Nuclear airliners

Terraformer · 2012-06-26 04:43:00

GW: Methane is renewable, either from Biogas or by producing it directly from CO2 and Hydrogen. I wonder if there is sufficient market for a "green" hypersonic transport to fund the development of the first stage of a TSTO system...

RobertDyck · 2012-06-26 05:56:10

Terraformer: I have long argued for a nuclear SSTO. Start with a lifting body based on HL-20 (now DreamChaser), powered by the nuclear jet engine. To fly hypersonic, it wouldn't use a fan jet, instead a turbojet based on the J58 engine used by the SR71 Blackbird. That engine had multiple modes, using doors to redirect air flow. At fastest speed it became a ramjet. Studies in the 1990s showed new technology could permit a new engine to fly up to mach 6 with jet fuel (according to Wikipedia). SCRAMjet designs have predicted much higher speed, the X-43A flew at mach 9.8 with hydrogen. At one point the Air Force hoped to achieve mach 17, and ATK has a contract from NASA to explore SCRAMjet technology to mach 20. A nuclear engine should achieve that. An article published in New Scientist in October 2000 talked about "plasma magic". This technique used a microwave beam to heat air ahead of the aircraft, so the aircraft flew threw a weak plasma. This reduced aerodynamic drag permitting higher speed. Later study showed this only works with poorly streamlined aircraft, a well streamlined aircraft has little if any benefit. But a lifting body is very blunt, designed for minimum skin surface area that has to be protected by a heat shield. So "plasma magic" would benefit a lifting body shuttle.

Add to this a solid core nuclear rocket for the final push into orbit. Then conventional RCS thrusters, using MMH/N2O4 just like the (now defunct) Shuttle. However, you could replace the thrusters with hydrazine arcjet thrusters for improved Isp. They require electricity, but do not need oxidizer. Avoiding N2O4 eliminates at least one toxic propellant. But arcjet thrusters manufactured by General Dynamics use pure hydrazine, Apollo used UDMH and Shuttle used MMH because they're more stable. Could an arcjet work with MMH?

The reason for using a nuclear jet engine for most of the way, then a nuclear rocket for the final push to orbit, is to reduce propellant. A jet engine uses air as its only propellant. The Shuttle re-entered the atmosphere at mach 25, so achieving mach 20 with a jet engine before starting to use propellant means a very small LH2 tank.

RobertDyck · 2012-06-26 07:54:53

Natural gas is not renewable. Natural gas is mostly methane, but it's not made from bio-gas, it's drilled from the ground. Some oil deposits have a pocket of natural gas on top, but now they're using coal bed gasification. Some companies are pumping down water into coal beds, claiming it isn't gasification since they're not pumping down steam, just water to flush natural gas out of the coal. Montana found they were using ground water, and just spilling the condensed waste water on the ground, resulting in salt that comes up with the natural gas/hot water mixture contaminating surface soil. Salting the soil turns arable soil non-arable. I've seen a TV ad from one company in Alberta that claims they use brine from deep underground as their working water, claiming they don't contaminate ground water. I don't know, but in any case it's not renewable.

And the current method of producing cost effective hydrogen is by processing natural gas. Since hydrogen comes from natural gas, or oil distillates, then making natural gas from it would be counter-productive.

Terraformer · 2012-06-26 10:20:15

Natural gas is methane, but methane isn't necessarily natural gas.

Hydrogen is generally considered to be reusable, in that it can be produced using "renewable" energy sources. With hydrogen and CO2 you can produce methane. The methane isn't renewable as an energy source, but it works fine for storage.

Leave the reactors on the ground and produce methane for use as fuel if you want a hypersonic craft.

RobertDyck · 2012-06-26 15:45:11

Chemical rockets are roughly 1% payload. Even with LH2/LOX, which has a much higher Isp than methane. You'll never get an SSTO or any truely cost effective reusable shuttle using chemical rockets.

But I started this discussion with airliners. Oil is way too expensive, and will only get more so. Kerosene based jet fuel, or any substitute that uses oil based fuel to make it, will also increase.

I started this by asking if you are ready to accept a nuclear airliner. I guess the answer is no. That means you will experience ever increasing air fare, and a dramatic jump when the US federal government no longer pays for airports.

louis · 2012-06-26 15:58:13

RobertDyck wrote:

Chemical rockets are roughly 1% payload. Even with LH2/LOX, which has a much higher Isp than methane. You'll never get an SSTO or any truely cost effective reusable shuttle using chemical rockets.
But I started this discussion with airliners. Oil is way too expensive, and will only get more so. Kerosene based jet fuel, or any substitute that uses oil based fuel to make it, will also increase.
I started this by asking if you are ready to accept a nuclear airliner. I guess the answer is no. That means you will experience ever increasing air fare, and a dramatic jump when the US federal government no longer pays for airports.

There are more realistic alternatives than dangerous nuclear power:

http://www.guardian.co.uk/environment/2 … green-fuel

Terraformer · 2012-06-26 16:50:55

I started this by asking if you are ready to accept a nuclear airliner. I guess the answer is no. That means you will experience ever increasing air fare, and a dramatic jump when the US federal government no longer pays for airports.

Not necessarily. I can easily accept nuclear power on the ground, and manufacturing renewable fuels for airliners using the energy. As long as the total cost of manufacturing the jet fuel remains the same as it does to extract and refine it now, the airfares shouldn't increase.

Rune · 2012-06-26 17:28:38

Terraformer wrote:

I started this by asking if you are ready to accept a nuclear airliner. I guess the answer is no. That means you will experience ever increasing air fare, and a dramatic jump when the US federal government no longer pays for airports.
Not necessarily. I can easily accept nuclear power on the ground, and manufacturing renewable fuels for airliners using the energy. As long as the total cost of manufacturing the jet fuel remains the same as it does to extract and refine it now, the airfares shouldn't increase.

I was going to reply to say that, but it would be repetitive now. So +1. As to flying fast, why would you want scramjets if you have a nuclear thermal engine: run it with atmospheric air until there is either not enough or it's coming too fast (/too hot) if you wanna get fancy, then switch to onboard propellant. Or just use a VTOVL NTR, Isp is enough. As to arcjets, they run on anything but have very bulky power needs. Nowhere near T/W>1.

Rune. I doubt safe NTR's can approach chemical engines' power to weight ratios anyway.

RobertDyck · 2012-06-27 01:47:22

Rune wrote:

why would you want scramjets if you have a nuclear thermal engine: run it with atmospheric air until there is either not enough or it's coming too fast (/too hot) if you wanna get fancy, then switch to onboard propellant.

That's what I said. The SCRAMjet analogy was just to show an air breathing engine can propel an aircraft that fast.

As to arcjets, they run on anything but have very bulky power needs.

one example: General Dynamics MR-509
thrust: 254-213 mN
valve power: 8.25 Watts max at 28 Vdc
system input power: 1800 Watts
input voltage: 65-96 Vdc
specific impulse: >502 sec
flight status: INTELSAT VIII, ECHOSTAR, ASIASAT
document revision date: 5-8-99

One key feature of an arcjets is it needs a catalyst to get the hypergolic propellant to break down. Different fuel, different catalyst. Is there a catalyst that'll work with MMH?

I doubt safe NTR's can approach chemical engines' power to weight ratios anyway.

Did you not read my comments about Timberwind? Very high Isp, very low engine mass. It was able to do this with a "fast" nuclear reactor. That means it consumes uranium very quickly, so it doesn't need nearly as much. Nuclear fuel will not last as long, but hot fire time is still enough for several trips to Mars and back.

Rune · 2012-06-27 03:54:56

There are arcjets running on hyrogen, and there are arcjects running on ammonia. Being an electrical engine, they can run on any fluid suitable to be heated by electrical discharge. You could call them thermal electric engines if you prefer. They use hydrazine now, I guess, to run monopropellant RCS from the same tanks, or for fuel commonality, or for some other similar reason (like hydrazine breaking down into hydrogen and ammonia on the engine through the use of catalysts). The figure of merit for propellant is molecular weight: the lower it is, the higher the isp and the lower the thrust.

As to fancy nuclear reactors, fancy chemical engines get to T/W about 150, as SpaceX has just shown with the Merlin 1D. Show me the NTR that, even on paper, gets close. I remember Timberwind as being about 30. Add the airbreathing gizmos, and see T/W decrease again.

Rune. Not really a fast reactor either, I think... that means fast neutrons are dominant, not anything about the energy density of the core (which on Timberwind is incredibly high, BTW).

RobertDyck · 2012-06-27 09:17:48

Yes, a fast reactor uses fast neutrons instead of epithermal neutrons. That means it doesn't need a moderator. It only works with highly enriched uranium, >10% U-235/233, but solid core nuclear thermal rockets use >99% enrichment. That's what the word "fast" means, but the important effect is lighter weight due to lack of moderator, and most importantly it consumes uranium very quickly. Fast consumption of uranium means you can launch a reactor that has much less to start with.

When you talk about thrust/weight ratios, engine mass is not that important. What is important is total stage mass: engine, tank, and propellant. Some more numbers:

Nerva 2, study 1991
engine mass: 8,500 kg
thrust: 333.40 kN (74,951 lbf)
Specific impulse: 925 s
stage mass (unfuelled): 27,000 kg
stage mass (fuelled): 158,400 kg
burn time: 3,575 s

Timberwind 45, developed 1990
engine mass: 1,500 kg
thrust: 441.30 kN
Isp (vacuum): 1,000 s
stage mass (unfuelled): 7,500 kg
stage mass (fuelled): 28,000 kg
burn time: 449 s

Centaur V2 stage
2 engines: RL-10A-4-2
fuel: LOX/LH2
thrust (total): 198.2 kN, 20,231 kgf
Isp (vacuum): 451 s
stage mass (unfuelled): 2,250 kg
stage mass (fuelled): 23,050 kg
burn time: 435 s

It would be nice to get closer matches, but this does illustrate the point. A Timberwind Centaur stage provides more thrust and more burn time than a Centaur V2 stage. NERVA is nice, but it has the problems you think are endemic with all NTR engines. NERVA's engine mass is just way too high.

Last edited by RobertDyck (2012-06-27 15:52:13)

GW Johnson · 2012-06-27 14:22:18

An alternative fuel for existing subsonic aircraft could be biodiesel-jet fuel blend. That's a drop-in fuel. I even know a good antifreeze agent for cold soak at high altitudes. I've been involved with flight tests of 20 and 30% biodiesel blends back about 1999. I think stiffer blends could be flown, if the higher viscosity and stickier surface tension characteristic don't screw up the metering and spray patterns. Biodiesel in existing turbines needs a thinner, and an anti-freeze agent.

For piston diesel, biodiesel a drop-in fuel, period. Viscosity resembles no. 2 diesel as-is. Just needs the antifreeze agent.

As I pointed out earlier, why screw around with supersonic airliners, or hypersonic airliners? It's easier to build a skip-glider, or even an antipodal rocket spaceplane. SR-71 equipped with attitude thrusters was successfully operated exoatmospheric, as a skip glider. Decades ago. Improved the range, it did, over straight supersonic cruise.

That choice would be true regardless of the propulsion scheme you choose to make it happen. Nuke rocket might be pretty good for an antipodal spaceplane transport. That's what Heinlein put in his science fiction about 1945-ish. Based on what we know about NERVA and the others, we really could do it from a technical standpoint.

GW

RobertDyck · 2012-06-27 16:48:21

Biodiesel is a good point. The local electric utility is Manitoba Hydro, they use biodiesel for their service vehicles. They found they have to adjust their fuel mix during the year: more biodiesel/less petroleum diesel during summer, more petroleum during fall/spring, and pure petroleum diesel in the depths of winter. So they're using petroleum diesel as anti-freeze and thinner.

There is a strong trend for farms to use biodiesel. Case IH is the merger of Case with International Harvester, a US manufacturer. They already build tractors and combines that use 100% biodiesel. It can only operate in 5°C (41°F) or higher. B6 to B20 (6% to 20% biodiesel) can be used if outside temperature is above -9°C (16°F). Farmers don't do much when there's snow on the ground anyway. And special care is necessary to store the fuel, keep all moisture out. And you have to replace fuel filters often if switching between petroleum diesel and biodiesel, because biodiesel acts as a fuel line cleaner. Any gunk stuck to the inside of fuel tanks or fuel lines will be removed, collecting in the filter. Case IH says 100% biodiesel contains 8% less energy than No. 2 diesel, and can reduce power and torque by 7 - 12%. I think it's worth it if the cost per km (per mile) is lower. Apparently so does Case IH and their customers; their news announcement says they're 100% committed to biodiesel.

Buhler Versatile is a Winnipeg manufacturer of tractors. Unfortunately they're only rated for B20, which is 20% biodiesel.

An SR-71 was equipped to operate exoatmospheric? I keep learning new things. What other military developments haven't been released to the public yet?

But I recommended nuclear airliners for the same speed they operate now: about mach 0.85. The high speed stuff would be only for a space shuttle. The point is to gain as much speed as possible without use of any propellant at all. That's why I said operate a nuclear jet engine from wheel stop to somewhere between mach 17 and 20. Then switch to a solid core NTR with LH2 propellant for the final push to orbit. This is something some people have trouble wrapping their mind around: a nuclear jet engine uses no propellant what so ever. To be picky propellant is air, but that isn't carried. So the past ratios of propellant for this vs propellant for that just don't apply.

GW Johnson · 2012-06-28 09:41:11

Yeah, Kelly Johnson let it slip at his retirement celebration from the old Lockheed "Skunk Works" that an attitude thruster-equipped SR-71 reached a one-million-foot altitude. That's 189 statute miles, 165 nautical miles, or 305 km.

He probably shouldn't have let that slip out, but he did. Not long after, we started hearing stuff about Mach 3 skip-gliding across the Pacific essentially unrefueled. You hear all kinds of things, now that the plane has been retired. Most of them aren't true, really, but the trend of these things is startling. That's why I do believe the skip-glide trans-Pacific thing.

But no faster than about 3.5 Mach, really, the J-58's are not true air turboramjets. About M3.8 on a very short transient is all the turbomachinery can stand, and even that shortens its life. They really have pushed it that fast. The air bypass to the afterburner is max 25%, and it comes off compressor stage 3 or 4, not the supersonic inlet. That's public info now. Long ago, that was classified.

Essentially what they did was add the same attitude thrusters to it that the X-15 had. Both were designed about the same time, and had similar strake-equipped fuselage shapes, just different wings and engines, aerodynamically. Like the X-15, slower exit Mach at near-vertical path angle leads to very high apogee and very short range, while higher exit Mach at lower path angle leads to a much longer range to re-entry. It's an energy-management thing before you leave the air.

GW

Terraformer · 2012-06-28 10:32:32

So, the SR-71 can technically be called a spaceship...

A quick question. GW: at what Mach number does it make sense to transition to a Ramjet?

Rune · 2012-06-29 07:41:41

That's a nice SR-71 trivia bit I never heard about either. The first thing that popped through my mind is that the old Blackbird would have made a hell of a delivery system for a KEV without any additional rocket stages. Would make a very responsive and cheap system to take out satellites in LEO, but I guess if you are worried about stopping salvos of ballistic missiles you would stick with missiles as delivery systems like they ended up doing with the upgraded Standards on Aegis destroyers (SM-3, is that the one?).

Terraformer wrote:

GW: at what Mach number does it make sense to transition to a Ramjet?

Well, a Ramjet will work (efficiently) from M>1.something to M~6, depending on design (a ramjet optimized for M2 flight won't do well at M6). Since a jet typically maxes out at about M2.5 (you can get faster, maybe 3.5 but you are pushing it and you will pay the price in T/W), I would say at about that speed, from M2.5-3 to M5-6, is when Ramjets make the most sense. Don't forget that to get a single engine to perform adequately over a big Mach range, you would have to use tricks like variable geometries, and those things complicate stuff, and you have to get into specifics to find out how they would turn out. But sometimes technological solutions complicate these analysis a lot. I'm thinking about SABRE, for example, where their magic heat exchanger changes the design parameters greatly (they make a freaking turbojet fly at M5.5!).

Rune. Sometimes I get surprised that armies make the rational decision when buying weapons. Sometimes. ^^'

Last edited by Rune (2012-06-29 08:56:15)

GW Johnson · 2012-06-30 09:36:18

Transition to ramjet? Depends.

For the SR-71, they would be bypassing a bunch of air and afterburning it, from about M2.5 on up for high thrust. At high fuel consumption, of course.

Thinking fixed geometry like the good missile guy that I was, there are two basic types of simple ramjets, a lower speed-range design, and a higher speed-range design.

The lower speed range is transonic or high subsonic to about Mach 2, or at most 2.5. These were the old "stovepipe" designs with the pitot or normal-shock inlets, a baffle flameholder, and a convergent only nozzle. Peak performance is about M1.5-ish. Min speed is well subsonic, limited by fuel consumption more than thrust. Looking slower, Isp starts looking worse than early-technology solid rocket somewhere around 300 mph (near 0.5 M). These were most successful if boosted to about Mach 1 before taking over on ramjet. If I were to do one of these today, I would do it as a dump combustor, not a baffle flameholder, especially if it had to fly at various speeds. Max speed is mainly limited by vehicle drag and low normal-shocked inlet recovery.

The high speed designs feature external-compression devices for inlets, such as the spikes on the SR-71. These can be axisymmetric, axisym-sectors, or 2-D ramps. There's all-external shock compression, or mixed external-internal compression. (All-internal really doesn't work in any practical sense.) Most of these today are dump combustors, although the early ones, like the RJ-43's on the old Bomarc, were baffle flameholders. The inlet design with its attached shock system inherently restricts you to Mach > 1.5 just for takeover, but internal engine pressures are correspondingly high enough for an always-choked exit, so these have very mild expansion bells. Throats are still very large, around 65% of the engine cross section. This kind of thing can be (and has been) flown to speeds as high as M6. Vehicle drag is the key limiting issue, followed closely by ionization instead of temperature rise in the combustor at speeds above M5-ish. (Recombination is not converted to velocity increase in a nozzle.) M5 is pretty easy to reach. M4 is a pretty common design cruise for missiles propelled this way.

GW

GW Johnson · 2012-07-01 10:59:17

Biodiesel antifreeze:

I used ethyl tertiary butyl ether (ETBE) for that. Discovered it completely by accident. We were flight testing 20-30% biodiesel in Jet-A in a PT-6-equipped Beech KingAir. I was doing cold fuel screening tests on the ground, and put 0.5% by volume ETBE into a B-30 fuel blend. Jet-A by itself starts depositing waxes about -58 F. B-30 typically did that around -20F.

The B-30 with 0.5% ETBE went to -68 F with no sign of deposits at all. All the biodiesel-jet fuel blends showed a really ugly color change at about 28 or 29 F, but no deposits at all until somewhere under -10F. The more jet in the blend, the lower they would go, but all showed deposition earlier than plain jet fuel, except the ETBE stuff, which went way colder than even plain jet fuel.

I never looked at using ETBE as a trace additive in plain biodiesel in diesel engines. But someone should. You don't need a fancy expensive freezer to do the work, unless you plan on running the real ASTM tests. For "jake-leg" screening tests, I just used 4 gallons of ethanol in a 5 gallon bucket, plus roughly 15 pounds of dry ice. Set the bucket on styrofoam, wrap it with an army blanket or two, and in less than 15 minutes you have a cold bath at -110 F. You'll need a thermocouple thermometer, stirring sticks, and some clear glass beakers. Safety warning: if you drop a tool in the bucket, go in after it bare-handed, DO NOT WEAR GLOVES! You have 7-10 seconds immersion time before frostbite begins. The gloves will freeze to your hands, you cannot get them off in time to prevent frostbite from the cold gloves.

GW

GW Johnson · 2012-07-01 11:25:34

As for the airliner problem, you still have to make the decision on how fast you need to fly.

There's well-proven subsonic cruise. It will always be cheaper than anything supersonic. I think there will always be a market for that. Renewable replacements for petroleum jet fuel will make a lot of sense in that application, especially after we finish sucking all the petroleum and natural gas that we can reach out of the ground. Hydrogen has a storage volume problem, very serious in practical aircraft design. Renewables like biodiesel will have a production capacity limitation, although it is not today's limitations.

We're pretty sure based on the Concorde and a whole slew of fighters and bombers that we don't want to do supersonic cruise. Hypersonic would be even worse. Way too expensive, way too hard to do in a mass-producible commercial aircraft design. Although, there's a lot of supersonic transport fans that have always ignored those inconvenient realities. (Scramjet has missile applications, but I just don't see the utility of it for space launch or air transport. The characteristics are just too poor a match to the mission.)

That leaves the "relatively un-proven" modes of supersonic exoatmospheric skip glide, and supersonic/hypersonic exoatmospheric ballistic trajectories. The thruster-equipped SR-71's already proved skip-glide works, they're just not widely known. The real question is only how much savings the range extension confers to offset the super-high costs of supersonic flight, in a transport size. ICBM's long ago proved the practicality of ballistic trajectories, it's just that no one considered replacing the rockets and warheads with rockets and airplanes. That one could be "proven feasible" by something like the X-37, if they were to fly it that way. And they could.

We don't have a clear path to a nuclear air-breathing engine. We do have a clear path to a nuclear rocket.

I'd go for chemical propulsion in the skip-glider, very little different from what we did with the SR-71. You could build a real air turboramjet (100% max bypass, off the supersonic inlet ahead of the compressor face), or you could just put two kinds of engines on the plane: turbojets and ramjets. Either would work just fine. We're talking exit Mach in the 3-4 range. About the same as SR-71 and B-70.

For the ballistic bird, we're faced with staging if we go chemical. Typical aircraft are roughly half structure, and half payload plus fuel, anything less structure is too fragile, we've learned that over the last century of flight experiences. The most "reusable" way to stage chemical is the carrier airplane. But I don't see that as ever being inexpensive. Too many people, too much equipment. Dropping rocket stages is even worse. So, there's your commercial application for the nuclear rocket, if and only if you have the chutzpah to fly it in the atmosphere! One or the other of those old NTR designs has the Isp and T/W to support a single stage airplane leaving the atmosphere at about M6-10 and 45 degrees, coming back down 6,000-12,000 miles away.

Voila! Nuclear airliner.

GW

RobertDyck · 2012-07-04 12:55:29

Great idea! According to Wikipedia, ethyl tertiary butyl ether (ETBE) is not only antifreeze, it also does not absorb moisture from the atmosphere. Does that mean it inhibits absorbing moisture, or just doesn't add to the problem? It also does not promote gasoline to evaporate, while ethanol causes both problems. But diesel really doesn't evaporate, so I don't think that's an issue for biodiesel.

Ps. Remove a tool from liquid at -110°F (-78.9°C) with bare hands? No thank you. I have changed a tire at -38°C, with 80% humidity and a stiff breeze. Winter gloves didn't have enough dexterity to work with lug nuts, but bare hands caused frost bite on contact with bare metal, and fingers become so numb I could barely move them. I've gotten frost bite enough times. I would use Bar-B-Q tongs.

Last edited by RobertDyck (2012-07-04 13:14:23)

GW Johnson · 2012-07-04 17:46:35

ETBE: I found that it simply did not absorb moisture itself, and neither did hydrocarbons, so mixtures didn't either. No real inhibition, just no tendency to accelerate moisture absorption. Ethanol does promote moisture absorption, but only up to about its azeotrope at 95% ethanol, 5% water. So it's not as bad an actor as "everybody" claims.

I've done gasoline-ethanol blends in on-the-road cars for about 6-7 years now, and I have found the problems "everybody cites" to be way overstated. I have found no problems at all (none !!!!!) up to about 35% ethanol in gasoline, in completely-unmodified engines, right down to fuel consumption, which is identical to 100% gasoline. This result I found in 3 completely-different vehicles in real (statistically-repeatable) road tests. Including what amounts to "cold weather" in central Texas (18 F = -8C). The only real problem occurs at about E-42, and it looks like late ignition timing. Which is unsurprising, since the book values for ignition delay time are longer for ethanol than gasoline.

Cold immersion at -110 F (-61 C): liquid pool immersion is quite a different environment than atmospheric, and it is not the same as "standard" cryogenic stuff. I was able to touch and pick up blocks of ethanol-wet dry ice with my naked hands, something absolutely impossible out in the dry air. Touching pieces of -30 F (-34 C) steel with bare fingers out in our atmosphere is something I would never, ever attempt. The behavior of surface skin moisture is completely different in those two scenarios (wet vs dry).

This is "school-of-hard-knocks knowledge" coming from a person who has actually worked outside more-or-less steady state at conditions as cold as -31 F (-35 C) and as hot as 130 F (54 C), or as hot as 300 F (149 C) if you count transients in jet blast. (Those jet blast transients are quite temporary, of course. No more than an hour at most.)

GW

SarK0Y · 2012-09-18 19:31:51

Terraformer wrote:

Oh, I think it was much earlier than that. 1910 at the latest.
Anyway, the trouble with nuclear airliners is that the shielding weight is going to be prohibitive, and if you solves that using shadow shields and other such fancy tricks, you're not going to be able to land the craft without powering down the reactor. Which is going to be expensive.
You could, of course, keep the craft in the sky and ferry passengers to it. Or, even better, you could use nuclear power on the ground to synthesise fuel, and use that...

*Thorium reactor is quite useful to've no problems with overweight of construction.
* magnetic plasma ramjets are the best choice to accelerate pretty bird, the no've moving parts.
-------
in fact, those lovely birdies w/ mp-ramjets & thorium reactor can drastically reduce the price to put a payload onto LEO.

GW Johnson · 2012-10-07 10:58:52

I have no idea what a "magnetic plasma ramjet" is in Sark0y's post 47 just above, but you can use chemical ramjet, chemical turbojet, and chemical rocket propulsion together, if you wish, in one vehicle. I have less faith in combined-cycle engines, and more faith in just using two or even three separate types of well-developed engines run in parallel.

For a skip-glider limited to Mach 3, turbojet/ramjet is good enough to get the job done. I am not at all sure about the economics, but I am sure they would be better than sustained Mach 3 cruise in the atmosphere. Both engines burn the same kerosene or kerosene-like fuel (even liquid methane or biodiesel). You get to reject air friction heat while exoatmospheric on the glide. At Mach 3, you can do the skip atmospherics with a metal airframe. Stainless and titanium skins seem adequate, based on prior experience. You could up that to Mach 5+, but no more than Mach 6, using Inconel skins, as in X-15, but those are pretty heavy.

For the single-stage rocketplane ballistic vehicle, the velocity requirements are just too high for large payload fraction and a large structural fraction at the same time, which is required for economics and safety-of-flight. That speaks to nuclear Isp levels. The final form of NERVA ca 1973 was pretty well free of core erosion, which means it was pretty well free of exhaust plume radioactivity. That's quite an improvement over the earlier Phoebus and Kiwi experiments.

The problem is reentry at destination. Is the reactor turned off? That's a little safer, but you still must deal with core containment in the event of a crash. If it is off, you're deadstick, so how do you handle emergency needs to divert to another runway or to go-around? This is a big heavily-loaded airliner full of people, remember? While an intriguing and promising technological approach, there are some serious design and safety issues here.

The solution might be a gas-core reactor concept, something not out of the academic lab demo stage. But it could be. I'd suggest the open-cycle approach as the lighter of the two (the other being the "nuclear light bulb"), which also automatically has no core to contain when you shut it down. But, you have to accept a radioactive exhaust at the launch site if open cycle. How bad? No real data, but consider LH2/uranium (-235 or -233) mass flow ratio 30:1 is "perfect" containment at the nuclear burn-up rates needed. There's an upper bound estimate of the daughter-product "fallout" you spew. Those are much more benign if you use Thorium-bred U-233. Or so I hear.

If you go with "nuclear light bulb" instead, the exhaust is clean, and you dump the core at burnout in space, retaining nothing on board. Unfortunately, that core dump takes place at suborbital speed, so it comes down very quickly. Just on a different target. Or, you land with a retained core, just like the solid core engine. Take your choice.

GW

SarK0Y · 2012-10-07 14:37:03

Johnson, mp-ramjet is improvement of classical ramjet: plasma flow is directed by EM fields & RF heats air up to plasma states. actually, mp-ramjet can use chemical fuel too. it makes possible to boost power of propulsion because nuke reactor cannot provide high power.

Tom Kalbfus · 2013-11-14 07:49:29

A nuclear jet engine would be quite useful in the upper atmosphere of Venus, as that atmosphere has no oxygen to burn jet fuel with.

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#26 2012-06-26 04:43:00

Re: Nuclear airliners

#27 2012-06-26 05:56:10

Re: Nuclear airliners

#28 2012-06-26 07:54:53

Re: Nuclear airliners

#29 2012-06-26 10:20:15

Re: Nuclear airliners

#30 2012-06-26 15:45:11

Re: Nuclear airliners

#31 2012-06-26 15:58:13

Re: Nuclear airliners

#32 2012-06-26 16:50:55

Re: Nuclear airliners

#33 2012-06-26 17:28:38

Re: Nuclear airliners

#34 2012-06-27 01:47:22

Re: Nuclear airliners

#35 2012-06-27 03:54:56

Re: Nuclear airliners

#36 2012-06-27 09:17:48

Re: Nuclear airliners

#37 2012-06-27 14:22:18

Re: Nuclear airliners

#38 2012-06-27 16:48:21

Re: Nuclear airliners

#39 2012-06-28 09:41:11

Re: Nuclear airliners

#40 2012-06-28 10:32:32

Re: Nuclear airliners

#41 2012-06-29 07:41:41

Re: Nuclear airliners

#42 2012-06-30 09:36:18

Re: Nuclear airliners

#43 2012-07-01 10:59:17

Re: Nuclear airliners

#44 2012-07-01 11:25:34

Re: Nuclear airliners

#45 2012-07-04 12:55:29

Re: Nuclear airliners

#46 2012-07-04 17:46:35

Re: Nuclear airliners

#47 2012-09-18 19:31:51

Re: Nuclear airliners

#48 2012-10-07 10:58:52

Re: Nuclear airliners

#49 2012-10-07 14:37:03

Re: Nuclear airliners

#50 2013-11-14 07:49:29

Re: Nuclear airliners

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