Fusion for Ground Launch - Is it theoretically possible?

GCNRevenger · 2004-07-28 21:55:29

Since the exhaust velocities are much lower, the ramjet will be using less power to produce the same amount of thrust. Less power=smaller fusion reaction.

I think you've got SeaDragon and chemical rockets stuck in your head Jim, which will kind of makes you an uninteresting person to debate with... I am talking about relativly small vehicles powerd by small reactors without shielding at a custom-designed airport to minimize neutron radiation hazards during reactor operation.

As you have so kindly pointed out, the reactor and the engine will likly be the largest/heaviest componet of the vehicle. Since you need far less energy since you don't need a T/W of >1, say you only needed about T/W of 0.5... the power plant just got three times smaller than a hypothetical T/W=1.5 pure rocket. Yeah, theres your achillies heel.

Then, since thrust is important but Isp is not with a nuclear ramjet, Euler has noted how the ramjet will produce much more thrust than a hydrogen rocket, so the power plant can be even smaller. Say about half the size, which is generous given the difficulty of achieving high thrust with a pure hydrogen propellant. So now, the spaceplanes' powerplant is six times smaller than a pure hydrogen rocket.

And don't forget, you don't need to carry your own reaction mass since you have the ramjet, which makes the vehicle another 10% lighter compared to a 10,000sec Isp fusion rocket, not counting the insulated tankage and associated structure mass for handing liquid hydrogen... Are we seeing a trend here yet?

The Europeans want to build a test fusion reactor, the ITER, using the Tokamak confinement fusion concept either in Canada or Japan most likly. The reactor itself is about five stories tall and about half and again wide. It will be the same scale and perhaps be the first fusion power plant, just sans coolant loop and turbines. This reactor will probobly generate a little more energy than you need for the engine, plus higher technology permitting higher power densities. A sphereical confinement reactor would be substantially smaller than this, probobly around half the size. And you would not need any "giant magnetic bottle" nonsense, just a magnetic pipe from the reactor to the Ramjet nozzle, which will probobly be short given the integration of the Ramjet with the spacecraft fuselage.

It will be a fairly large aircraft as they go, but none of this giant mega rocket nonsense, which is becoming quite the tired hangup... The airport will probobly have to be custom built, but the neutron radiation will not be substantial with a little distance. Remote/automated tractors, possibly a meter-thick tank of water between the terminal and the tarmac. Tunnels to the planes with cement walled boarding ramps. And of course, the small man-made dirt hills with an S-turn between the tarmac and takeoff runway.

Sea operations? Come now... big tanker ships carrying explosive liquid hydrogen, carrying the cargo on boats, carrying passengers on boats, loading the cargo from boats, etcetera etcetera... Too much trouble, especially with a lack of landing accuracy, for rapid turn around.

JimM · 2004-07-29 03:12:53

My point, which you continue to disregard, is that the fusion reactor will most likely not be very scaleable. It will be huge anyway, and ridiculously too big to fly through the air on an aircraft.

There will be no "relativly small vehicles powerd by small reactors without shielding at a custom-designed airport to minimize neutron radiation hazards during reactor operation."

And it doesn't matter how much shielding you install at the airport, you still plan to fly an unshielded reactor through the atmosphere for thousands of miles. You cannot be serious!
-- You will not be allowed to do this, period.
-- That is the end of your nuclear spaceplane, period.

So, get real. The vehicle has to be fully shielded, and it has to be VTOL, to get it out of the atmosphere and into space as rapidly as possible. And do the same in reverse on re-entry. No other version will be allowed to fly.

Therefore, because of the intrinsic size of the reator and the mass of the shielding, the vehicle has to be huge.

You say Euler has noted how the ramjet will produce much more thrust than a hydrogen rocket but this is (1) not so, and (2) not relevant anyway.

(1) Not so, because for an equal amount of energy, heavier propellant will deliver lower exhaust velocity than a lighter one, and thus less thrust per pound of propellant (our old friend 0.5mv^2 again) so in the end it's a wash. That's exatly why LH2 is so popular as a rocket propellant. If I'm wrong, why then the answer is to use H2O or even compresed air as propellant if that makes you happier. (In fact i'd favour H2O because it's so much easier to obtain and to store than LH2, assume there's enough spare mass budget for heavier propellant.)

(2) Not relevant, because a fusion vehicle with an Isp of 10000 (or more) will have a propellant fraction so small (see earlier on this thread) that the benefits of flying through the atmosphere are well and truly wiped out by the weight and complications overheads--if you were ever allowed to fly one, which I don't think you will.

So we are forced into having a gigantic, heavily shielded, VTOL vehicle.

Any object weighing from 20 to 50 thousand tons is not going to be moveable across any sort of ground surface; and unless the surface is solid granite, it's more likely to sink in so far it'll never fly again anyway.

Therefore it has to be a sea-creature.

That is how I arived at this conclusion, not because of an obsession with Sea Dragon.

Sea operations? Come now... big tanker ships carrying explosive liquid hydrogen, carrying the cargo on boats, carrying passengers on boats, loading the cargo from boats, etcetera etcetera... Too much trouble, especially with a lack of landing accuracy, for rapid turn around.

The concept of bringing the thing into port has never occured to you?

And incidentally what sort of vehicles carry the largest cargoes and passenger manifests right now? Why, ships!

BTW, if the propellant is H2... or H2O... or air...why ship it to the space vehicle? There's water and air all around and a fusion reactor on board. Make your own... (Tougher on dry land if you want LH2 or H2O, of course.)

And if it lands a few miles away from target, so what? It can be towed or even propel itself to where it should have been. Try that on dry land.

You know, it's obvious, GNCR. The only rational place any really large earth-to-space vehicle has to land and take off from is the sea. That's a truth realised by the designers of Sea Dragon, but quite separately it's also true of this thing.

Why are you fighting the blatently obvious?

GCNRevenger · 2004-07-29 12:55:35

Ah yes the "oohhh nooooo, radiation!" argument, come on...

In a nuclear fusion reactor, the neutron or residual gamma (from secondary emission) radiation will be gone in minutes after shutdown, making the plane rather safe to handle. The radiation also is simply not that signifigant, as the neutron release will be only moderate under operating condition... but anyway if it is such a concern, the vehicle can be launched over the ocean from a runway on the East Coast over miles of open water.. just like rockets today.. before it reaches sufficent altitude to negate harmful effects on the ground (100,000ft is 18 miles). Plus since it will be going so fast, accidental exposure of a passing plane will be so fleeting that the actual dose recived would be trivial. So, the plane can take its time getting out of the atmosphere, permitting a smaller reactor with even less radiation.

The only place that will be of concern is near the airport, which can be fenced off for a few miles and the strip' proper shielded with dirt berms or small manmade ponds. So, the only shielding you really NEED is for the passengers, which can be made very light with boron or hydrogen impregnated polymers, making the spaceplane even lighter than the megarocket... Just as safe as flying your mega rocket with a reactor ten times the power (have to lug all that shielding & propellant) over everyones' head. Problem solved.

Oh the fusion reaction is quite scaleable, the question is just in the reactor... you could make a nuclear fusion reaction on your coffee table. This one is proven, and not related to sonofusion. http://en.wikipedia.org/wiki/Farnsworth … rsch_Fusor
A commertial power plant sized reactor (rough figuring from ITER volume) with the sphereical confinement arrangement will be around 20-30ft around X 30-40ft tall/long (they stand taller because of magnet arrangement) which would fit neatly inside of a large aircraft's hull. One that is smaller than this, since they will undoubtably be more advanced and you probobly don't need that much energy, will probobly be smaller still. Oh, and this wouldn't be the first time nuclear energy has been applied to flight, this little anticent measly 3MWt reactor was going to power the NTP jet engines of a strateigic bomber... with all that free reaction mass... http://www.military.com/Content/MoreCon … icairpower

"(1) Not so, because for an equal amount of energy, heavier propellant will deliver lower exhaust velocity than a lighter one, and thus less thrust per pound of propellant (our old friend 0.5mv^2 again) so in the end it's a wash."

Actually no, you are still thinking rockets and not airplanes... since there is no propellant, a Ramjet inhaling heavier air can produce more thrust for the same amount of energy, now that exhaust velocity is unimportant. This factor can reduce the total weight of the vehicle by perhaps several fold, since the reactor, the heaviest piece, need not be as powerful. This is not like comparing the 8MT SSME to the 10MT RS-68, where the mass difference is trivial compared to the propellant, but rather hundreds of tons!

Are you even listening to me Jim?

The heaviest piece of the rocket, weighing more than likly hundreds of tons... suddenly only like half the size? Just "wash" indeed. And now that the reactor is smaller, you need less shielding. And without propellant tankage, you knock off another double-digit percentage of vehicle mass, which makes the whole vehicle lighter so you need less reactor to begin with... Oh! And don't forget how you need double or tripple the thrust in a rocket than an airplane, making that mega massive 1000's tons reactor get down to 100's and 10's reaaal quick.

Sea operations are not suited in any form for rapid turn-around. If you can't land within meters or at least hundreds of meters, then the time required to tug the thing to port becomes nontrivial. Or the time required to bring everything to it... Using the reactor to generate your own propellant onboard by cracking sea water would require you to lug along a huge distilation and electrolosys rig and a Hydrogen liquification plant, in order to crack 100's of tons of hydrogen from 1000's of tons of seawater in a relativly short time. If you use the water straight as propellant, your propellant fraction probobly will increase several fold, which makes your rocket even less practical. Oh! And don't forget the fun of working around a RUNNING fusion reactor making explosive gas mixtures under high pressure.

I could go on and expound some more about how the brute-force fusion rocket is silly respective of a fusion ramjet airplane, but I need to get back to work and analyze today's copolymer batch...

GCNRevenger · 2004-07-29 13:42:19

And who says a MCF fusion reactor can't be designed and lightend for flight? Steel vessel replaced with composit that is internally reflection coated. Liquid helium double-walled cryostat superconductor magnets replaced with HTSC magnets cooled by LN2 single walled, composit cryostat. Power provided by MHD generator wrapped around the ramjet plasma injector "pipe" with very high efficency for nominal mass. The whole thing a nice compact sphere vessel rather than a big torus. Etcetera etcetera, it won't be like a commertial reactor... alot of weight can be shed from the machine if it were designed from the ground up to fly and generate heat and/or high voltage power rather than to sit and make low voltage power for cities...

Oh yes, and there might be a liiittle environmental catastrophy if you were to fly a Fusion rocket from the ocean without shielding (save crew bioshield), since the millions of gallons of vaporized water would soak up alot of the neutron flux, and may itself become a radioactive cloud... a little air though, no big deal.

SBird · 2004-07-29 13:44:43

Of course, the whole discussion is somewhat moot since none of us know what an operational fusion engine might look like in terms of performance, mass or volume...

Eveyone is working on an assumption that the fusion reactor will wigh at least dozens of tons. That's not necessarily the case:

http://depts.washington.edu/rppl/programs/stx.html]here

There are lots of alternate fusion designs that got left behind the Tokomak design. Many of them are far better for proplulsion purposes.

As fr the Farmsworth Fusor, it's a dead-end design. It makes a great neutron generator but the trajectory of the particles is an inherent limitation. Because the fusion fuel nuclei are constantly yo-yoing through the center of the device, you very quickly lose all your kinetic energy to Brehmstrallung (sp) radiation. It's only even theoretically feasible for light nuclei like He3.

GCNRevenger · 2004-07-29 13:53:33

Hey SBird, welcome back, long time no see

Somthing like that, but the reactor will undoubtably weigh signifignatly more than the engine, and the reactor will probobly be the heaviest piece of the vehicle, so its not entirely unreasoanble to think about it.

Yes I know the fusor will not be useful for power production, I dug it up as illustration that the fusion reaction itself can be scaled, and it is more a matter of engineering to get it down that small than it is a matter of fundimental physical limitations, like critical mass in fission reactors.

JimM · 2004-07-29 15:47:19

You havn't got it yet. It does not matter how well you may or may not be able to sell the concept of an airborne atomic reactor to me or the other people on this forum: it has absolutely ZERO chance of getting approved politically.

I am well aware of the NB-36H. Its history was one of the factors I considered when coming to the conclusion that your flying fusion plant was a dead duck from day one. Despite the pre-nuclear-risk-panic times it lived in and despite the failure to mention this on the site you referred us to, the project was killed for basically the same reason Orion was killed: the risk of catastophic failure was too great to justify the project. (Orion also had a problem with the Nuclear Test Ban Treaty, true, but that could have been got around, politicians willing: they were not.)

Since these times, the percieved risk of nuclear projects in general has grown manyfold. Even you must have noticed this. Even if every word you say is true, the chances of the sort of project you propose ever getting approved is just non-existant.

Before I move on from your "USS Dead Duck", I can't resist one final comment:

Actually no, you are still thinking rockets and not airplanes... since there is no propellant...

What a quaint, unreal world you must live in. If there's no propellant, what's the point of the ramjet? Or, how much better the thing would fly in a vacuum...

Get real! Of course there's propellant! It's what you call the hot expelled air comeing out through the rear of the ramjet; that's what propels the thing, which is why, strangely enough, it's called 'propellant.'

(What you may have meant in your confusion is that there is no oxydiser, but gee, there's none of that in the VTOL vehicle either.)

BTW, the exhausted air (the 'propellant' to most people) does have a specific implulse, you know. Funkily enough, it's measured in just the same way as it is with conventional rocket propellant: the number of seconds of thrust equivalent to (say) a pound of the propellant that is delivered per pound of propellant, ie. (lb(f).sec)/lb(w), with the answer in seconds. Sound familiar?

The only real difference is that the propellant is grabbed out of the atmosphere rather than drawn from an onboard tank. And because, at constant temp & pressure, air is heavier than H2, it will have lower--actually, a much lower-- Isp than H2.

So you are entirely wrong: for the same energy, the jet will deliver less thrust per pound of (yes) propellant, as exhaust velocity is all-important--still. The jet's only saving grace is that it's got a bigger 'tank' to draw on.

Therefore the jet will be significantly less efficient than the rocket--and that's before taking into account drag, the really minimal lift you'll get at 100,000 feet, and the scarcity of air to suck in in the first place. And of course there's all the fun of hypersonic heating...

So, the real efficiency comparison is this: how much additional vehicle mass (engines, wings, and so on) good old fashioned complexity (did I mention KISS?) maintenance, etc., etc., will it cost you to save having a bigger tank of LH2 (or whatever) (because you will need one anyway if you want to be able to operate in space) than needed with jets, wings, undercarriages, etc., etc., etc.?

Now since the pure rocket version at an Isp of 10000 has a propellant fraction of less than 10%, let's say going your way let's us cut back propellant fraction to, oh, 3%... (Boy, am I being generous in my estimate of the velocity your ramjet will manage.)

So unless you can add wings, ramjets (BTW, how do you get the thing up to ramjet speed in the first place? Turbojets? That would be a lot more weight...), undercarriage and all the rest for less than 7% of what the mass would be if there was just a bigger tank, it's a certain, undeniable looser. And surely not even you can really believe you could add all these extra things and extra complications and extra things-to-go-wrong for less than 7%?

Because, apples and apples, at Isp 10000, that's all you've got to play with. And of course at Isp 20000 (for who knows what's possible?) you'd have about 3% to play with; at Isp 30000, just over 1%... shall I go on, or have I made my point?

But to finish this: in the end it does not matter that it's a silly engineering concept, because you'll never in a month of Sundays be allowed to build it anyway.

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

So what can be built that we'd get away with?

How about this trail concept?
(It's not the vehicle I'd actually build, but it's to demonstrate a point.)

A VTOL vehicle sized to match your HTOL, but rocket only with no wings, jets, etc., and a bigger propellant tank. No additional shielding over what you supplied.

Since energy is essential unrationed, it's not a limiting factor. Using LH2 instead of air, it's Isp will be several times better. I'm not sure exactly how much better, but at a guess, 8 times (O2/H2 = 16/2 = 8) better so a vehicle that delivered 25% of its mass as thrust in horizontal flight using air as propellant should have no trouble in principle managing a T/W of more than 1 using LH2.

So since the bigger LH2 tank will weigh less (probably a lot less) than ramjets, wings, etc., why bother with all the complecity of horizontal flight? It's quite clearly utterly irrelevant and inefficient to boot, as well as unsellable politically.

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

Sea operations are not suited in any form for rapid turn-around

I gather you've never seen a moden oil terminal in operation, or a container port?

100,000 to 200,000 tons loaded or unloaded in significantly less than 24 hours--and at an oil terminal, that's highly inflamable oil or gas. Is that fast enough for you?

And as for electolysing H2O: given that this vehicle would obviously live in the days of fusion power, a full-time water-splitter reactor at the dockside would seem rather obvious, to speed turnaround.

Not one of your objections to ocean launch holds water (sorry!) for an instant; go on, just admit it. Are you biased against the ocean because you get sea-sick, or something? Well if so, I've got good news for you. A modern stabilisation system on a big ship (and this would be big: about the size of the great pyramid, perhaps) means you don't have to worry about sea-sickness again. And with it's fusion reactor(s), it will have no trouble making it's way to port at a fair rate of knots. And anyone in a real hurry can be picked up or landed by helicopter using a helipad that opens up on the vehicle once it's safely on the ocean.

-----------

In fact, I'd give my vehicle at least five reator/rocket systems and a T/W ratio better than 1 with only four operating . Then, if one fails, the mission could continue into LEO.

Euler · 2004-07-29 19:12:37

You are all underestimating the energy savings that you get from using air as part of the rocket engine. As I said, a pure fusion engine would have an isp of around 2*10^6. If you combine the plasma from the reactor with the air at a ratio of around 1 million:1, you would be able to reduce the isp to around 1000. This would allow you to produce the same amount of thrust with a reaction that is 2000 times smaller. The amount of hydrogen used up by a medium sized vehicle during it's ascent would be measured in grams, and a small, light reactor should be able to lift a large payload.

Scott G. Beach · 2004-07-29 20:51:42

A NASA grant provided $250,000 for fusion experiments with a focus fusion device. This device might be able to provide electrical energy to power a Variable Specific Impulse Magnetoplasma Rocket (VASIMR). This kind of rocket could get to Mars quickly but it probably could not be used for launching payloads into Earth orbit.

For more information, go to http://www.focusfusion.org]http://www.focusfusion.org

“The goal of the Focus Fusion Society is to help to develop an environmentally safe, cheap and unlimited energy source. We will do this by funding research into hydrogen-boron fusion using the plasma focus device. The successful development of Focus Fusion Energy will eliminate the environmental destruction of fossil fuel use, free the world economy from the crushing burden of high energy prices and eliminate the continual wars aimed at maintaining control of oil resources. It will give to everyone decentralized, small-scale power, eliminating corporate control of energy. It will provide the cheap energy needed to eliminate world poverty. It will make possible a new space propulsion system that will radically cheapen and speed space exploration. It will free the world from 19th-century energy sources and provide power for the Third Millenium.”

SpaceNut · 2004-07-30 09:44:51

I am really not sure which post to put this under for Project Prometheus since I know very little about nuclear powered rockets.

Advanced Electric-Propulsion Technologies R&D Teams Selected

http://www.nasa.gov/home/hqnews/2004/ju … teams.html

JimM · 2004-07-30 09:47:41

The amount of hydrogen used up by a medium sized vehicle during it's ascent would be measured in grams, and a small, light reactor should be able to lift a large payload.

Quite so. So why would anybody want to bother with complicating the vehicle with airbreathing jets and wings and making it unsellable politically by flying it horizontally?

So let's stick to the sane side of this street and go with the pure rocket.

Gennaro · 2004-07-30 11:49:41

Thanks Jim for pointing out the reason you advocated sea launch. GLOW and not radiation or anything like that which I had expected. This means you can basically take off from the shoreline, right?

Need to read this through a couple of times more before I make up my mind who's probably on the right track here. Just going to say what an awesome sight your sea monster would be! Taking off from water like a 22nd century flying boat, oh I mean, rocket ship behemoth. Check out those ripples!
:laugh:

Euler · 2004-07-30 14:24:23

The amount of hydrogen used up by a medium sized vehicle during it's ascent would be measured in grams, and a small, light reactor should be able to lift a large payload.
Quite so. So why would anybody want to bother with complicating the vehicle with airbreathing jets and wings and making it unsellable politically by flying it horizontally?
So let's stick to the sane side of this street and go with the pure rocket.

I was talking about the ramjet. As I have said, a rocket where all of the thrust comes directly from fusion products will use up around 2000 times more fusible material than the ramjet would. This still puts you in the kilograms range rather than the tons, but if you are using an expensive fuel like tritium, it could still get expensive. More importantly, since the reactor would have to be 2000 times more powerful, it would also have to be bigger, heavier, and more expensive. It would likely end up being too heavy to lift it's own weight.

An alternative to the ramjet would be to carry some inert propulsive material onboard the ship. The amount that you would have to carry would be small compared with chemical rockets, probably around 1/4-1/3 of the vehicle's mass. This approach would also have the advantage of being more useful than the ramjet outside the atmosphere.

JimM · 2004-07-30 14:33:45

This means you can basically take off from the shoreline, right?
Just going to say what an awesome sight your sea monster would be!

I must confess, I think the thing would indeed be an awesome sight. However, I don't think a shoreline launch would be a good idea, actually.

For two reasons, I'd launch it over the horizon from the shore.
One, a reasonable water depth would be a good idea; can you imagine the mess it would leave behind from a land-based launch?
Two, the noise would be unbelievable, well over the pain threshold; eardrums would be burst by the score. (People on board would be insulated from the worst of this.) Have you heard Shuttle blast off? Imagine that, squared.

Imagine a second sun, rising rapidly from over the horizon, accompanied by the sound of hell itself as it dissapears into the heavens...

------------
"I am become death, the destroyer of worlds..."
...from the Bhagavad-Gita, quote by J. Robert Oppenheimer after Trinity, the first atomic bomb test

JimM · 2004-07-30 14:53:39

As I have said, a rocket where all of the thrust comes directly from fusion products will use up around 2000 times more fusible material than the ramjet would.

Whatever leads you to suppose all the thrusts come from fusion products? LH2 doesn't have to be 'fusion product', you know. It's just got the highest Isp. And as for using trituim as propellant? Come on, get serious!

An alternative to the ramjet would be to carry some inert propulsive material onboard the ship.

Gosh, just what I suggested in the first place. It took you a while...

The amount that you would have to carry would be small compared with chemical rockets, probably around 1/4-1/3 of the vehicle's mass.

At an Isp of 10000, the amount would be about 9% of the vehicle's mass. I worked it all out in some detail earlier in this thread. Do try to keep up!

This approach would also have the advantage of being more useful than the ramjet outside the atmosphere.

A ramjet in a vacuum is noted for its utter uselessness. Another point I made some time back.

Euler · 2004-07-30 15:46:27

Whatever leads you to suppose all the thrusts come from fusion products? LH2 doesn't have to be 'fusion product', you know. It's just got the highest Isp. And as for using trituim as propellant? Come on, get serious!

You might use tritium because it is easy to fuse (because of this it is used as the active ingredient in fusion bombs). After all, if you can't get the fusion to occur, your rocket won't work.

We aren't trying to get the isp as high as possible. We are trying to get it as low as we can without the propellant becoming an unreasonably large proportion of the ship's mass. That is the point of carrying the extra inert propellant: it lowers the isp.

JimM · 2004-07-31 02:30:32

You might use tritium because it is easy to fuse (because of this it is used as the active ingredient in fusion bombs). After all, if you can't get the fusion to occur, your rocket won't work.

It seems you STILL havn't got it:

The reactor fuel will probably be tritium.

The propellant is NOT the reactor fuel. The two are completely separate and distinct. Got that now?

The system I envisage is: the reactor heats the propellant (NOT the reactor's fuel) which is then exhausted through magnetically-contained rocket nozzels.

We aren't trying to get the isp as high as possible. We are trying to get it as low as we can without the propellant becoming an unreasonably large proportion of the ship's mass. That is the point of carrying the extra inert propellant: it lowers the isp..

Don't be daft. Of course we're trying to get the Isp as high as possible!

The propellant may well be LH2 because LH2 delivers the highest Isp--exactly what's wanted.

Euler · 2004-07-31 03:29:42

It seems you STILL havn't got it:
The reactor fuel will probably be tritium.
The propellant is NOT the reactor fuel. The two are completely separate and distinct. Got that now?
The system I envisage is: the reactor heats the propellant (NOT the reactor's fuel) which is then exhausted through magnetically-contained rocket nozzels.

That is what I have been saying all along. Notice how I used phrases like "fusible material" and "inert propulsive material" to distinguish the two.

I may have misunderstood you, but I thought that you were advocating using reactor fuel as the propellant.

Don't be daft. Of course we're trying to get the Isp as high as possible!
The propellant may well be LH2 because LH2 delivers the highest Isp--exactly what's wanted.

If we wanted the isp to be as high as possible, then we really would use the reactor fuel as propellant.

When you are limited by the amount of propellant that you can carry, you want the isp to be as high as possible. However, if you are limited by the amount of power that you can produce, then you want a lower isp because it is more energy efficient.

JimM · 2004-07-31 04:42:53

If we wanted the isp to be as high as possible, then we really would use the reactor fuel as propellant.

I think we're almost there.

If I put it, "We want the Isp to be as high as possible, short of using the reactor fuel as propellant." Can we agree on that?

When you are limited by the amount of propellant that you can carry, you want the isp to be as high as possible.

Well yes, of course. And using any propellant delivering an Isp of 10000, only 9% or so of GLOW need be propellant to get to and from LEO, which I was using as my reference mission, of course. So clearly, if (let's say) 50% of GLOW was propellant, the ship could probably get to Mars and back in a month or so, or cruise the solar system for years, or whatever you fancy.

Alternatively, if we stuck to the LEO requirement of 9% of GLOW, the cargo-carrying capacity would be immense. Or the luxury passenger carrying capacity could rival the largest ocean-going vacation liners of today--and a ticket might not be all that more expensive!

None of this would be possible with a silly ramjet aerospace plane.

----------------------------------------
I keep coming back to this, because we all keep forgetting: spaceships are ships, not planes.

ANTIcarrot. · 2004-07-31 04:45:33

Point withdrawn.

ANTIcarrot.

GCNRevenger · 2004-08-03 10:01:41

...Turn my back on the place for three days...

"What a quaint, unreal world you must live in. If there's no propellant, what's the point of the ramjet? Or, how much better the thing would fly in a vacuum...

Get real! Of course there's propellant! It's what you call the hot expelled air comeing out through the rear of the ramjet; that's what propels the thing, which is why, strangely enough, it's called 'propellant.'"

Hey Jim, you know a while back when you became just a little bit too.. how to say it.. offensive about that fellow who wanted to make the super laser engine and change E=MC^2? Well guess what, your doing it again. You making me out to be an idiot because I don't precisely spell out every single detail the way you expect it, not to mention how you ignore my arguments repeatedly for whatever reason and reguard me as stupid for making them, is not justifiable. Don't even think about giving me the "well YOU should have worded it more exactly, its YOUR fault" line, no no, you knew very well what I was talking about and made cracks about me anyway, this cannot be interpreted any other way than intellectual bigotry. I expected more civility from you, particularly since you have done this before, but apparently you failed debate class miserably or somthing because you forgot the need for respect in peer discussions.

Now then... the list of things you have essentially dismissed out of hand:

-The FUSION reactor will, unlike the fission reactor on the NB-36H, be competly safe almost immediatly after shutdown because the conditions needed to sustain the neutron-releasing reaction will no longer exsist. If the thing crashes, the reaction stops. If it blows up in the air, the reaction stops. If the plasma is vented the reaction stops... It will not be a radiation hazard if it fails. The fusion spaceplane will be no more unsafe than your giant rocket as it will only have to operate its reactor over water, not to mention generate less radiation to begin with. The argument that it is a radiation threat is ignoring the facts.

-The reactor, being the heaviest, largest piece of either vehicle most likly, will not have to be as large on a spaceplane because less thrust will be required in an spaceplane with lift versus a rocket without. The difference in thrust needed is so signifigant, that the spaceplane reactor will be several times smaller. It would not be unreasonable that, even considering the mass for wings, that it would cut the vehicle mass in half right there.

-Using ambient air as propellant, the Isp of the ramjet is so high it is essentially irrelivent compared to a rocket because no propellant (fuel or oxidizers) is carried onboard, only a few kilos of plasma feedstock; therefore the engine can be tailored to trade Isp for thrust, enabling a ramjet with a smaller reactor to generate similar thrusts. Hence, the reactor on the spaceplane becomes even smaller, perhaps by several times.

-The shielding required to block ~100% of all neutron and secondary release radiation from all direactions produced from a large fusion reactor will be quite heavy. Since your rocket is supposed to be shielded entirely, this will raise the mass of the rocket substantially, thus requiring an even larger reactor, which will in turn need more heavy shielding, that really cuts into the payload mass fraction which requires making the vehicle larger still

-The propellant mass of the rocket with its tankage/structure & handling equipment is nontrivial, contrary to your suggestion, which will account for at least 15-20% of the total vehicle mass. For a vehicle weighing thousands of tons, the added mass is a signifigant factor, particularly if the LH2 must be insulated from the sea water that conducts heat better than air... Don't forget the mass for landing fuel.

So, overall, the Ramjet spaceplane will weigh only a modest fraction of the weight of a fusion VTOL rocket, perhaps only a small fraction depending on the weight of the reactor shielding. Frankly, I think you have ignored these facts almost entirely because you dismiss the concept of a spaceplane no matter what... Preparing a rocket for takeoff is a little harder than filling an oil tanker with relativly stable liquid too... Ocean propulsion propellers? Heavy ocean wave stabilizing system? Liquid hydrogen plant onboard? Helecopter pad onboard?. The cost to build a space vehicle of that terrible size would be astronomical if it were possible at all.

JimM · 2004-08-03 16:05:13

GCNRevenger I expected more civility from you, particularly since you have done this before, but apparently you failed debate class miserably or somthing because you forgot the need for respect in peer discussions.

JimM And there I was, brought up to believe debates were the verbal equivalent of war, and those who can’t stand the heat should stay out of the kitchen…

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

JimM Intellectual bigotry? No, I don’t think so. But there is a limit to how long anyone can be expected to suffer weird or untenable notions without pointing out in a manner that leaves little room for doubt that they are weird or untenable. Now I do NOT take you for a fool, not by a long shot, but you have expressed some weird notions (such as airbreathing jets lacking propellant, as I have already pointed out) and I do consider you fixated on HOTL even when it is as clear as can be that, at least in this case, it is entirely untenable.

The FIRST and LAST reasons it will never fly is that no matter what the truth about radiation risk, there is more chance of a snowball surviving to old age in a furnace than of you ever getting the thing approved. Thus everything in between is of curio value only. Please face up to it: Nuclear HOTL is entirely unsellable politically.

So anyway, let’s look at the list of curios:

GCNRevenger The FUSION reactor will, unlike the fission reactor on the NB-36H, be competly safe almost immediatly after shutdown…. etc., etc…

JimM How can you know this? No-one has yet achieved a self-sustaining fusion reactor; this is pure speculation.

GCNRevenger The argument that it is a radiation threat is ignoring the facts.

JimM Perhaps the engineering facts. But it’s the argument that will sink the project.

GCNRevenger Using ambient air as propellant, the Isp of the ramjet is so high it is essentially irrelivent compared to a rocket because no propellant (fuel or oxidizers) is carried … Hence, the reactor on the spaceplane becomes even smaller, perhaps by several times.

JimM Not true. The Isp of LH2 is several times more than ambient air. (Go work it out. It’s an inverse function of the square of the propellant density at NTP.) This means that for a reactor of the same size, a vehicle using LH2 as propellant will deliver (let’s say) about 10 times the thrust of an airbreather. (Theoretically it could be much more, but I’m being my naturally modest self here.) Hence an otherwise identical vehicle than can fly horizontally with T/W = 0.5, would, if placed on its tail, be able to take of vertically with ease if provided with LH2 to use as propellant, since its T/W would be something like 4 or 5.

We already know the propellant fraction need to get to LEO with some to spare is about 9% of GLOW at an Isp of 10000. So when you take away all the air-breathing and flying-through-air gubbins, the ship would actually be significantly lighter than the HOTL version. So taking all this into account, the reactor output and hence reactor size for VTOL would in fact be several times smallerthan what a HOTL would need, everything else being equal.

GCNRevenger The shielding required to block ~100% of all neutron and secondary release radiation from all direactions produced from a large fusion reactor will be quite heavy. Since your rocket is supposed to be shielded entirely, this will raise the mass of the rocket substantially, thus requiring an even larger reactor, which will in turn need more heavy shielding, that really cuts into the payload mass fraction which requires making the vehicle larger still.

JimM Uh, actually it would probably still be less massive than your HOTL. (See previous answer.)

GCNRevenger The propellant mass of the rocket with its tankage/structure & handling equipment is nontrivial, contrary to your suggestion, which will account for at least 15-20% of the total vehicle mass.

JimM I never said it was trivial, but, at Isp 10000 propellant is 9%, so with tankage, etc.? 10%? 11%? And that’s not a ‘suggestion’. It was worked out correctly, as I demonstrated. Whereas your 15-20% is, so far as I can see, no better than what we used to call a ‘wild-a—ed guess’.

GCNRevenger …the LH2 must be insulated from the sea water that conducts heat better than air...

JimM Another attraction of the VTOL’s cylindrical geometry. The propellant would be far from the exterior of the ship; so that’s not an issue.

GCNRevenger Don't forget the mass for landing fuel.

JimM Already allowed in the 9%, if you recall.

GCNRevenger So, overall, the Ramjet spaceplane will weigh only a modest fraction of the weight of a fusion VTOL rocket, perhaps only a small fraction depending on the weight of the reactor shielding.

JimM Nonsense, as I have shown above. For the same payload, it will probably be significantly more massive. I do wonder how you're going to manage to fly an aircraft weighing thousands and thousands of tons through the atmosphere.

GCNRevenger Frankly, I think you have ignored these facts almost entirely because you dismiss the concept of a spaceplane no matter what...

JimM Why are you so hydrophobic?

GCNRevenger Preparing a rocket for takeoff is a little harder than filling an oil tanker with relativly stable liquid too...

JimM What, like LPG? I gather you know nothing of that business either.

GCNRevenger Ocean propulsion propellers?
GCNRevenger Heavy ocean wave stabilizing system?

JimM Why ever not? Thrusters, my dear fellow, thrusters. A doddle with a fusion plant to power them.

GCNRevenger Liquid hydrogen plant onboard?

JimM The fusion plant is already there. What’s your problem?

GCNRevenger Helecopter pad onboard?.

JimM Why ever not? My vehicle would be about as big as the great Pyramid. I already said that.

GCNRevenger The cost to build a space vehicle of that terrible size would be astronomical…

JimM Obviously. Going on the history of (abandoned) HOTL to-orbit projects so far, so would your concept--especially when we factor in its massive size. The difference is, mine could work—and be allowed to work.

JimM You have still to enlighten us on how you’re going to accelerate your HOTL from a standing start to ram-(or scram-)jet speed. Elastic bands? Turbojet engines? (Dare I say it) rockets?

JimM And how exactly are you going to finally get it into a regularized orbit—and deorbited again later, without rockets?

dicktice · 2004-08-03 17:01:11

JimM: Your first mistake was to rise to GCNR's bait. Did I miss something, or are you simply assuming the availability of some kind of mobile fusion reaction thingy, to levitate and accelerate your grandious flying pyramid into deep space? Excuse my pouncing on that one detail. Otherwise, it would make a great sci-fi, and GCNR should be ashamed of himself for trying to discourage your efforts.

Euler · 2004-08-03 19:17:25

But there is a limit to how long anyone can be expected to suffer weird or untenable notions without pointing out in a manner that leaves little room for doubt that they are weird or untenable. Now I do NOT take you for a fool, not by a long shot, but you have expressed some weird notions (such as airbreathing jets lacking propellant, as I have already pointed out) and I do consider you fixated on HOTL even when it is as clear as can be that, at least in this case, it is entirely untenable.

JimM: you have a reasonably good understanding of physics. However, you still don't understand physics nearly as well as you think you do. You often put forward "weird or untenable notions" yourself, but you will never admit that they are weird and untenable no matter how much evidence there is against you. Worse, you often end up insulting people for not agreeing with your sometimes untenable ideas. A case in point:

The Isp of LH2 is several times more than ambient air. (Go work it out. It’s an inverse function of the square of the propellant density at NTP.) This means that for a reactor of the same size, a vehicle using LH2 as propellant will deliver (let’s say) about 10 times the thrust of an airbreather.

Here is your basic mistake. You see, because the H2 has a higher Isp (higher exhaust velocity), the reactor with the H2 will deliver less thrust.

You see, the kinetic energy of an object is equal to 1/2*mass*velocity^2. The momentum of the object is equal to mass*velocity. This means that if you double the velocity, the momentum would also double. However, you would require 4 times as much energy to get the object to that speed.

Now lets apply this to rockets. If you increase the exhaust velocity (Isp), then you will require more power to produce the same amount of thrust. If your reactor stays at the same size and power output, then it will have less thrust if you increase the Isp.

GNCRevenger: I agree with most of what you have said in this thread. However, you always seem to be pessimistic about everything. Many ideas that you see on this forum are impractical, but you should try looking for solutions to problems, rather than just more problems. If you occasionally expressed optimism for someone else's ideas, more people would listen to what you say.

JimM · 2004-08-04 02:11:59

Here is your basic mistake. You see, because the H2 has a higher Isp (higher exhaust velocity), the reactor with the H2 will deliver less thrust.
You see, the kinetic energy of an object is equal to 1/2*mass*velocity^2. The momentum of the object is equal to mass*velocity. This means that if you double the velocity, the momentum would also double. However, you would require 4 times as much energy to get the object to that speed.

I’m sorry, but you’re wrong. What matters is not momentum but change in momentum (the impulse).

I = F * t

… where I = impulse, F = force, and t = time. When t = 1 second, this is Isp.

So change in momentum, I = m * deltaV

… where m is mass, and deltaV is change in velocity

Now we can apply directly the specific impulse of the vehicle (which we can assume we know for the purpose of this exercise) so…

If the VTOL rocket has an Isp of 10000 (using LH2) and the HOTL vehicle has an Isp of 1000 (using ambient air), but both have exactly the same GLOW of say 1000, then …

If Isp = 10000 and m = 1000, then deltaV = Isp/m = 10 (VTOL)
If Isp = 1000 and m = 1000, then deltaV = Isp/m = 1 (HOTL)

Thus, if both vehicles have the same reactor power (ie., energy),the vehicle with the Isp of 1000 would have to weigh just 10% of the vehicle with the Isp of 10000 in order to achieve the same deltaV…

If Isp = 10000 and m = 1000, then deltaV = Isp/m = 10 (VTOL)
If Isp = 1000 and m = 100, then deltaV = Isp/m = 10 (HOTL)

Therefore, if the HOTL vehicle can achieve a T/W of 0.5, the same vehicle sat on its tail and using LH2 as propellant instead of air could achieve a T/W = 5.

QUED.

(In fact, it’s worse than that for HOTL. The rocket equation applies to the VTOL but not HOTl, which would make deltaV higher, everything else being equal.)

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