Externally heated SSTO launch vehicle

[Carreidas 160]

I've seen concepts of externally heated launch vehicles (see http://escapedynamics.com/) claiming 10% payload fraction. I'm trying to figure out how such a design would work and what the tradeoffs are.

Basically, your launch vehicle only holds reaction mass (preferrably LH2) that is heated using an outside energy source, such as microwave antennas or lasers. These sources point at heat exchangers on the launch vehicle which heat up the LH2 such that it exits the vehicle at high velocity. Benefits are:

- simpler tank structure (only LH2 necessary, no LOX)
- higher Isp (700-900 range)

The questions I have are:

- How efficient is the energy transfer (microwave or laser -> heat -> kinetic energy), i.e. how many mega/gigawatts of power will you need to beam?
- how heavy is the heat exchanger?
- How precise can you make the sources point toward the launch vehicle, even when the latter is at high velocity
- Design: traditional VTOL rocket or HTOL spaceplane a la skylon?
- Could you add a heat storage unit, such as a molten salt container, that acts as a heat source when lasers are ineffective?

[RanulfC]

I'm surprised that they don't have any mention of Keven Parkin, the person who actually came UP with the idea. You would think they would at least have brought him onboard as an advisor

I've seen concepts of externally heated launch vehicles (see http://escapedynamics.com/) claiming 10% payload fraction. I'm trying to figure out how such a design would work and what the tradeoffs are.

First here's a good overview of both laser and microwave thermal propulsioin by the people who came up with the ideas: "A Comparison of Laser and Microwave Approaches to CW Beamed Energy Launch"
http://authors.library.caltech.edu/5349/1/KARaipcp06.pdf

And Kare's original NIAC final report:
http://www.niac.usra.edu/files/studies/final_report/897Kare.pdf

And Parkins original MTP paper:
http://authors.library.caltech.edu/3303/1/PARaipcp04a.pdf

Basically, your launch vehicle only holds reaction mass (preferrably LH2) that is heated using an outside energy source, such as microwave antennas or lasers. These sources point at heat exchangers on the launch vehicle which heat up the LH2 such that it exits the vehicle at high velocity.

LN2 has been suggested for the early part of the flight for extra exhaust mass and from what I understand the vehicle has to be "in-the-air" first so either some kind of launch assist (air-drop, catapult, etc) or booster would be needed to get it off the ground. Other than that...

Benefits are:

- simpler tank structure (only LH2 necessary, no LOX)
- higher Isp (700-900 range)

The questions I have are:

- How efficient is the energy transfer (microwave or laser -> heat -> kinetic energy), i.e. how many mega/gigawatts of power will you need to beam?
- how heavy is the heat exchanger?
- How precise can you make the sources point toward the launch vehicle, even when the latter is at high velocity
- Design: traditional VTOL rocket or HTOL spaceplane a la skylon?
- Could you add a heat storage unit, such as a molten salt container, that acts as a heat source when lasers are ineffective?

The payloads I've read about are pretty small, around 100kg (440lbs) with estimated vehicle weights coming in around 908kg (2000lbs). Estimated power for the microwave system is around 300MW, while the laser version is estimated to need 100MW. The microwave system has a higher accelleration and shorter flight time meaning you "might" be able to get away with only one power array, the laser system would probably require a couple.

Heat Exchanger designs vary, as do materials. About the only "solid" number I could find was in this presentation:
http://www.hobbyspace.com/AAdmin/archive/SpecialTopics/Events/2011/Resources/SpaceAccess2011Thermal.pdf
Which estimates the HE mass at between 30kg and 60kg. It really depends on the material and design. Kare's HE design tends to be lighter since it is both expendable and use laminar flow for the propellant while Parkins is reusable and uses chaotic flow. (Kare's HE would be lighter but have much finer "channels" to allow the laminar flow while Parkins is heavier but more robust with larger channels)

"Pointing" is rather straight forward in theory but harder in practice. Kare uses a vehicle beacon and "point-forward" device to keep the beams aligned on the vehicle while Parkin uses both active and passive feed-back loops to keep the beam aimed at the vehicle. In either case it would have to have clear air and orbit space between and beyond the vehicle for safety.

VTL or HTL? Could be either but as I noted before the vehicle pretty much HAS to be in the air already to catch the "beam." First of all you do NOT want to point all that energy horizontally, especially in the case of the microwave beam because of the high power "side-lobes" generated, but for safety sake you want the beam to have as few chances of hitting unintended areas as much as possible. As I pointed out this will probably require some sort of "launch-assist" or "booster" to get the vehicle into position to ride the beam. I note the cited website states that their vehicle will "operate from a runway" which I'd take with a large grain of salt. It probably won't operate on beamed power from that runway

A heat storage system would add a lot of mass to the vehicle and I'm not sure what you mean by "Ineffective" since the beamed energy MUST be "effective" all the way to orbit. It is estimated that the laser system would require power for around 400 seconds (about 7 minutes) out to a range of about 600-800km from the "launch" site. Hence the probable need for more than one laser array along the trajectory. The microwave system on the other hand is based on higher accelerations with a powered flight time of around 200 seconds (about 4 minutes) out to a range of around 200km which MIGHT allow the use of only a single transmission site. (My "opinion" is both will need more than one transmission array downrange to optimize the efficency of the system) So there is not point where the vehicle is not in "effective" range of the power beam.

Anyway here's some more various papers on beamed propulsion that I ran across that might be of interest:
Enhancing the performance of solid rocketw with microwave beams:
http://www.eas.uccs.edu/aketsdever/EaST%20Laboratory_files/AIAA%20Region%20V%20Student%20Conf_2009_Microwave.pdf

A different "take" on the pulsed propulsion rocket similar to the Lightcraft but using pulsed microwaves:
http://www.kml.k.u-tokyo.ac.jp/lecture/CP05_BEP_MicrowaveRocket.pdf

Randy

[Asteroza]

Kare's laser SSTO rough rule of thumb is 1MW of laser for every 1Kg to LEO. Kare seemed to favor a two beaming station approach, with the initial station 200km downrange of the liftoff point, and I believe the other station was 400Km downrange, with the second station needing less power since you are approaching burnout acceleration limits. As RanulfC points out, either microwave or laser system has a "first mile" problem of getting within line of site of the first beaming station if it is located downrange. Locating a station at the liftoff site means it is limited in range (think a radius distance, versus a downrange station which can use a diameter distance). The situation favors something that gets altitude fast. Unfortunately, that would usually be the domain of either conventional vertical takeoff rockets, a borehole catapult tunnel in a mountain, some sort of tower catapult, or a floating vertical catapult pipe in the sea (sorta similar to the Quicklaunch gun). The other options are air drop/launch from a mothership aircraft or a balloon.

The unconventional option is a LEO relay (laser mirror or beam catcher and director like the Tactical Laser Relay)(microwave might be a wave guide?) in orbit to transfer power from a station below the horizon if attempting a ground launch.

There has also been suggestions of laser assist Skylon, which that alternative shape Skylon that was proposed recently might work well with as it has a large flat area in the form of the expanded wing/body area to mount the heat exchanger. It also would neatly solve the first mile issue as SABRE takes care of the ground launch to altitude regime. Laser assist Skylon can potentially boost payload significantly as it reduces propellant (particularly LOx) requirements during the final rocket phase to orbit.


One could make the argument that with a containerized laser system, the infrastructure costs could be spread out in a way such that an initial 100Kg payload prototype vehicle using air drop launch can validate the system and provide the initial income to continue to build out the beaming site as well as construct the ground catapult for a larger vehicle.

[Jim]

Some corrections:

Laser assist Skylon may potentially boost payload significantly as it reduces propellant (particularly LOx) requirements during the final rocket phase to orbit.

The argument has flawed logic.  The prototype has to be able work before one can talk about a larger vehicle.

[Avron]

Some corrections:

Laser assist Skylon may potentially boost payload significantly as it reduces propellant (particularly LOx) requirements during the final rocket phase to orbit.

The argument has flawed logic.  The prototype has to be able work before one can talk about a larger vehicle.

And be able scale the prototype.. again simple logic  and physics need to be adjusted or ignored for the concepts to work.. Small things like dust in the atmosphere and air friction.. 

[Asteroza]

Avron, you did read Kare's reports right? He fairly clearly lays out the conditions for the beaming station and atmospheric performance issues, and states his assumptions which at face value look realistic. Whether a Skylon/SABRE based vehicle is a workable and/or preferred solution to the first mile problem for a full sized vehicle, and your feelings regarding Skylon, are separate from the beaming issues. I'm merely stating the alternate Skylon vehicle configuration has a conveniently large and mostly flat surface that could be used for the heat exchanger mount. Kare has proposed a conventional rocket shape with a side mounted hexagon plate, slightly in the vein of a Pegasus rocket. Parkin has proposed a half cylinder wrapper heat exchanger on a somewhat bulbous rocket as well.

As far as a bricklifter class vehicle for a prototype, I don't think anyone believes Skylon can scale down well enough for such as application. If Skylon as a system can fly, it's going to start big to begin with, and at that point then people will start pushing towards enhancements such as laser assist.

Jim, are you objecting to discussing a larger practical payload beamed SSTO vehicle before someone demos a bricklifter, or the shift from a conventional Skylon to a laser assist Skylon (which potentially reduces vehicle size)?

[Jim]

Jim, are you objecting to discussing a larger practical payload beamed SSTO vehicle before someone demos a bricklifter, or the shift from a conventional Skylon to a laser assist Skylon (which potentially reduces vehicle size)?

Both. the viability of a beamed vehicle making it to orbit is questionable.

[R7]

And if the beamed vehicle makes it into orbit the political fallout could be massive due to ABM/ASAT potential.

[Oli]

Kare's laser SSTO rough rule of thumb is 1MW of laser for every 1Kg to LEO.

 

[cordwainer]

Yeah, I really don't see the practicality of laser launch to orbit systems. Even if you get over all the issues that have been brought up, (the need for launch assist to place the rocket at high enough altitude to train the lasers on, guidance and weather issues that have to be overcome to maintain a proper lock on the heat exchangers for instance) you still have to look at the fact that lasers are not terribly efficient. 1MW of laser for each kilogram for a sizable payload works out to a lot of electrical energy, obviously you only have to run your lasers for a 4 to 8 minutes but still that's a lot of energy and a lot of cooling you will have to provide the lasers. A double pulse laser thermal system with a ground based microwave launch array and rockets powered by solid propellant vaporized by an onboard laser might work efficiently enough for small payloads though.

[RanulfC]

Jim, are you objecting to discussing a larger practical payload beamed SSTO vehicle before someone demos a bricklifter, or the shift from a conventional Skylon to a laser assist Skylon (which potentially reduces vehicle size)?

Both. the viability of a beamed vehicle making it to orbit is questionable.

Well, as usual the "math" works And the concept has been around and in subscale testing since the late 60s. It has only been very recently that anyone has looked at CW instead of pulsed beams and noone has actually done an serious prototyping. In essence the heat exchanger material and durability is the key factor in making either microwave or laser HE launch possible.

I'd like to note that the person who came up with the "Laser-Assisted" Skylon concept did an awful lot of "hand-waving" in mixing the technology. My "take" on the concept was that it would not work as suggested due to the way the SABRE engine works.

And if the beamed vehicle makes it into orbit the political fallout could be massive due to ABM/ASAT potential.

That "issue" already exists actually. In order to test-fly his "Lightcraft" models, Leik Myrabo has to have a steel plate suspended above the models flight path because the laser he uses has the capability of blinding a satellite in orbit. (We also have to inform the Russians when testing just in case something happens)

Randy

[RanulfC]

Yeah, I really don't see the practicality of laser launch to orbit systems. Even if you get over all the issues that have been brought up, (the need for launch assist to place the rocket at high enough altitude to train the lasers on, guidance and weather issues that have to be overcome to maintain a proper lock on the heat exchangers for instance) you still have to look at the fact that lasers are not terribly efficient. 1MW of laser for each kilogram for a sizable payload works out to a lot of electrical energy, obviously you only have to run your lasers for a 4 to 8 minutes but still that's a lot of energy and a lot of cooling you will have to provide the lasers. A double pulse laser thermal system with a ground based microwave launch array and rockets powered by solid propellant vaporized by an onboard laser might work efficiently enough for small payloads though.

Read the Kare report I posted above, he address' many of the issue brought up. Parkin points out that the microwave concept actually avoids many of the problems with lasers though it too has its own issues that need to be addressed. For the most part what really needs to happen is a small scale "proof-of-concept" test program to develop and test the various components to see how "viable" the concepts are in the real world.

I should probably point out at this point that I recently stumbled across a book called "The Big Lifters" by Dean Ing:
http://www.amazon.com/The-Big-Lifters-Dean-Ing/dp/0812516141

This is probably the first mention of a "heat-exchanger" propulsion system and though it is at the end of the book used for space launch the ORIGINAL puprose is to boost heavy cargo LTA vehicles. (Don't ask, just read the book its pretty good ) Similarly concepts have been advanced in the past to use ground based lasers to boost the performance of "regular" rockets both solid, (cited in my original post) and liquid as well as using them to move satellites with HE thrusters. (As well as using them for "skysweeping" debris) Not sure how "good" microwaves would work for the same purpose but there is always the MASER beam:
http://en.wikipedia.org/wiki/Maser

One idea I'd had was suborbital Point-To-Point using laser HE boosted vehicles with normal jet engines for take off and landing. But as I said what really needs to happen is more prototyping and research to see how well the basic concepts work before speculation gets TO far along

Randy

[Solman]

Kare's laser SSTO rough rule of thumb is 1MW of laser for every 1Kg to LEO.

 

That's 1 MW for what? seven or eight minutes?
Even if 15%(?) efficient overall, that's still roughly 1 MW-hr or about $100.00 of power which is under $50.00/lb.
Microwaves would presumably be more efficient and therefore have an even lower electric power cost.
One big advantage is the potential flight rate in the case that on-orbit assembly is used. A given size installation could find itself rapidly amortized by almost continuous use.
Another potential use is removal of orbital debris. Also orbit raising with a heat exchanger equipped ROTV is a possible money maker.
A beam power station in orbit around the Moon or Mars could facilitate much faster trips since beam power can be used to slow incoming spacecraft down as well as send them out at high efficiency.
IMO beam powered propulsion is what will be used routinely for a long time into the future and will be what takes us to the stars. After all it even beats antimatter.
Also much higher efficiency propulsion is possible than that afforded by heat exchanger type BPP.
 When that first station becomes operational in the Centauri system routine trips at a high fraction of c will become possible I think.

[Oli]

Even if 15%(?) efficient overall, that's still roughly 1 MW-hr or about $100.00 of power which is under $50.00/lb.

Its still $100k for one ton. Only the power.

But I was thinking more about the cost of the lasers. Even if you get a 100kW laser for $1m (questionable), that's $100bn of lasers for getting 10t payloads into LEO.

[Asteroza]

Jim, are you objecting to discussing a larger practical payload beamed SSTO vehicle before someone demos a bricklifter, or the shift from a conventional Skylon to a laser assist Skylon (which potentially reduces vehicle size)?

Both. the viability of a beamed vehicle making it to orbit is questionable.

What concerns do you have that you feel are inadequately addressed by Kare's reports? Are the technology choices and assumptions invalid, and if so why? If it is a costing model and economic viability issue, are you essentially arguing that this is a nonviable "build it and they will come" big infrastructure situation? I see the containerized laser system Kare proposes as a great way to short circuit the technology development curve as it becomes a matter of mass production of containers after one has been successfully built, and technically has bricklifter potential after only a few containers, and scale out is comparatively simple. The microwave system is not as modular due to the sync issues pushing for an all up approach.

And if the beamed vehicle makes it into orbit the political fallout could be massive due to ABM/ASAT potential.

While people have concerns about a high powered directed beam being used in an anti-satellite role or even accidental damage, the fact is any advanced propulsion system is effectively a weapon in one way or another due to the energies involved, the most basic version being a cargo rocket/ICBM. With a laser ground station and adequate space tracking data from USAF, accidental damage can be avoided. Also, a ground based laser station is much like a runway, as both are a bomb magnet during a conflict. If those containers were mobile truck mounted, I imagine there would be more complaining, but that might be a moot point if the recent US army truck mounted HEL demos turn into fielded systems.

[darkbluenine]

the viability of a beamed vehicle making it to orbit is questionable.

The ABL (Airborne Laser) was a 1MW-class laser that had to traverse "hundreds of kilometers" of the Earth's atmosphere to destroy ICBMs during boost-phase from an aircraft loitering at 40K ft.  The actual range that MDA demonstrated during ABL's tests remains classified, but if they got in that ballpark, the precedent exists to deliver MW-class power on a laser from ground to orbit.

It's also worth mentioning that the job of incoherently combining multiple beams on a heat exchanger to vaporize LH2 is actually much easier than the ABL's job of coherently focusing one large beam on a solid rocket casing long enough to punch through the metal.

Of course, an ABL-type laser system would be prohibitively expensive, and the point of Kare's study is to identify a non-militarized and affordable laser system for delivering that kind of power.

Kare's laser SSTO rough rule of thumb is 1MW of laser for every 1Kg to LEO.

 

A MW isn't that much in terms of demonstrated military lasers these days.  The ABL mentioned above fit a 1MW system on a airplane, and the High Energy Laser Mobile Demonstrator (HEL-MD) is a 100kW system that fits on a truck.  (There are others, too.) 

I like Kare's approach a lot.  It transfers much of the complexity of a launch vehicle to the ground, it's highly scalable, and it leverages Moore's Law-like trends in semiconductor lasers.  It's the lazy man's (read "easiest") approach to power beaming and laser launch.  The precision required of Myrabo's Lightcraft and the fact that the thinning atmosphere at higher altitudes worked against it always seemed like snake oil to me.  And the inability to test a microwave launcher without installing a huge microwave array first makes Parkins' approach seem like a non-starter.

That said, I'm unaware that anyone has tested a heat exchanger for either Kare's or Parkins' approach, and until their claimed efficiencies are validated, there's no reason to go further with either concept.  But unlike Parkins' approach, Kare's could easily leverage any number of military lasers for this test -- I'm sure they'd appreciate having the target to shoot at.

If Kare's heat exchanger was validated, sub-scale, 1-10kg payload testing would make sense.  But proceeding to the full-scale, $2-3B array for 100kg payloads couldn't be justified unless there was a huge market, like unitized propellant (re)supply or massive swarmsat array deployment/replenishment.

[Jim]

The ABL (Airborne Laser) was a 1MW-class laser that had to traverse "hundreds of kilometers" of the Earth's atmosphere to destroy ICBMs during boost-phase from an aircraft loitering at 40K ft.  The actual range that MDA demonstrated during ABL's tests remains classified, but if they got in that ballpark, the precedent exists to deliver MW-class power on a laser from ground to orbit.

No, only from 40k ft.

[Jim]

It's also worth mentioning that the job of incoherently combining multiple beams on a heat exchanger to vaporize LH2 is actually much easier than the ABL's job of coherently focusing one large beam on a solid rocket casing long enough to punch through the metal.

It didn't need to punch through, just weaken.

[darkbluenine]

No, only from 40k ft.

AFAIK, ABL intercepted during boost phase, not mid-course.

It didn't need to punch through, just weaken.

Regardless, incoherently combining beams to heat up a metal plate is much easier that creating a beam coherent enough to damage a metal wall.

[Jim]

Regardless, incoherently combining beams to heat up a metal plate is much easier that creating a beam coherent enough to damage a metal wall.

What metal?  The casings are composite.