You are not logged in.
I've been thinking lately about beamed propulsion for launch. In the long term, I think that this idea makes it possible to drastically reduce launch costs down to extremely low levels.
I'm not going to look at the specific propulsion methods just yet. I have some ideas about that but I'd rather look more generally at what path will be taken to orbit.
The two big issues with trajectories for beamed propulsion are tracking and line-of-sight. Tracking refers to the difficulty of keeping the beam focused and keeping the craft in the right position to be at the receiving end. When your velocities get up to several km/s, it's understandable that this could be a pretty big issue.
The second is line-of-sight. If you are launching to LEO, your craft will end up over the horizon pretty quickly and thus either multiple beaming arrays will be needed to keep it in the path (this makes tracking even more of a nightmare) or you need to accelerate your rocket at ungodly high accelerations. This can push your array power to unacceptably high levels.
My basic idea is that instead of boosting to LEO, it makes sense instead to boost to GEO. Although the required delta-V is higher, the firing time is also much higher, which makes the acceleration lower and the required power lower. It also means that the beaming array can be stationary, which would be a nice plus because bearings that large and that accurate would be expensive.
However, there are still some issues. The T/W of the craft at liftoff has to be at least 10 N/kg, and really a bit more because some of the thrust has to go towards increasing its tangential velocity so that it stays over the same spot.
I estimate that the total delta-V will be about 12-13 km/s, with the exact amount depending strongly on the trajectory taken and less strongly on other factors. The issue is that if your thrust to weight ratio is larger than one, your firing is likely to end before you reach GEO and then you'll be in an atmosphere-intersecting elliptical orbit.
Does anyone have any ideas about the thrusting profile to manage the issue, or other comments?
-Josh
Offline
Repost...?
"I'm gonna die surrounded by the biggest idiots in the galaxy." - If this forum was a Mars Colony
Offline
I'm sure I've seen this post before?
"I'm gonna die surrounded by the biggest idiots in the galaxy." - If this forum was a Mars Colony
Offline
You could use beamed-energy (probably super-high power laser) for launch to LEO, if you solve the line of sight problem by using multiple laser installations. There would be one near the launch site, and one or two more downrange, or even one already in orbit.
The problem with this is that we're talking about lasers big enough to be death rays. Death rays and atom bombs fall in the same sort of fear category as nuclear pulse propulsion. The ignorant fear will kill the concept.
Technologically, we're just about there. The same kinds of tracking and pointing skills that enable ICBM intercept with anti-missiles can point the laser. You build this beam launch system, you've also built an anti-missile defense system, no way around that. Political nightmare.
The downside of laser for this launch or weapon application is poor damage-coupling into the target. The beam heats the target, which smokes, and the smoke blocks the beam. There's a different beam that gets around this limitation: the electron or proton charged particle beam. They have 100% unimpeded damage coupling, but they wander all over the sky in the atmosphere, like the lightning bolts they essentially are.
The way around this is to combine the two: fire a laser bolt followed immediately by the particle shot. It'll be a series of discrete hits, no way around that. The laser creates a straight line ionization trail, which the particle shot follows as if it were a copper wire.
There I went and told you how to do it. And I also told you why you really don't want to. Which is why the the anti-ICBM defense weapons are missiles, not the energy weapons they first investigated in the 1980's (originally ca. 1950 in Project Seesaw).
GW
GW Johnson
McGregor, Texas
"There is nothing as expensive as a dead crew, especially one dead from a bad management decision"
Offline
Terraformer- I vaguely remember that, but a quick google reveals nothing. It's possible that you recall a post from between 2008 and the Great Crash. I may have even made it, but regardless I'd say this is an important enough topic to merit a thread.
GW-
IMO it really depends how you sell it. Technically speaking, it's quite silly to think of the beam for beamed propulsion as a "death ray" because it's pointed in the exact opposite direction as all of human civilization. Beyond that, since there's nothing nuclear involved I would think clever branding (especially if we're talking about a private company) mold public opinion to our will.
What do you think about the idea of going straight to GEO?
-Josh
Offline
Hi Josh:
For LEO I'm pretty sure you would need multiple laser sites to keep line-of-sight on the vehicle during ascent. For GEO, I'm not sure. I just know the radar (line-of-sight) horizon is about 7 nautical miles (nmi) for the typical eyeball height of 5 ft, maybe 12 nmi from the bridge of a destroyer.
If you had to base one of the lasers on-orbit to keep line-of-sight through the trajectory, then you have a laser that shoots down from space, with the power to be a "death ray". That's what you have to avoid to "sell" a system like this.
The "shoots up only = defensive only" argument is how missile defense was sold, and it's potentially an anti-satellite technology, although in practice it is really not. (But with some changes, it could be.)
GW
GW Johnson
McGregor, Texas
"There is nothing as expensive as a dead crew, especially one dead from a bad management decision"
Offline
For LEO you'd either need several lasers or extremely high accelerations, I'd imagine. Again, that's why I'd rather go to GEO because it eliminates a huge expense (multiple lasers) while increasing the energy use only slightly.
-Josh
Offline
Perhaps you can inform me if the following has any merit:
Just Lasers at the launch of a chemical rocket.
The purpose is to remove condensate from the outer walls of the rocket.
Also, I wonder if the outer surface heated could at first give a slight additional lift through the updraft of air around the rocket.
Also, I wonder if (After the updraft was irrelavant due to rocket speed) just heating the walls would cause the air around the rocket to be thinner and so less friction.
If any of these would help, then a method like that (Just using lasers the first few thousand feet) would be a humble but practicle starter technology to introduce a later greater process.
I have also speculated on adding additional heat the chemical rocket nozzle during flight, but I am aware that that might have bad effects, such as a malfunction, but if you could add a few percent of extra energy during any part of the flight, it might be worthwhile.
Done.
Offline
Well whatever gain you got from eliminating condensate would be paid for in the price of more fuel evaporating. I don't think that you could produce significantly more lift or reduce the drag significantly with these processes, either.
Adding energy to a chemical rocket with beams means that you add the issues of beamed propulsion to those of chemical propulsion. They're strong technologies by themselves but I don't think there would be much benefit to be had from using them together.
-Josh
Offline
Well thank you for responding, I did request it.
I do not want to interfere with the path that you want to take with this thread, but I will at least argue for a short time on some of this.
I consider your arguments to be valid from a point of view, but also see another angle or two.
As for removing condensate, I think that could be accomplished with a certain wavelength of light, pulsed lasers, which would not heat the cryrogenic fluids very much at all, during the time the rocket is not in flight. The value is debatable.
As for the notion of actually heating the tanks intentionally during flight with a laser, for a rocket constructed with that intention, it could be beneficial, since during flight fluids are being directed to an engine where they are intended to become very hot. It would simply be needed not to overpressurize the tanks to a failure. It would be a laser powered steam engine effect more or less.
The updraft thing is very weak, and likely is not worth the persuit, but is interesting to think about.
As for shining laser beams into the inside of a running rocket engine, it would likely produce a greater push, with the same amount of mass, but could damage engines intended not to operate that way.
As for the argument against combining two powerful technologies, I will say that their was a transition between sailing ships and steam ships where both technologies were used, and that may have made it possible to develop steam engines for ships in the first place.
If any hybrid method could deliver extra amounts of payload to orbit for a reasonable added cost, then I think it can be a continuing consideration.
Cost would be the test.
Done.
Offline
The simple act of heating the condensate would inevitably result in some heat being transferred to the cryogens by conduction and convection. The condensate does serve to insulate, to some degree; Further, recondensation would likely begin pretty quickly which tends to add a lot of heat energy to the cryogenic propellants.
The reason why I would expect cost to increase is that you are constrained by the limitations of both the beamed propulsion system and of the chemical rocket systems; Your mirror alignment has to be good and you'll probably need a lot more "stuff" on your rocket to make it work.
-Josh
Offline
What if you adopt a pop-up pop-down laser powered lower stage, and a high performance near-SSTO for the upper stage? Use the laser only for 1-150km vertical, so you don't have to track it...
"I'm gonna die surrounded by the biggest idiots in the galaxy." - If this forum was a Mars Colony
Offline
The de-condensation process could stop at the point where the fueling apparatus was disconnected, so it would not be that bad of a problem since any increased venting could be topped off. I also think that the heating would be more of a zap type of a pulsed thing, scanning across the tanks to inhibit buildup. Some wavelengths are particular to ice, but conduction would put some heat in. But it is not a priorty, just an option to inventigate.
I actually only think of using the laser to engine coupling for the first 5000 to 10,000 feet, just to see if any good could come from it.
Of course long before committing to such a thing you would do an experiment with a small engine on a testbed, to see how this works, the problems it creats, does it give a benefit? Also in fact what are the additional costs in modifying the engine, and in the laser system itself. Of course it has to pay it's way.
Done.
Offline
But in order to prevent condensation you would need to keep the surface of the tank above freezing, or at least substantially above its natural temperature. This will lead to significantly more boiloff. Topping off is of course possible but not really desirable. That kind of tempersture gradient is also not really good for surface materials.
It's not impossible, but why? Condensate isn't s problem, except on the former space shuttle with its insane parallel staging.
-Josh
Offline
OK, I am going to presume that you are correct on the condensation thing, and give it up.
But as an ending I will suggest that a hybrid propulsion method as suggested previously "Might" be useful.
What I am speculating on is that the chemical rocket would be fully capable of orbit without the assist from the laser, but if assisted by the laser might achive orbit with fuel and oxydizer left over. Supposing that that leftover could be handled in some manner to be stored, then it would be an orbital resource.
So, if the laser system failed during launch, from technical or atmospheric issues the rocket and payload would not necessarily be lost. But if a laser assist was successful, then leftovers for orbital activities.
Done.
Offline
I don't want to risk hitting any debris. 200km up should be relatively safe, though. Plus, the higher you go, the more you have to worry about drifting downrange, making recovery harder.
"I'm gonna die surrounded by the biggest idiots in the galaxy." - If this forum was a Mars Colony
Offline
Void,
That's an interesting idea, but it seems like you're suggesting a relatively low (but still quite high) power laser. Do you have any guesses for how the mass of the additional beamed propulsion systems will compare to the fuel savings?
Terraformer,
What debris are you talking about?
-Josh
Offline
Satellites, bits of satellites... but if that's not a problem, you might as well boost that high. Except it's probably not going to be as efficient, because you're going to be adding both legs of the triangle together. But if your craft is cheap enough, then sure. Water heated by laser, perhaps, with an Isp of 500s? All straight up, then deploying a high performance upper stage.
"I'm gonna die surrounded by the biggest idiots in the galaxy." - If this forum was a Mars Colony
Offline
Really no idea about the mass. I was just hoping that existing engine designs could be evaluated on a testbed while firing, and that shining a modulated laser into them would reveal how they would have to be modified to tollerate it. I am sure they are all tuned for a certain power limit before failing, so I would start by adding 1% extra power with a laser beam, and then move it up until the engine showed stress. Then you would have to figure out how to redesign the engine to tollerate operating in both modes. Chemical only or Chemical + Laser Boost.
Of course you would have to figure out if it actually gives more thrust.
I am not competent to give a added hardware weight evaluation. Of course that has to be considered.
A Oxygen + Hydrogen flame is transparent I belive (To visible light), so I would suppose that you would need a wavelength which would be absorbed by very hot steam.
If it was a solid rocket then you would need something else. I don't think I would persue it with solids though.
Also you would have to figure out your aiming. I suppose that for a altitude of 0-10,000 feet, that might not be impossible, but then you have to consider what damage it might do to the rockets other parts, if the aim is wrong.
I Don't know if you would use solid state or chemical lasers. Chemical Lasers can be pretty powerful, although as used in aircraft, they have issues, but they exist, and have been tested. I am sure we don't want something so powerful that it can knock down a ICBM though.
As far as aiming it is favorable that the lasers and the rocket would be co-operating with each other, unlike using a laser to intercept a ICBM.
These would be ground based I would think.
Done.
Offline
Well I don't think that there's any chance at all that you could use a standard engine. I mean 1% of the (maximum, e.g. 109% thrust) power of the SSME (Space Shuttle Main Engine) is 5 GW. 1% of this would be 50 MW. I kind of doubt the engine could withstand a 50 MW laser being pointed at it. It's perhaps not impossible to design a hybrid but it would be expensive and with not much payoff. I mean it would increase your effective Isp by about .5% (Because power goes up with velocity squared and the square root of a 1% increase in thrust power is about a .5% increase in Vex), which corresponds to 2 s for an H2/LOX engine. Meanwhile, a 50 MW continuous pulse laser will be extremely expensive.
Void, you may not agree with me-- and that's fine! I'd encourage you to state your disagreement if you don't agree, because I'm far from being right all the time.
Terraformer,
Actually I think higher Isps are reasonable, if we can solve the power issue for the ground beam. I've suggested that this could be done with Sodium Vapor lamps, which produce monochromatic light much more cheaply than lasers.
Logically, if you're using sodium vapor lamps you want to use sodium vapor in your rocket engine because you can guarantee an identical emission and absorption spectrum. An approximate equation for the exhaust velocity of a rocket engine is given on the Wikipedia page for a De Laval nozzle. It's pretty accurate, giving a calculated Isp of the SSME of 384 s at sea level compared to 363 s in reality. Using a molar mass of 23 g/mol for Sodium and a Ratio of Specific Heats of 5/3 (Assumed the same as for Mercury Vapor and Noble Gases, I calculate that an Isp of 1000 s can be achieved using Sodium at 65,000 K.
Now, this seems like an obscenely high temperature, and it kind of is. But i's not as bad as it sounds. Because you don't need a heat exchanger the high temperature sodium doesn't need to touch the walls. In fact, if you control your beam properly you could probably keep the material along the chamber walls to 2000 K or less.
Now, if you transition to Water seeded with Salt (Assumed average molecular weight 20, ratio of specific heats 1.2), the required temperature falls to 23,000 K. However, because water would certainly break into its constituents at this temperature, you're probably looking more at 11,000 (The molar mass goes way down, but the ratio of specific heats heads towards the 5/3 for monatomic gases).
If you use seeded Ammonia instead, you can get down towards 9,000 K.
There are some interesting trades going on here.
-Josh
Offline
Hmmm, you could get pretty high with those sorts of Isps for your Zeroth pop-up stage. Entirely eliminating air drag, and majorly reducing gravity drag? Would you be able to get into orbit from such a position, given a delta-V of 8km/s... say a high-performing hydrogen stage with an Isp of 450s. Mass ratio of about 6.5, ~15% for everything other than fuel.
"I'm gonna die surrounded by the biggest idiots in the galaxy." - If this forum was a Mars Colony
Offline
Well I don't see why not. But again, why stage? If your Isp is 1000 s, you don't need to and it adds complexity.
I'm sure we've both seen it suggested that a laser pointed at a block of carbon could hit 5000 s. I don't know if I believe that though.
-Josh
Offline
Because you're going straight up - so you only need one base station, and you can recover the popup first stage easily. Whilst you allow your near-SSTO to actually achieve orbit by removing 1-1.5km/s of the requirement.
"I'm gonna die surrounded by the biggest idiots in the galaxy." - If this forum was a Mars Colony
Offline