Michio Kaku's steam powered rocket

In this video, esteemed theoretical physicist Michio Kaku proposes a rocket powered by steam. The steam is created by using ground based lasers heating the water carried by the rocket. In the video he claims that the problem with rockets is that 90% of the weight carried by a rocket is it’s own fuel and that one solution to that problem is to use ground based lasers to create a steam powered rocket. But you still have to to carry the water. As far as I know, rocket propellant is made up of fuel and oxidizer, both of which contain a lot of hydrogen, which is what water mostly is as well. I’m not seeing the big savings. What am I missing here?

One advantage. Water is much more compact that hydrogen and oxygen contained seperately

But given the problems involved in using bigg assed lasers to power such a beast, I’d say this idea is bad. Not that it wouldn’t WORK…just it wouldn’t be worth the trouble.

There’s a difference between “propellant” and “reaction mass”. In a chemical rocket, the propellant supplies both the energy to drive the reaction mass out the back, and the mass itself. in laser propulsion, only the mass is onboard, the energy is beamed in. Since mass is energy, not having the energy on board saves mass. In a nutshell.

Lets look at it this way.

You got X pounds of hydrogen. And Y pounds of oxygen. You “burn” the two and get an exhaust of with an XYZ temperature. Throw that shit into a rocket exhaust (or local quarry) and there you go.

Now, instead you have a tank of water. Which you use lasers to heat to temp XYZ and throw into a rocket exhaust.

ALL other things being equal, the two have performance.

The upside is the water tank is significantly smaller. The downside is you need to hit the water tank rocket with a shitload of laser energy to transfer said energy to the water.

If you could magically transfer the laser energy to the water you might have something.

But you need some thing to actually do that. Basically the equivalent of the worlds most expensive and over engineered car radiator. That is so hot it is glowing red/white.

It’s not a new idea, and it’s one that’s just a variation on a thermal rocket, but consider how difficult it would be.

First off, it’s a liquid. Imagine the dynamic forces on the rocket when the damn things start to slosh about. Then, don’t forget, the thing needs to tip over to a horizontal position.

Then you’ve a vessel that’s going to go from 0 to some hundred km away in the space of 5 minutes. So not only does the cross section you need to hit decrease rapidly, you have to maintain an even distribution of heat and take into account the required tracking of a small craft curving away from you. I suppose you could lay out a string of laser sites along the flight path to ensure a smooth hand off but that only adds complexity and locks in your initial choice for a flight path.

I’m not seeing the great value.

The difference between the two is that with the laser/steam rocket, you aren’t limited to temp XYZ as you are with H2/O2 chemistry. In theory, you could heat it up to a much higher temperature than is possible with H2/O2 combustion, thereby achieving much higher exhaust velocities (and therefore greater thrust) than would be possible with an H2/O2 rocket operating at the same mass flow rate. So for the same mass of “propellant” (where “propellant” is H2+O2, or water, depending on which rocket you’re looking at), the laser/steam rocket can deliver greater thrust, or can deliver the same amount of thrust for more time.

In theory, there’s no difference between theory and practice. In practice, there is.

The laser/steam rocket would be a bitch to implement IRL. It would be insanely expensive to develop laser(s) with sufficient power output, sufficiently accurate targeting (this includes collimation and aiming) at ranges of hundreds or thousands of miles, and the hardware on the rocket to collect that energy and transfer to the water to generate the needed steam.

You would almost be better off grafting a block of ice to the end of your rocket and directly heating it to steam. It eliminates the sloshing and heat transfer the water design would require. Still makes it a terrible idea, at least for ground to orbit work. A steam kettle design would work better in space.

You had me at this. Perhaps my favorite internet quote of all time. If I had a uterus I would happily have our love child.

But to get serious. Yeah IN THEORY a steam rocket powered by ground based rockets “might” be a smidge better than other rockets.

But in practice? The downsides are serious and the upsides are minimal.

Ground based lasers not ground based rockets.

Because ground based rockets would be stupid :smack:

This is a variation on the Laser Sustained Detonation Wave “rocket”. I worked on one of these in my first job out of grad school. We demonstrated pretty impressive impulse when we used Acetal platic (such as Delrin or Celcon) as the ablation mass, instead of water (We used it because it was an easily-handled solid, and it had a huge absorption at the wavelength of the carbon dioxide lasers we were using.

The plan was ultimately to use Ice, rather than plastic, with some inconsequential stuff mixed in. It’s pretty eco-friendly, and a helluva lot easier to handle than water. I describe the operation in my short story The Flight of the Hans Pfaal. we never got it past the lab stage, though.

Prof. Leik Myrabo, of Rensselear Polytechnic, took an even more exotic path, not even carrying the reaction mass. His plan for his Apollo Lightship was to use the surrounding air itself as the reaction medium, so he wouldn’t even have to carry that. He built several test models, and shot them high into the air down at White Sands, NM



This is an idea with an old pedigree – Arthur Kantrowitz (who was advisor on out project) suggested it back in the 1970s. It’s shown up in several science fiction stories since – Jerry Pournelle’s High Justice, Michael Kube-McDowell’s The Quiet Pools, Dean Ing’s The Big Lifters. The basic idea is very different from Laser Light Pressure craft, since the laser energy is used to ablate a layer of material, then feed energy into it via inverse bremstrahlung. It acts like the detonation wave from an explosion, hence the term Laser-Sustained Detonation. After the gas clears away, you take another shot. Lather, Rinse, Repeat. Eventually you get into orbit. That was Kantrowitz’ dream (and Myrabo’s) – Ground (well, mountaintop, actually) to orbit without rockets. Obviously, Myrabo’s would have to get enough velocity before he ran out of air to use as reaction mass.

Or you could use a Myrabo design as a first stage, with either a conventional rocket or a Kantrowitz ablative design on top of it. That way, you’d still need to carry some reaction mass, but not nearly as much as for a ground-to-space rocket.

Let us look at it this way.

What gives you a buttload of thrust? For almost a nothing fuel/weight wise. That is engineered to heck and back. That has been probably perfected using state of the art materials?

Jet engines. And their relatives scram jets and ram jets.

Heck, nothing better than a ram jet. Run some air down a tube, inject some fuel and viola, thrust baby.

If rocket makers are not using these yet, they sure ain’t (IMO) using laser based hot water tanks anytime soon.

OK, what’s wrong with both?

That is, you take a standard H2 + O2 rocket. Burn the stuff to get the energy and blast the resulting water out the end. Standard rocket. Then you take some big-ass lasers and heat the already superheated reaction mass even hotter. What’s the purpose of starting with lukewarm water when, if you separate the water into H2 and O2 you can start with superhot water instead?

Well the exhaust is moving over 4000 m/s. Over the time frame of 1ms the ejected mass is going to move 4m. Good luck getting anything out of that.

That also leaves aside the problem is constraining the bloom. You’re heating a volume of water vapour and it’s going to want to expand in all directions. To get a meaningful return on your laser power consumption you’ll need to add on some sort of reflector…which will naturally interfere with the regular engines/nozzles.

The advantage of the LSD-wave “rocket” is that all you have to send up is your payload and your reaction mass. The rocket engine stays on the ground, with all its weight, and you don’t have to waste precious fuel and reaction mass to lift it. You can make it as big and heavy as you want.

Furthermore, your reaction mass doesn’t have to be highly volatile, explosive, and often frighteningly hazardous, like many rocket fuels*. It can be as innocuous as specially-prepared ice, or a block of plastic (I don’t see the advantage of using water – it’s messy to control and and has a huge heat capacity. Use a block of ice – it stays in one place, and if you add the right chemicals it’ll absorb your laser pulse in the first few microns and willingly sublime and ablate.) Much easier and safer to handle. If you use Myrabo’s scheme, you don’t even have to worry about the reaction mass – it’s free as air.

*Some charming candidates:

Devil’s Venom – https://en.wikipedia.org/wiki/Devil’s_venom

Hydrazine – https://en.wikipedia.org/wiki/Hydrazine

Lithium + Fluorine

Nitrogen Tetroxide – https://en.wikipedia.org/wiki/Dinitrogen_tetroxide

How would laser heated nitrogen triodide work? :slight_smile:

Probably go BOOM!

Yeah - but provided the rocket keeps accelerating (which is pretty much its main purpose) the local G vector is still aligned with its long axis.

True but you still have to deal with the various dynamic forces on a thin tin can as well as tons on water moving about. Not a lot of fun.

I had a friend, way back in college, who designed a steam-powered radio. I don’t know if he ever actually built it. We also worked a bit on designing a sundial with digital read-out.