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Hm, I need to recapitulate on rocket science. Once I proposed that if you feed a steady state fusion rocket's exhast with heavier elements, it might be able to produce high thrust and hence become a VTOVL of choice.
Yes, I know flying tokamaks are likely to be very heavy systems, but perhaps size and mass intensity can be worked upon? Would the concept be viable at all? What do the experts say?
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I would speculate that any kind of flight based fusion system be Inertial Confinement based. Have an area of LASERs fire at a pellet of lithium deturide in a combustion chamber filled with tritium or deuterium as a 'starter' this would initiate fusion and you could then use a magnetic nozzel to direct the trust.
As far as adding additional reaction mass to the plasma stream to improve thrust, it's an idea, you would be sacraficing exhaust velocity and temprature but the increased thrust might make sense for VTOL opperations,
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My guess: Not possible.
The kinetic power released by a rocket's exhaust is equal to a half of the thrust times the exhaust velocity. If you raise the exhaust velocity of the rocket without changing the thrust, all you're doing is making more heat.
A lot of useless heat.
Fusion powered rockets represent scant gain in efficiency for ground launches even if they work, and they'd run so hot that any current design is as likely to blow up as to actually launch. Fusion rockets capable of ground launches are a technology for the distant future, if it's possible at all.
"We go big, or we don't go." - GCNRevenger
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Actually no, if you increase the velocity while maintaining the same same thrust, you will get an increase in fuel efficency.
I am picturing some kind of fusion plasma ramjet... the superhot plasma from the fusion reactor is leaked/sprayed into the center of the engine and the propellant (probobly ambient air) gas is admitted through the intake. The inside of the engine is tailored to reflect the radiant energies from the plasma, and as the plasma mixes with the gas, the gas gets very very hot.
If the gas is released carefully, it could be possible to reach high exhaust temperatures without damaging the engine, particularly if it were cooled activly somehow.
[i]"The power of accurate observation is often called cynicism by those that do not have it." - George Bernard Shaw[/i]
[i]The glass is at 50% of capacity[/i]
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Only interesting and meaningful replies in this thread.
Actually no, if you increase the velocity while maintaining the same same thrust, you will get an increase in fuel efficency.
Yes, that sounds about right according to my (limited) understanding. How do you picture the operation at higher altitudes by the scramjet? I mean, airflow will drop to zero at some point, so to speak.
Will there be some secondary propellant system?
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I think that you would encounter rapidly reducing returns as you get to higher altitudes or lower air pressures (ie Mars). What it might be good for is for distant-future spaceplanes, kind of a fusion scramjet arrangement. I've heard of ideas for regular fission ones, but these present problems (nasty radiation & heavy shielding). 0-3000mph using fusion/mhd heated turbines, then cutting in the fusion plasma scramjet and off to orbit on only kilos of deuterium perhaps...
[i]"The power of accurate observation is often called cynicism by those that do not have it." - George Bernard Shaw[/i]
[i]The glass is at 50% of capacity[/i]
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What about permanent propellant tanks? Filled with something I don't know, hydrogen, ammonia, whatever? Of course, if you could cruise the rest to LEO on pure plasma that would be better. (Was that correctly understood?)
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I dunno, that would get too heavy pretty quick I think, you couldn't produce enough thrust that way without a huge reactor I don't believe.
The plasma in the reactor has to predominantly stay in the reactor for the thing to keep running, the little bit you would bleed off to use for propulsion wouldn't give you alot of thrust either if you just wanted to run the engine without a seperate propellant.
[i]"The power of accurate observation is often called cynicism by those that do not have it." - George Bernard Shaw[/i]
[i]The glass is at 50% of capacity[/i]
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There's just that pesky little problem about radioactive exhaust...
Plus such a ship would be somewhat difficult to stick in a museum after it had reached the end of its operational life.
ANTIcarrot.
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The kinetic power released by a rocket's exhaust is equal to a half of the thrust times the exhaust velocity. If you raise the exhaust velocity of the rocket without changing the thrust, all you're doing is making more heat.
A lot of useless heat.
Not so.
Kinetic energy is half mass time exhaust velocity squared…
E(k)=0.5MV^2
… which is why it is worth trading exhaust mass for ehaust velocity.
… which is why hydrogen, the lightest element, is so often a popular candidate propellant for a nuclear rocket: it weights less, but is expelled at higher velocity, so the result is higher E(k) and so higher delta-V for the rocket, everything else being equal.
I am picturing some kind of fusion plasma ramjet..
I have my doubts about this concep, for three reasons.
(1) It's a considerable complication to the engineering for (I suspect) little or no gain in performance. And anyway the mass ratio of a fusion-powered vehicle should not be nearly as critical a matter as we are used to thinking of it today.
(2) I suspect the Isp from a fusion rocket working as a rocket using (say) H2 will not be so much lower than from the ramjet using atmosphere, with its so much heavier mass.
(3) With a single pure rocket mode of operation, the vehicle can just get itself out of the atmosphere and away from that nasty drag as soon as possible; since this could be done in no more than a minute or two, the doubtful and at best marginal gain from a ramjet mode is clear.
By the way, I strongly suspect a fusion-powered ship whould be huge—probably in the 20 to 50 thousand tons range. I'd not propose a ground launch for it, but from the ocean.
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The advantage of a nuclear ramjet/scramjet though, is that you would not only avoid the need to carry oxidizer like a chemical rocket, but any propellant at all. Think about that for a minute...
[i]"The power of accurate observation is often called cynicism by those that do not have it." - George Bernard Shaw[/i]
[i]The glass is at 50% of capacity[/i]
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The advantage of a nuclear ramjet/scramjet though, is that you would not only avoid the need to carry oxidizer like a chemical rocket, but any propellant at all. Think about that for a minute.
I believe I already have.
The usual and best argument in favour of a hybrid system like this is that the Isp of the ram/scramjet is much better than the rocket phase, because a chemical rocket has to take its oxydiser with it while an airbreather does not.
But that does not apply in this case. A fusion-powered vehicle does not need any oxydiser anyway, whether a rocket or a jet--and air, as propellant, has a very poor Isp when compaired with H2. What's more, the air-breather has to fly through the air--obviously--which means drag, so any given delta-V will take so much longer to get to, and will start to fall away again as soon as the engines stops firing. Hence, taking all this into account, I'd hazzard a guess that the actual real-life efficiency of the jet mode will be very much worse than the rocket mode
And then of course if you do manage to achieve any significant portion of orbital velocity while still using the jet, your vehicle is going to have a very serious heating problem.
Furthermore, to have a two-mode propulsion system as you suggest will mean a very significantly more complex vehicle that a single-mode system need be. And for what? Nothing very much if anything at all, I would suggest.
Remember the KISS principle: Keep It Simple, Stupid.
There's another thing. If a fusion vehicle would be in the 20 to 50 thousand ton range as I suggested earlier, I just can't see it taking off from anything resembling a runway and flying horizontally through the atmosphere, with wings and all the rest of the stuff that an aircraft needs but a spacecraft does not.
I think it would have to be sea-launched, and launched vertically. That means it would certainly not be taking off (from a standing start) under ram/sramjet power. It would have to be launched in rocket mode; in which case the only sensible trajectory for it to follow is to climb vertically until it can be rotated to the horizontal--clear of the atmosphere--and accelerated to orbital velocity as quickly as possible. Re-enter it tail-first and 'land' it back on the ocean that way too, firing the rocket to slow the last few thousand feet as necessary (and give it a little cross-range) and we have a lean mean fusion machine.
It would be a spacecraft that does not try to play at being an aircraft.
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F = m’ * v.e
P = 0.5 * m’ * v.e^2
Where F & P are the rocket engine’s thrust and power, m’ is the reaction mass, and v.e is the exhaust velocity.
Therefore:
P = 0.5 * F * v.e
So, if you keep the same amount of thrust (in this instance, the thrust needed to lift a rocket off of the ground), then raising the exhaust velocity raises the power requirements. For a rocket, this also means raising the operating temperature.
Raising the operating temperature requires an engine design that can deal with the extra heat. For rockets that operate at temperatures that are low compared to nuclear fusion (chemical rockets, etc.), raising the temperature doesn’t require much change in engine mass to deal with the extra heat. However, for a purely fusion powered rocket engine, that’s not necessarily the case. While it is true that there would be a substantial reduction in mass ratio (the ratio of launch mass to final mass), the final mass would have to be enormous to support an engine capable of working with that much power.
Lowering the mass ratio does indeed mean less fuel per rocket mass. However, there is a limit of returns where the extra engine mass needed to raise the exhaust velocity exceeds the fuel savings.
Ground launched nuclear fusion rocket engines are probably beyond that limit of returns.
There is hope, though. Rocket engines in which the heat-producing reaction occurs within the nozzle (chemical rockets, gas core nuclear rockets, etc.) are much easier to cool. A fusion rocket engine could conceivably (though not necessarily) operate similarly.
"We go big, or we don't go." - GCNRevenger
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No Jim, I don't believe you have... you keep on comparing the nuclear ramjet to a rocket engine using Hydrogen, but I can find little more than flippant remarks glossing over the fact that the "efficency" of the vehicle can hardly be compared since the ramjet would need no propellant at all. Isp is not even a valid concept.
Just leak a little reactor plasma into the ramjet and use the ambient (possibly compressed) air as the reaction mass... Why would you want to leave the atmosphere rapidly? And waste all that free propellant and lug your own? Well thats silly...
And with a nuclear reactor and the essentially unlimited amount of energy it has now that it no longer need worry about propellant supplies, who cares if it takes longer to accelerate using the atmosphere for reaction mass? Drag is also not such a problem, since you no longer have to design the vehicle for absolute maximum propellant economy.
This time, the performance of complex and high-tech whips KISS.. well.. stupid.
This is all dependant on a light weight and efficent fusion reactor able to create signifigant quantities of plasma of course, which is a fairly far-out concept, but how the reactor is essentially harmless when not operating with a light weight neutron shield for the crew, there are advantages. By then, i'm sure that materials will exsist that can handle the 4000-5000K temperatures. We have Tantalum ceramics today that will withstand ~4000K.
As for ocean launch or recovery, such schemes are simply senseless when considering true reuseable craft. It is simpy silly given how much trouble it would be to handle and manage such a launch and recovery scheme rapidly on a regular basis... oh, and if it were rocket powerd, you must lug along the landing propellant too. Nuclear SeaDragon this is not.
Edit: On further reflection, I guess it depends largely on the efficency & operating temperature of the reactor and its ability to pass off the extreme heat to the Ramjet intake gasses, and bring that up to very high temperatures without harming the engine... This is a question mark for me.
[i]"The power of accurate observation is often called cynicism by those that do not have it." - George Bernard Shaw[/i]
[i]The glass is at 50% of capacity[/i]
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No Jim, I don't believe you have... you keep on comparing the nuclear ramjet to a rocket engine using Hydrogen, but I can find little more than flippant remarks glossing over the fact that the "efficency" of the vehicle can hardly be compared since the ramjet would need no propellant at all. Isp is not even a valid concept.
Of course Isp is theoretically infinity if you grab all your propellant mass from the air, but that's a long way short of saying efficiencey is irrelevant.
Flippant? No, not me, not this time.
GCNR, I've run the numbers: have you? Because I believe they can show you why you are wrong.
Now I don't know what the Isp of a ground-launched fusion rocket would be, but presumably it would be significantly higher than a chemical rocket could manage. Also, the normal deltaV assumed necessary to get to LEO is about 28,000 fps.
So, let's assume the Isp for a fusion rocket is somewhere between 2,000 and 10,000, and the deltaV requirement is a (generous) 30,000 ft/sec--this should be more than sufficient to power the final landing phase, as most deltaV will be lost during re-entry.
The following equation derives the propellant fraction at GLOW for various valuse of Isp, where…
Fp = propellant fraction @ GLOW
deltaV = velocity increase
Isp = specific implulse
(EXP = exponentional)
Fp =1-(1/EXP(deltaV/(Isp*32.2)))
(This is the standard rocket equation, turned around a bit.)
Results for selected values of Isp:
Isp Fp
---------------------
2000 0.37
3000 0.27
4000 0.21
5000 0.17
10000 0.09
I think these numbers show quite clearly that certainly from about Isp = 4000 onwards, and probably much less than that (3000? ... even 2000?) the combined mass overhead of any form of atmospheric propulsion system such as ramjet, etc., plus wings and so on, makes them pointless as they would almost certainly weigh more than the extra LH2 used instead by the rocket if they were not there.
In fact, they would be worse than pointless, because the whole mass of the ramjet, wings, etc., would have to be lifted up into orbit and them back down again, while the extra LH2 propellant needed by the rocket would be burnt off and so not there to act as a drag on the whole journey.
Then again, a few extra lbs or even tons of LH2 is going to cost a lot less and require a lot less maintenance than a scramjet plus wings, etc.
As for ocean launch or recovery, such schemes are simply senseless when considering true reuseable craft. It is simpy silly given how much trouble it would be to handle and manage such a launch and recovery scheme rapidly on a regular basis... oh, and if it were rocket powerd, you must lug along the landing propellant too. Nuclear SeaDragon this is not.
Of course I agree this is not Sea Dragon, but it's going to be a monster all the same. Where else but the ocean can you imagine launching and recovering such a behemoth? And where else but a shipyard would you--could you--build such a beast?
To quote yourself, it would be 'simpy silly given how much trouble it would be to handle and manage such a launch and recovery scheme rapidly on a regular basis' on land. In fact it would be vastly simpler at sea. And as I said before, can you really imagine such a giant taking off and landing on a runway?
No, it's a natural sea creature (like Sea Dragon, although otherwise very different).
Oh-- I have already allowed for landing propellant earlier, as I'm sure you noticed.
This time, the performance of complex and high-tech whips KISS.. well.. stupid.
Well...no, actually. KISS wins hands down, as you can see above. Why make the thing more complex for the sole result of making it less efficient and effective and more expensive and complex to build and more likely to go wrong?
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I find the latter exchanges between GCN and JimM challenging indeed.
It could be of interest that what I initially had in mind would be a magnetically confined fusion rocket with hydrogen entered into the plasma stream from an integrated propellant tank. Exactly what Jim is proposing. It would take off and land vertically by its own power, reaching LEO (dock and exchange cargo) and go back again, ideally on a single fuel tank. It would be assumed that it also could have its H2 (or alternative) refueled from interplanetary tankers.
Of course, the setting is quite futuristic, but if the mode of operation would be Deuterium/Helium-3 it would have to get the latter component by interplanetary shipment in any case.
Many of the points presented by JimM were somewhere in the back of my head (atmospheric drag, wings and similar extra structure, horizontal landings by massive craft), but I've neglected reading up on these issues lately, which means I wasn't ready to spell it out.
The basic rationale was that if you already have such a heavy, yet high Isp propulsion system as a MCF rocket, the relative extra bulk due to a propellant tank would diminish in importance.
High Isp would also be sacrificed generously in order to achieve a T/W safely above 1 with the underlying assumption that the mass fraction would be notably improved in comparison to a chemical VTOVL/SSTO, which hardly manages to take anything but its structural dry mass to orbit. The trade-off in terms of capacity and total vehicle mass was an unknown.
The only thing I was prepared not to agree with JimM on would be the provision to launch it from sea, whatever that means - an island or some huge launch rig? Not the latter, I hope.
If seclusion is essential, I would have preferred what would amount to a launch intensive spaceport in some desolate place, like a desert or something, mainly due to logistical considerations. I somehow fail to see how a place in the middle of an ocean would be preferable, but maybe Jim is willing to explain in further detail what he means.
As for the air breathing concept, I'm still not sure how the craft is supposed to manage all the way to LEO once it leaves the stratosphere (the propellant) behind, even though it supposedly would hang out a while within this range to gain momentum. The stratosphere extends up to an altitude of 80 km while LEO extends from 350-1400 km (the ISS orbits at a height of 386 km). That’s quite a gap.
It would be awfully nice if GCN cared to elaborate on the issue in order to enlighten a dilettante like myself. Maybe my assumptions about the atmosphere are incorrect.
If any craft like this would ever be feasible, I suppose fusion engines would have evolved, becoming lighter and possibly have a different configuration or geometry than presently envisaged for fusion power plants. For example, the RIGGATRON engine features a reaction chamber curved inwards, forming a sort of spiral or circled device. This might potentially save on the ship’s design volume if nothing else.
Illustration:
http://www.islandone.org/APC/Nuclear/12 … ar/12.html
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Ah but Jim, there is one achillies heel to a fusion rocket versus a ramjet spaceplane that Gennaro made me think about... that the rocket must achieve a thrust/weight ratio of substantially greater than 1 in order to operate. While a spaceplane can get along with much less thrust, especially since fuel efficency is not so vital so that it can accelerate more liesurely. Since the reactor and associated engine hardware will undoubtably be the most signifigant contributor of the vehicles' mass, then getting away with a much less powerful reactor and getting rid of the onboard reaction mass and return reaction mass could very well make up for the extra mass of the wings and wheels versus a pure ballistic rocket... especially if the structure is built from CNT composits or amorphous metals and other advanced materials that will make the extra struture mass quite small.
And with the spaceplane, you can launch and land on runways if the vehicle is kept to within a reasonable mass, vastly simplifying operations versus any of this water landing nonsense. It must fly like an airliner, rapidly often and routinely, which simply puts the water option out of the question. We have airplanes today that weigh hundreds of tons, and I am not suggesting that we go and stick a giant Tokamak city-powering reactor in one, but somthing smaller, lighter, and unshielded (save for a crew bioshiled). This would of course require a technological generation or two beyond the first Fusion power reactors, but I think that a powerplant of reasonable mass could be built if we can pull off large scale reactors.
As for the altitude gap... well, when you start talking speeds north of Mach-20, there really isn't one. If you can reach Mach-25 with the fusion ramjet in the atmosphere, all you really have to do is pitch the nose up and coast to orbit, followed by a tiny OMS burn. The X-30 "Orient Express" was intended to use this trajectory, speeding up all the way to orbital velocity at the top of the atmosphere, then nose up and enter orbit from sheer inertia. This is all acedmic of course, an interesting thought excercise, as spaceplanes will undoubtably use chemical engines for the forseeable future, and a fusion powerplant of any sort may not be practical. But, this is the only concept I have heard of for a useful spacecraft that does away with the propellant entirely and enters space only by nuclear energy... which is why its interesting, to me anyway.
[i]"The power of accurate observation is often called cynicism by those that do not have it." - George Bernard Shaw[/i]
[i]The glass is at 50% of capacity[/i]
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One of the big problems I see in MC fusion propulsion is that they all hearken back to the 'Mirror Machine' arrangement favored by the Princeton plasma physics lab in the 60s and 70s. The geometry makes allot of sense for a rocket engine but because of the boundary layer turbulence that the inline configuration and circulation of plasma causes it is amazingly hard to get the confinement times needed for ignition. That is why the focus has been on IC fusion (EG the National Ignition Facility) Pulse fusion (EG the Z machine at Lawrence Livermore [I think]) and Tokomaks (EG the ALCOR at MIT or the JET in the United Kingdom)
In any event we all these technologies are between 5 and 15 years away from implementation for power plants on the ground so it's anyone’s bet whether they will ever be light enough for ground launch. The important use of these technologies will be in space propulsion and power generation on the surface of the moon and Mars. The moon and gas giants have He3 and Mars has deuterium, voila. (Deuterium/Tritium fusion doesn't make economic sense, the free neutron issue aside)
In the really long term I would guess that there will be craft that are fusion powered can take off and land on the surface and make the Earth/Mars 'milk run' in three of four days, but they will have to be based on materials science and a understanding of plasma physics that we can't even begin to approximate currently.
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An efficient fusion rocket should have an Isp of about 2*10^6. In order to get a T/W ratio of 1, it would be necessary to have a power output of 10^8 Watts/Kilogram. The point that some people have been trying to make is that it would be extremely difficult to have such a high power output without melting the engine.
This is where the ramjet idea comes in. Since it uses air from the environment the ramjet actually has a higher isp, but the exhaust velocity is lower. More importantly, since the exhaust velocity is lower, the ramjet can produce more thrust while needing less power. The idea is that hopefully it will be able to provide enough thrust to get off the ground without having prohibitive power requirements that would destroy the engine.
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Another good point... all the NTR engines to date have needed extreme (gigawatt) power levels to generate substantial thrusts. I don't see how a fusion/hydrogen rocket will reach sufficent thrust levels to get off the ground.
[i]"The power of accurate observation is often called cynicism by those that do not have it." - George Bernard Shaw[/i]
[i]The glass is at 50% of capacity[/i]
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The problem with heat seems to stem from the necessity for confinement.
What if there were no confinement at all, but rather an ablative nozzle/chamber, like the old Orion nuclear pulse propulsion idea? One could conceivably have an "Orion Lite" vehicle, but then the main limitation becomes the power source.
If you don't use fission, how do you prime a fusion reaction that isn't self sustaining?
"We go big, or we don't go." - GCNRevenger
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Ah but Jim, there is one achillies heel to a fusion rocket versus a ramjet spaceplane that Gennaro made me think about... that the rocket must achieve a thrust/weight ratio of substantially greater than 1 in order to operate.
This is your best shot at my achilles heel?
Well that's a relief; for a moment I thought you might have a serious point.
Clearly the rocket has to have a T/W better than 1. So?
We are talking about a fusion rocket that does not exist yet, except as a glint in some engineer's eye. But whether the fusion power unit is Tokomak-based or laser-blasted pellets or anything in between, it's going to have to contain the fusion-generated plasma by means of magnetic bottles of some sort or another. So--obviously--by the time we have the technology to do that, we can also contain the rocket blast in a magnetic blast chamber and funnel. So overheating rocket chambers is not an issue--so, by default almost, getting a T/W better than unit is not an issue either. I don't think it's likely to be even a minor issue. Far from it, I suspect that on the day, keeping the thrust of the thing down is more likely to cause problems.
But one other certainty seems clear to me. The thing, magnatiec bottle and all, is going to be huge. I suspect it will stand several hundred feet high and be sort of Apollo CM-shaped, with a base diameter of several hundred feet also. (You might think 'flying saucer' but that's not my intention.) It's sure to weight many thousands of tons especially with the shielding I'd not dispense with as you seemingly would. (I think the idea that this thing could take off and fly through the air without shielding is a sure-fire killer for your aircraft scheme, BTW. Talk about an achilles heel!) In any case, the notion that it takes off and lands horizontally like an aircraft is frankly laughable; even without shielding, it would be so massive it would make the Airbus A380 look like a featherlight.
It is also quite unsuited to dry-land VTOL, because (a) most surfaces would not support the mass, and (b) it would be unmoveable. So, the only sensible place to launch and land it is the ocean. All the necessary services could come to it, or it could be towed to them. And anyway, as I said earlier, the only realistic place to build this monster is a shipyard.
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But whether the fusion power unit is Tokomak-based or laser-blasted pellets or anything in between, it's going to have to contain the fusion-generated plasma by means of magnetic bottles of some sort or another. So--obviously--by the time we have the technology to do that, we can also contain the rocket blast in a magnetic blast chamber and funnel. So overheating rocket chambers is not an issue--so, by default almost, getting a T/W better than unit is not an issue either.
It seems reasonable that it would be easier to contain a small thermonuclear explosion than it would be to contain a large one. Hence, the problem is still there.
But one other certainty seems clear to me. The thing, magnatiec bottle and all, is going to be huge. I suspect it will stand several hundred feet high and be sort of Apollo CM-shaped, with a base diameter of several hundred feet also.
What makes you so sure of this?
I think the idea that this thing could take off and fly through the air without shielding is a sure-fire killer for your aircraft scheme, BTW. Talk about an achilles heel!
The fusion reaction produces much less radiation than fission reactions. Shielding should not be a huge problem, especially since the fusion reaction in the ramjet idea would be much smaller than in a pure fusion rocket.
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It seems reasonable that it would be easier to contain a small thermonuclear explosion than it would be to contain a large one.
Even a small thermonuclear explosion is not small. But I don't think a fusion reactor is the same thing as a thermonuclear explosion in any case.
I suspect it will stand several hundred feet high and be sort of Apollo CM-shaped, with a base diameter of several hundred feet also.
What makes you so sure of this?
If I was 'so sure' of this, I'd not be saying I 'suspect' it.
Whatever, the mass overhead of a flying fusion reactor, containment bottles, shielding and so forth would be enormous; probably thousands of tons. That's the downside. On the upside, the mass it could lift would be even more enormous. The bottom line is this: a truly huge vehicle--more like the size of an ocean liner than an air liner. But a spaceship that can transport thousands of tons of payload, or thousands of passengers in the sort of luxury found on cruise liners today, and at quite modest cost, perhaps no more than a few thousand dollars each. Now that's space tourism.
Shielding should not be a huge problem...
Sez who?
Quite apart from anything else, the lack of shielding would kill the horizontal take-off idea stone dead politically. That I would say I'm sure of.
...especially since the fusion reaction in the ramjet idea would be much smaller than in a pure fusion rocket.
Sez who?
I have already indicated that I'd expect the propulsion unit will be massive, not because it would not be nice to have a small one, but because that's what the physics and the engineering will deliver. And that just makes the aircraft idea impossible--as well as pointless, as I've already pointed out earlier. If the vehicle can take off vertically, having jets and wings is even worse than pointless, it's actually counterproductive.
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...especially since the fusion reaction in the ramjet idea would be much smaller than in a pure fusion rocket.
Sez who?
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.
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