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Say you have a your VTOL with isp=10,000, and an HTOL with isp=1000. They both have identical reactors, producing identical amounts of power.
... except that HOTL couldn't, where it mattered.
Firstly, at take off: without yet another (expensive and heavy) means of propulsion, its thrust at the proposed start of its run down the runway would be zero.
Secondly, at 'cruise' at 100,000 ft, where it could not find enough atmosphere to turn into propellant—except perhaps by having enormous intakes that would (a) multiply the drag problem manyfold and (b) add significantly to the mass and complexity of the vehicle.
The HTOL uses propellant at a rate 100 times greater than the VTOL does, and therefore is able to produce 10 times more thrust. That is its advantage.
Except that it would not; it could only only produce the same thrust, at best. And for various practical reasons already referred to, it certainly could not use propellant at a higher rate than the VTOL during 'launch' (take off) or probably, at 'cruise'.
And in any case, even if you were right (which I do not accept) the power reactor in the VTOL can be made bigger with much greater ease and almost no weight penalty—not possible for HOTL
All-in-all, HOTL is an over-complex, impractical and self-limiting way to get to orbit, if it could ever be made to work.
Furthermore VTOL is amost endlessly scaleable; HOTL is not.
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VTOL is a true spaceship; HOTL is not, but no more than an aircraft that pops momentarily out of the atmosphere before dropping back down again (how?) to where it's at home.
You would do better to consider the behavior over a range of specific impulses that will actually give a power output, rather than at the one value that you know will not. For example, consider rocket engines with exhaust velocities of 10 m/s (about what you'd expect from a child's squirt gun), and 1000 m/s (about what you'd expect from a rocket powered dragster).
If you're like me, your first impulse is that the rocket powered dragster produces the most thrust. But if the one with 10 m/s were the Hoover Dam instead? It has an exhaust velocity similar to a squirt gun. Adjust the gates just right, and it can put out the same power as the rocket powered dragster. (You would have to stopper the Hoover Dam down quite a bit to reduce it to the output of a measly dragster rocket, I might add.)
Sorry, you're wrong too.
The example you give is misleading. Suppose, instead, we compared one squirt gun with an exhaust velocity of 1 m/s and another squirt gun, otherwise identical, with an exhaust velocity of 100 m/s. The volume and mass of water squirted is identical; only the velocity differs. So which delivers the most thrust?
Clearly, the answer is the gun with the most powerful spring (or however squirt guns do it) In other words, exhaust velocity is directly proportional to thrust, when mass of water (or, in our case, propellant) is identical.
And so it becomes clear that exhaust velocity and hence thrust, and hence deltaV, is directly proportional to Isp, everything else being equal.
Thus, a VTOL machine using LH2 propellant (let's say, Isp 10000) at the rate of (let's say) 1 ton/sec, will deliver greater—significantly greater—thrust (and deltaV) than a HOTL using ambient air (let's say Isp 1000) also at the rate of 1 ton/sec.
So it is clear (I'd say, obvious) that the only putative advantage a low-Isp HOTL has over a high-Isp VTOL is that it can expell a greater mass/sec, or the same mass for longer, or perhaps both.
But this is a purposeless advantage, given the low mass of LH2 propellant needed to get the VTOL to LEO, already demonstrated to be less than the mass-penalty of adding on wings, ramjets, and whatever else is needed to make a HOTL work...
...plus all the other reasons I listed earlier.
Actually, if you take special note you will see that the 'm' in question is not the mass of propellant but the mass of the entire vehicle, propellant and all.
Obviously. However, to derive it you assumed that the propellant will have a mass of 1. If the propellant does not have a mass of 1, then the formula is completely bogus.
I'm sorry for the delay in answering, and also sorry for my error...
deltaV = Isp/m
...should have been written...
deltaV <is proportional to> Isp/m
This could also be written, perhaps more clearly, as...
deltaV = (Isp/m)k
... where k is a constant of unspecified valus.
You said a little while back...
...the engine with lower Isp can produce the same amount of thrust while using less energy.
I now put it to you that, by the process of reductio ad absurdum, the logic of this statement can be shown to be absurd and thus false:-
What you are telling us is that if the vehicle' propellant (not engine) had an Isp of zero that would deliver the same thrust while absorbing zero energy—and also that if the propellant had an Isp of infinity it could only deliver the same thrust if it absorbed infinite energy.
So, discarding your earlier proposal and given that deltaV is shown to be directly proportion to the Isp of the propellant and in inverse proportion to the mass of the vehicle, this means that the higher the Isp for a given ship mass the better the acceleration.
Now of course in the case of a rocket the ship mass will decrease as propellant is used up, but as I have earlier explained, in the case of the VTOL ship with a presumed Isp of 10000, this would be a relatively small effect and has been ignored to ease comparison with the HOTL sc/ram/jet-powered alternative. (This also works to the advantage of the HOTL, BTW)
A HOTL with an Isp of (say) 1000 from ambient air would thus be at a very considerable disadvantage; it's deltaV (and hence acceleration and hence impulse) would be very much poorer than the VTOL. On the other hand it has the apparent advantage of what might be called an 'infinite propellant tank' but to be set against that are the following, largely overlooked here, disadvangages:
(1) It has to fly through that 'tank', creating considerable drag (mostly in the supersonic and hypersonic speed ranges) which it has to fight against. This is probably equivalent to at least a halfing of the effective Isp.
(2) At 'cruising altitude', said to be 100,000 ft, there is going to be very little ambient air to use as propellant, which will have the effect of curtailing thrust considerably.
(3) It has to apply several thousands of degrees of heating to air that at least in the case of the scramjet, will only be exposed to the heater for a minute fraction of a second--and will have already entered the champer at a very high temperature due to collision with the hypersonic vehicle, but what matters is what I might call the deltaHeat.
(4) It is still going to need to be able to fly as a rocket anyway, to regularise its orbit and to re-enter at the end of its mission.
(5) It will basically be confined to shuttle-style missions between earth and LEO, while the VTOL, with additional fuel (easily accommodated) could fly around the solar system more or less at will, possibly re"fuel"ing at Titan, Mars, or wherever else is appropriate.
(6) It can't start from rest at the end of the runway powered by sc/ram/jets. They don't work until considerable speed is already achieved. We still have not been told how this is to be done.
I = F * t
… where I = impulse, F = force, and t = time. When t = 1 second, this is Isp.
Actually, Isp is impulse/propellant mass. This is why your "deltaV = Isp/m" is only true if the mass of propellant used is 1, and even then it is only an approvimation because it does not take into account the changing mass of the vehicle.
Actually, if you take special note you will see that the 'm' in question is not the mass of propellant but the mass of the entire vehicle, propellant and all.
Further, if you care to read my most recent answer to SpaceNut, you will see that I chose to disregard the changing mass of the VTOL (changing mass does not apply to the HOTL in any case) for simplicity, and why.
Therefore, deltaV = Isp/m holds generally.
If the engines both use up propellant at the same rate. However, by using up more propellant, the engine with lower Isp can produce the same amount of thrust while using less energy.
Clearly, if the propellant (NOT the engine) with the lower Isp is used in sufficiently greater quantities, as high a deltaV (NOT thrust) can be achieved as with the higher Isp. But that would require the use of more--a lot more energy, not less. There is absolutely nothing to gain and a lot to loose by deliberately using a low-Isp propellant.
Indeed, I suspect that one of the ways to resolve the reactor efficiency problem is to use as high-Isp propellant as possible, as that will make a significant difference to efficiency in thermodynamic terms; sufficient to explain the apparent 'something-for-nothing' aspect of higher Isp delivering higher deltaV that's bothering you, I'd not be surprised. (But it's only a guess. I don't have the time or inclination to work that one out in detail.)
(Also, the heating of a highly hypersonic airflow sufficiently to deliver specified thrust, even with a fusion reactor and magnetic containment, is going to be asking a lot. At (say) 20,000 fps, it's difficult to see how the ambient air can be exposed to the fusion-generated heat for more than about, oh... say... one thousandth of a second--probably much less... and at 100,000 ft, there's not going to be a lot to use anyway. Once again, you're going to have a problem getting rid of waste heat. And think of all the global warming--not CO2, just raw heat--you'd be accused of making that doesn't apply to a VTOL. Ho-hum.)
QUED: when I was at school, this was the standard abbreviation of 'quad erat demonstrandum.'
In the equation is not the Mass changing also as fuel is burned?
You're quite right, that's what happens with a rocket, and that's what the rocket equation handles. But in the case of the HOTL vehicle, it's not a rocket, so mass is assumed to be constant. Also in this particular case, the propellant fraction of the rocket (the VTOL) is so small (about 9%) that for a first-order approximation for comparison purposes with the HOTL, it can be ignored.
deltaV = Isp/m
Actually, deltaV=Isp*ln(mass ratio)
Interesting point.
deltaV = Isp*ln(mass ratio)
...is of course the guts of the rocket equation.
However...
deltaV = Isp/m
...is something quite different. Here, 'm' is not the mass ratio (as it seems you have assumed in error) but the mass of the object, be it rocket or not. Recheck how I derived this from the equation for impulse, and I am confident you will see that I'm right.
Thrust is equal to impulse/time.
If T = thrust, Tsp = specific thrust, and t =time in seconds
T = I/t
...then if time = 1 second
Tsp = Isp/1
... thus specific thrust = specific impulse.
Therefore, the higher the Isp, the higher the thrust.
QUED (again)
BTW: We don't know what the actual power output of the reactor--or it's efficicency--is, so there's little point in speculating on 50% or whatever efficiency--although I must comment that if it's 50%, it's only a question of time, probably measured in a pretty small number of seconds, before there's a nice big mushroom cloud where the vehicle was. (All that heat has to go somewhere.)
Frequency is the inverse of time.
No.
frequency = c/wavelength (where c = velocity of light in a vacuum)
wavelength*frequency = c
wavelength = c/frequency
The frequency of light (also called more properly, electromagnetic radiation) ranges from zero to 10^22 herz, where you find cosmic rays. The full range is called the electromagnetic spectrum, and whatever the frequency, it all travels at the same velocity, c, in a vacuum.
Visible light occupies only a tiny sliver of the spectrum, just below the 10^14 frequency. As the frequency rises above visible light we go through the realms of ultraviolet light, x-rays, gamma rays and then cosmic rays. As the frequency fall below visible light, we go through the realms of infra-red, and then radio, from super-high frequency radio all the way to super-low frequency radio.
So light has limits of min and max for the equation based on energy form that is released.
The higher the frequency, the higher the energy of the radiation. Thus gamma rays are more dangerous than x-rays, and so on.
But e=mc^2 is always true.
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.)
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…
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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?
There is radiation for one thing...
Light IS radiation
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.
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I keep coming back to this, because we all keep forgetting: spaceships are ships, not planes.
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.
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.
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...
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"I am become death, the destroyer of worlds..."
...from the Bhagavad-Gita, quote by J. Robert Oppenheimer after Trinity, the first atomic bomb test
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.
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.
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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.
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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.
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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.
With soft currencies like the rouble, fixing a price in dollars seems risky.
Don't you mean, fixing a price in anything except dollars, euros, pounds or yens is risky?
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?
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.
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.
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?
GCNR, I feel your $500 million price tag is far too low. The Russians have never ventured beyond LEO and it's a third of century since the Americans have. They have no man-rated lunar excursion module; they have no deep-space communication system; they have never demonstrated a manned Soyuz hyperbolic re-entry.... the list is endless.
Sure, they can cut a lot of corners that delayed Apollo and cost a lot of money, but you can't launch an expedition like this (and that's what it would be) unless there was a reasonable chance of sucessful return for all on board, including the tourist.
Which raises another thought. Can this mission be flown with one tourist and just two crew? I frankly doubt the mission can afford a passenger.
$500 million? By the time you've finished all your vehicle development and testing, make that $2 or $3 billion, and 3 or 4 years to lift-off. And these numbers are on the side of optimistic, I'd say.
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.
The Soyuz would require a kick-stage for the cis-lunar run.
There's the rub. It wouldn't half need one. (And going via the ISS makes no sort of sense trajectory-wise, BTW.)
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.