inward, separating the wall from the much hotter fissioning plasma by a buffer zone about an inch thick.

The mixture of fuel and buffer gas needs to be continuously removed, separated and recycled - with a typical residence time in the NLB of about 4 seconds. The recycling machinery must of course also be high-temperature resistant.

The NLB description is quite misleading when it says that "The silica bulb just has to be transparent enough to let the gigantic power output of the fissioning core flow through."

This little detail is crucial, as is the fact that the heat radiation coming out of the NLB is only coming out of the outer part of the fissioning plasma, not its central region (again, due to the opacity of the high-Z uranium material, pressurized to about 300 atmospheres, in order to have sufficient density and mass for nuclear criticality).

This means that the NLB silica glass bulbs need to have a large surface-to-volume ratio, which leads to fairly thin, long things resembling oversize conventional nuclear fuel rods.

Indeed, the "fissioning core" comprises not a single NLB tube, but a series of them stacked next to each other, much like fuel rods in an ordinary reactor, with neutrons diffusing freely throughout the entire assembly in order to maintain the fission chain-reaction and criticality (ie. steady power output).

Taking into account the wall material and cooling, the highest practicable plasma surface temperature with this type of arrangement has in fact been found to be approximately 5000°K.

The center of the NLB may be slightly hotter - perhaps as much as 9500K, achieved in a test of a water-cooled NLB at the United Technologies Research Center - but again, its the outer part of the plasma that determines the temperature of the thermal radiation going out and the temperature of the tungsten-seeded hydrogen propellant.

Its the latter which determines the performance of the rocket, and it is clearly much lower than a "gas core reactor.... operating at temperatures of 25,000°C."

Consequently, the specific impulse of the GCNR NLB engine would be much less than the 3060 seconds claimed -- something like 2000 seconds would be about the highest you could hope for.

You could do way better than that with a VASIMR engine, and the reactor supplying the electric power for it can be made much more reliable than a pile of fragile glass tubes, IMO.

Reference:

Thermal Radiation in Gas Core Nuclear Reactors for Space Propulsion, S. A. Slutz, R. O. Gauntt, G. A. Harms, T. Latham, W. Roman and R. Rodgers, Journal of Propulsion and Power, Vol. 10, No. 3, May - June 1994, pp. 419 - 424.

GoogleNaut · Guru

I'm forced to agree with Jaro.

For liftoff from planetary surface, you need a thrust to weight ratio greater than one. For truly large chemically powered boosters the thrust to weight for the overall booster is about 1.1 to 1.2 to 1. An NTR, because of shielding requirements, is probably closer to 0.8 to 0.9 to 1, which makes it useful as an upper stage.

Thus a conventional chemically powered booster is useful as a 'lofting' stage to let the NTR do an 'altitude start.'

Philipum · Veteran Member

Thank you Jaro for the enlightement ->

I was at first very excited by the idea and then skeptical because of the lack of reference in M. Tate's essay. But you provided a reference, and thank you for that!

But then,

how can space exploration have a future?

What do you think of the laser beam idea, in particular Ablative Laser Propulsion seems promising, don't you agree?

10kBq Jaro · Senior Member

Yes, I agree that Ablative Laser Propulsion is a promising concept for ground-to-LEO launch application -- particularly for small payloads launched in great numbers.

But I also think there is a way to use lasers effectively using a non-ablative propulsion method, especially in combination with other, complimentary technologies -- and for launching even relatively large vehicles.

Its not a new concept, having been discussed a number of times in technical journals. The idea is to heat hydrogen rocket propellant by iluminating a heat exchanger on the belly of the launch vehicle, as it flies several miles above the laser station, and up to several hundred miles down-range from the launch point (i.e. the laser station would be located a hundred or more miles from the launch site). The launch itself is performed using a high-thrust technology, such as a maglev track in an evacuated large-diameter pipe or tunnel built on a steep slope of a tall mountain.

In this concept, the hydrogen is used for accelerating the vehicle from supersonic to orbital velocity, just like a nuclear thermal rocket (NTR), except that the "N" portion remains on the ground, in a nice safe powerplant (the power from the plant is used to run the E-M launcher, as well as the giant laser).

The combination is therefore optimised to provide high thrust at launch, and high efficiency (specific impulse ~900 sec) for orbital insertion, with minimal on-board propellant mass.

I think this is the best - and necessary - intermediate evolutionary stage between the chemical rocket launchers of today (and the near future), and the space elevators of a much more distant future.

GoogleNaut · Guru

I too think this is a good idea. However, the laser power required even for small payloads is substantial. Even for relatively small 1 ton payloads, the amount of beamed energy is on the order of 100-200 MW. For a laser system, this is about 2 orders of magnitude greater sustained power than anything ever built. A more practical near term solution may actually use microwaves.

Here is a nice CalTech paper on the subject of beamed power SSTO:
http://monolith.caltech.edu/Papers/ParkinLauncher.pdf

There is an interesting study that uses a microwave power beam from a space based power station (probably a solar power sattelite) to a fairly large saucer shaped craft. The craft is a helium derigable that intercepts and redirects a portion of the mocrowave beam. A portion of the beam is focussed on a spot above the craft--which causes the air to heat and ionize. The rest of the beam is converted into onboard electric power which is used directly to feed superconducting coils around the perimeter of the craft. The ionized air expands and the Lorentz Force directs the accelerated air aft of the craft generating thrust. The ionized air ahead of the saucer forms an aerospike in reverse---the saucer flys through the low density air with low drag.

It's an interesting concept--I don't know what stage it's at. But here's an article on it:

http://science.nasa.gov/newhome/headlines/prop16apr99_1.htm

I think something like this is probably the way to lift bulk materials into orbit for space contstruction projects. However, the problem here, is that some items may be too heavy or too complicated to not lift in one piece--necessitating a heavy lift launch anyways. Time will tell...

10kBq Jaro · Senior Member

GoogleNaut wrote:

Here is a nice CalTech paper on the subject of beamed power SSTO: http://monolith.caltech.edu/Papers/ParkinLauncher.pdf

Yes that is a nice paper - although I do have some reservations about it.

Whether one uses microwaves or lasers to achieve the heating result is of secondary importance. More important is that the "Parkin SSTO" takes off on microwave power from the ground up :

"The ascent trajectory itself consists of two segments. In the first segment.... the vehicle is steered vertically (β=90°) at 50% throttle to minimize drag losses as it ascends through the atmosphere. When a transition altitude of 65 km is reached, the second segment of ascent begins as the craft levels off, and thrusts horizontally (β=0°) at 100% throttle. [.....] 2 g’s initial acceleration, 19 g’s peak."

By contrast, the E-M launch scheme I was refering to is rather the opposite: the highest acceleration occurs on the ground, inside the evacuated launch tube, thus eliminating the initial large gravitational losses, as the vehicle slowly gains altitude. There is a price to pay for this, in terms of air drag, as the vehicle leaves the launch tube at high velocity. These losses can be minimised by launching near the top of a tall mountain, in low-density air (also required for the laser station).

I would also take issue with the claim that "Beaming energy sufficient to propel a ton into LEO requires 100 MW+ of energy transmission through the atmosphere.... today’s most powerful lasers are still two orders of magnitude weaker." By using an optimised combination of E-M launch and thermal rocket, the vehicle mass is considerably reduced, requiring less beam energy. Moreover, such beam energies are already available today in the AirBorne Laser (ABL) development program. Fire several of them in a salvo, and the mass to LEO can be increased to quite a few tons..... As the article says, its easier to focus higher-frequency beams over great distances -- that's why lasers would tend to be the prefered option, if available.

Lastly, it appears that the authors lack an understanding of nuclear reactors : "Nuclear thermal thrusters operate on the hydrogen heat-exchange principle using neutrons as an energy source, rather than microwaves." The energy source is of course the fission of uranium or plutonium -- neutrons merely serve to propagate the fission chain reaction.

GoogleNaut · Guru

Jaro wrote: "...its easier to focus higher-frequency beams over great distances -- that's why lasers would tend to be the prefered option, if available. "

Strictly speaking, this is true. However, the 'optical' clarity of air is improved somewhat in the microwave frequency regime, and so atmospheric disturbances and clouds will tend to be less dispersive than in the case of a true laser. Also, power conversion of electricity into beam power is far more efficient than any laser system. A CO2 gas dynamic laser operating in the infrared band could possibly achieve 10% efficiency, and advanced chemical lasers could possibly achieve closer to 20% that is really pushing it...microwave power conversion start off at about 50% or better for commercial microwave ovens. Operating with gyrotrons at superhigh frequency, it seems that conversion efficiencies could approach 70% or so.

A comment about the launch trajectory: it's a pretty efficient way to go if you aren't burning chemical propellants. Go straight up to clear the atmosphere, and then a high acceleration dash horizontally to build up the needed velocity to achieve orbit before the vehicle moves over the 'horizon' of the transmitter station. Since this is a small vehicle, it seems conceivable that locating a launch site on top of a high peak such as Mauna Loa in Hawaii, with a nuclear power plant at more conventional locations. It seems doable. Larger vehicles, of course, will require a lot more beam power.

10kBq Jaro · Senior Member

GoogleNaut wrote:

A comment about the launch trajectory: it's a pretty efficient way to go if you aren't burning chemical propellants. Go straight up to clear the atmosphere, and then a high acceleration dash horizontally to build up the needed velocity to achieve orbit before the vehicle moves over the 'horizon' of the transmitter station.

Its also interesting that the type of 'straight up' launch, and the velocity achieved prior to the horizontal acceleration dash, dictates the aerodynamic form of the vehicle, as much as the beam heat exchanger.

At the low-speed end of the spectrum is the "Parkin SSTO" with rocket-thrust take-off and airframe similar to the X-33 design.

At the other extreme is high-speed launch, where the vehicle is given most of its velocity on the vertical leg of the launch trajectory, and the beam-heated thermal rocket merely provides the last few km/sec to get into orbit. In this case the vehicle must be of the hypersonic waverider type, in order to efficiently turn the vertical velocity vector into a horizontal one. It does this in a purely aerodynamic, unpowered maneuvre. But it requires a high lift-to-drag ratio in order to not lose speed while performing the turn, at sub-orbital altitude.

From what I've read, it seems that the mass ratios of the launch vehicle (relative to payload to LEO) tend to be better for the high launch velocity concepts. But I must admit that I haven't seen any general optimisation study, so its not clear to me what the "best" design might be....

GoogleNaut · Guru

A moderate acceleration, vertical profile trajectory could clear most of the atmosphere in just a few minutes. This is the basic principle of almost all chemical launch systems. However there is a certain, optimized trajectory for which delta-v achieves a maximum. This is the origin on the synergic 'power turn' where a vehicle, after achieving some minum amount of altitude, begins to do a slow turn ending with the vehicle accelerating almost horizontally. This synergic turn is initiated fairly early on, almost from the moment the tower is cleared, and allows the vehicle to quickly climb out of the atmosphere without necessary having to do a 'right angle' turn somewhere later.

The trajectory described for the microwave thermal SSTO seems to be more of a straight up, and then turn kind of thing... I'm not sure what this affect has on the overall delta-v, certainly there is a penalty but I'm not sure how much. With 700-800 seconds of specific impulse available, the delta-v penalty does not translate into a very large payload or propellant penalty, probably not enough to matter. By delaying the initiation of a synergic 'power turn' until later, then aerodynamic forces can be minimized as well as heating due to atmospheric friction. Enough 'radial' velocity component (that component of the velocity vector pointing to the center of the earth) can be added, such that as the vehicle turns and accelerates tangentially (parallel to the ground) then at the moment that the vertical compenent of velocity goes to zero, then the vehicle will have achieved orbit. A clue with this is that the orbit mentioned was 1000 km high, which would put it really close to the innermost Van Allen belt. I'm not sure why the orbit is so high, unless it is to reduce atmospheric drag to almost zero (atmosphereic drag is significant on the International Space Station in its 160 mile high LEO. The ISS must be constantly reboosted as its orbit decays, sometimes as much as a couple of miles per day! This seems kind of silly to do it this way...)

Anyways, a "right-turn" trajectory could have some advantages, even though the synergic "power turn" is a little more efficient overall.

10kBq Jaro · Senior Member

GoogleNaut wrote:

Anyways, a "right-turn" trajectory could have some advantages, even though the synergic "power turn" is a little more efficient overall.

If you think about it, one-quarter of an orbit around the earth is a "right-turn" trajectory. A more obvious "right-turn" was the one performed by the Ulysses spacecraft, to put itself in solar polar orbit. This type of gravitational "right-turn" would be much more commonly used by interplanetary spacecraft, were it not for the unfortunate fact that the nearest planet with enough mass is Jupiter (which is why Ulysses is relegated to such a distant view of the Sun's poles, and why it makes such infrequent passes over them -- it swings way out to Jupiter's orbit after each pass...).

The fact that the inner planets (Venus, Earth, Mars) have too little mass to perform such maneuvres using gravity alone, has not escaped the scrutiny of hypersonic aerodynamicists, who have in the past proposed using the upper atmosphere of these planets to greatly augment their turning potential. A variety of interesting missions are made possible using hypersonic wave riders, performing high-Mach turns around one or more of the three planets (typical entry speed is on the order of M = 30 to 60).

In the context of an earth launch "right-turn" trajectory with thermal rocket boost to LEO, the flight environment is a relatively mild M ~ 10 to 15. But even if the launch velocity were sufficiently high to reach orbit directly (ie. without rocket boost), the payload mass ratio would still not be 1.0, because the hypersonic aeroshell itself would be pretty hefty, in order to take the mechanical stresses of launch, plus the very high aerodynamic heating rates (on top of which, a small rocket push for orbit circularisation is required anyway, since aerodynamic forces cease to function outside the atmosphere, obviously....).

But from the studies I've seen, something close to a 0.8 mass ratio may be possible using this technique. And that's a far cry from the 0.1 mass ratio of the "Parkin SSTO".

On the other hand, the very mild launch acceleration of the latter, make it practical for a whole host of applications, possibly including human crew launch.

So no single concept may be the right solution to all launch problems.....

GoogleNaut · Guru

I agree.

The hypersonic waverider concept is a fascinating one, and one with lots of potential I think.

Interestingly enough, the 'lift' for these craft actually points down for some hypersonic trajectories, because the turning radius is small enough that the centrifical acceleration would actually cause the craft to 'skip' and fly back out of the atmosphere. By doing a half roll into a "belly up" inverted flight mode and using the aerodynamic "lift" to push the craft 'down' so it stays in the atmosphere longer, then a greater angular turn is possible. So 'wave riders' can actually work both ways!

Philipum · Veteran Member

Thus, both the microwave-heating concept and the magnetic levitation+laser concept are doable? This gives me hope! Unlike chemical rockets, with these concepts you invest most of the money on the ground-based installations, which, once built, you can re-use at will providing enough power (we can assume cheap and clean nuclear power). This should be cheaper in the long run!