Nuclear Single Passenger Earth SSTO

[Spacescifi]

 

I was thinking of a way to beat the whole weight problem for space travel using both off the shelf tech and theoretical stuff we don't have.

 

A shape like an oblong pill is fine, but the passenger will be standing longways.

 

So... can we use a mix of nuclear airbreathing rocketry to get to orbit to get a human who likely only weighs close to 200 kliograms plus the mass of the vessel? Or is that too much for nuclear to handle? When air thins out we switch to liquid methane.

If so... how about antimatter infused air and later some liquid methane when the air thins out?

Before you think AM is bad, we only need to have enough to get to orbit a few times, how much AM that requires I dunno, but since it is only a small single person SSTO I think that is a huge mass savings plus.

In orbit it can dock with the big momma vessel and restock on antimatter if the need arises.

 

With nuclear you don't need to restock hardly ever, but the power is less, and fusion there is no way we xan get by without lots of mass... barring project orion, which this is not.

 

Discuss?

[kerbiloid]

Methane jets with antimatter-powered fans are a thing!

[DDE]

8 hours ago, Spacescifi said:

So... can we use a mix of nuclear airbreathing rocketry to get to orbit to get a human who likely only weighs close to 200 kliograms plus the mass of the vessel? Or is that too much for nuclear to handle? When air thins out we switch to liquid methane.

Nuclear airbreathing - given certain liberal assumptions about the tech - is being hailed as a practical technology for SSTOs with significant payloads.

https://drive.google.com/file/d/0B52GlVTiDp1AU2JxRlFxNFNXUXM/view

[wumpus]

6 hours ago, DDE said:

Nuclear airbreathing - given certain liberal assumptions about the tech - is being hailed as a practical technology for SSTOs with significant payloads.

https://drive.google.com/file/d/0B52GlVTiDp1AU2JxRlFxNFNXUXM/view

I'm going to have to study that slide deck, if only for all the other information for air-breathers it appeared to have (mostly trajectory analysis).

If you want one of these, I'd start with an air-augmented rocket.  Even side boosters can justify work learning how to build intakes that work on at least one mode of the multitude of modes that NTR requires.  By the time you can build the intakes, perhaps nuclear power won't be quite so demonized.

[Spacescifi]

9 hours ago, DDE said:

Nuclear airbreathing - given certain liberal assumptions about the tech - is being hailed as a practical technology for SSTOs with significant payloads.

https://drive.google.com/file/d/0B52GlVTiDp1AU2JxRlFxNFNXUXM/view

 

Awesome!

According to the document you provided it even beats the falcon 9 for payload capacity!

Not sure about Elon's starship though.

This has provided an interesting look into what could propel proper SSTO's one day.

It sure is a lot easier than AM, which is definitely scifi territory for now.

 

I brainstormed this and learned that plug aerospikes would be optimal for any spaceship that could shield itself from the force of gravity on a planet. Lighter than air, the vessel would begin to float like a balloon, and with the help of nuclear airbreathing rocketry turbojets or AM if I wish, it could leave the atmosphere with minimal propellant loss.

Not only that, but plug aerospike just look awesome and can double as a heat shield if need be.

 

Yet so long gravity is in play they won't be optimal due to extra weight abd heat management.

Both of which could be easily solved if we could block gravity from the vessel. Heat management would be solved by pumpinhlg cold propelkant through the aeropike to cool it, which is easier now that it weighs nothing so it won't cut into thrust.

Once the vessel reaches sufficient speed in atmosphere it won't need to use any propellant at all, since gulping up all that air with the nuclear power is all it needs to fly. Well to be sure, you won't need to use ANY propellant with gravity blocking so long there is sufficient air.

With gravity all you need is enough airspeed and a nuclear jet rocket hybrid and enough air. No propellant required except reaching sufficient speed to let the nuclear take over completely.

Edited March 9, 2020 by Spacescifi

[Guest]

You don't need magic gravity-blocking tech at all. A nuclear airbreather with a plug aerospike would be very much capable of dealing with the weight of the plug nozzle. You don't need antimatter, either, indeed, if you use your book to promote nuclear engines as a safe propulsion solution, you'll be helping the environment in the long run.  A well-designed nuke engine would be perfectly capable of lifting a single-person capsule using air as the first stage and methane as a second (same engine, just switch out propellant). 

Seriously, you can have perfectly serviceable SF without all these gravitic shenanigans. All you need to do is a bit of outside the box thinking, and a little math. Once you get down to numbers, a lot of things can be made to work.

[Spacescifi]

1 hour ago, Dragon01 said:

You don't need magic gravity-blocking tech at all. A nuclear airbreather with a plug aerospike would be very much capable of dealing with the weight of the plug nozzle. You don't need antimatter, either, indeed, if you use your book to promote nuclear engines as a safe propulsion solution, you'll be helping the environment in the long run.  A well-designed nuke engine would be perfectly capable of lifting a single-person capsule using air as the first stage and methane as a second (same engine, just switch out propellant). 

Seriously, you can have perfectly serviceable SF without all these gravitic shenanigans. All you need to do is a bit of outside the box thinking, and a little math. Once you get down to numbers, a lot of things can be made to work.

 

I believe you... but if I want to lift a payload capacity on par with a 747 jetliner, fiction probably is required.

 

Maybe... just maybe nuclear could pull it off.

 

My thing is, I much rather go with make-believe where needed than make the public believe that nuclear works better than it does.

But I really do like what you said and agree for the most part.

 

I simply have a setting where there is already fictional stuff like FTL jumping, so that kinda allows for the gravity blocking in my mind anyway.

 

But realistically, yeah, I do think nuclear airbreathing is the path to true SSTO spaceships.

Really hefty ones (navy destroyer weight) will need a propellant boost until their airspeed is high enough for the nuclear heat to give the thrust needed to stay in flight.

It won't hover without propellant, but it can sure help slow it for landing.

Edited March 9, 2020 by Spacescifi

[Guest]

9 minutes ago, Spacescifi said:

My thing is, I much rather go with make-believe where needed than make the public believe that nuclear works better than it does.

You can have a nuclear SSTO the size of a 747. It's just a matter of making it big. Of course, it would be a lot bigger than a 747, but that's a given. Check out Skylon, and then imagine you can replace the engines with nuclear ones. The real Skylon concept could carry 11T, with airbreathing nuclear engines it would be higher. Now, for a 747-class payload (100T), you would need to build bigger, but it's far from impossible. 

Oh, and you can use nuclear engines even from standstill. Leaving aside the fact that anything flying wouldn't weigh as much as a floating ship (because the latter doesn't need to worry about weight that much), it's just a matter of having a big enough runway. In fact, the propellant mode on most nuclear rockets will have less thrust than air mode, because air is quite heavy. Nitrogen is 28 g/mol, methane is 16 g/mol.

[Spacescifi]

2 hours ago, Dragon01 said:

You can have a nuclear SSTO the size of a 747. It's just a matter of making it big. Of course, it would be a lot bigger than a 747, but that's a given. Check out Skylon, and then imagine you can replace the engines with nuclear ones. The real Skylon concept could carry 11T, with airbreathing nuclear engines it would be higher. Now, for a 747-class payload (100T), you would need to build bigger, but it's far from impossible. 

Oh, and you can use nuclear engines even from standstill. Leaving aside the fact that anything flying wouldn't weigh as much as a floating ship (because the latter doesn't need to worry about weight that much), it's just a matter of having a big enough runway. In fact, the propellant mode on most nuclear rockets will have less thrust than air mode, because air is quite heavy. Nitrogen is 28 g/mol, methane is 16 g/mol.

You just might convince me yet to not use gravity blocking. I could just say that nuclear is like the wheel... so why reinvent a way to reach space?

Oh sure, the materials science will be much better, but the basics of getting to space do not have to change too much... although if I want to lift something truly massive... once again project Orion is the 500 pound gorilla in the corner of the room that nobody wants. Although pure fusion whuch has less fallout will most certainly would be available, if they have good antimatter production, which they would for FTL jump drives.

So you are more or less right, I don't need antigravity, so long people pr civilizations are willing to pay the necessary price in resources and potential hazards.

Which is rich story material right there. You have to have either a strong reason, or sufficiently advanced protection that using nuclear is not considered as hazardous as it is for us.

Wow. Was not aware that propellant had less thrust than airbreathing, but you are right on the mass. Throwing bigger, harder, pushes you faster, higher. Only way to compensate with smaller stuff is by throwing it faster and harder, which has heat limits on the vessel.

I suspected one could lift with nuclear, I just was stuck with the idea that NTR was weak because of the original tests, even though they were not optmizing it for high thrust and air breathing changes the game.

 

For what it's worth... your idea is competitive with Elon's for payload. Just more of a true SSTO than Starship is since you won't need boosters to reach orbit.

Actually... what if we mixed a solid booster with an airbreathing nuclear reactor?! Maximum thrust! Call me crazy but I am a little with rocketry.

Just drop the the solid fuel tank with parachute when exhausted. Ought to give quite a boost I suspect, while still providing that classic SSTO scifi look without all those boosters attached looking all old school like the space shuttle

Starship will be the world's most powerful launch vehicle ever developed, with the ability to carry in excess of 100 metric tonnes to Earth orbit.

SpaceX › starship

Starship | SpaceX

 

 

Feedback

 

 

Edited March 10, 2020 by Spacescifi

[Guest]

I wouldn't mix SRBs with nukes, though that's a very Kerbal thing to do. Exhaust from solid rockets is very heavy and very dirty, so it would muck up the reactor, and that would be bad. Honestly, solid rockets aren't all that great. They're good for missiles, and they're cheap, but when you have reusables, they effectively become obsolete.

A air-augmented nuclear SSTO with payload in 100T range would likely be bigger than Starship, but perhaps not by that much. Estimating performance on those is somewhat tricky, because to get the most out of an airbreather you need a Skylon-like flight profile, and calculating the actual performance becomes very complex. Vertical liftoff with a nuclear airbreather is a possibility, or you can do horizontal takeoff like Skylon, which would let you use smaller engines.

Edited March 10, 2020 by Guest

[Spacescifi]

15 minutes ago, Dragon01 said:

I wouldn't mix SRBs with nukes, though that's a very Kerbal thing to do. Exhaust from solid rockets is very heavy and very dirty, so it would muck up the reactor, and that would be bad. Honestly, solid rockets aren't all that great. They're good for missiles, and they're cheap, but when you have reusables, they effectively become obsolete.

A air-augmented nuclear SSTO with payload in 100T range would likely be bigger than Starship, but perhaps not by that much. Estimating performance on those is somewhat tricky, because to get the most out of an airbreather you need a Skylon-like flight profile, and calculating the actual performance becomes very complex. Vertical liftoff with a nuclear airbreather is a possibility, or you can do horizontal takeoff like Skylon, which would let you use smaller engines.

 

Vertical lift off but the flight will definitely curve upward to get the most air intake before leaving the atmosphere entirely.

Would magnetohydrodynamics further enhance thrust the airbreathing nuclear here at all? Or would the added weight just mean less cargo space?

Here is a really, really, crazy idea...

 

 

This looks a lot like.... 

 

 

Why not just combine the two concepts?

 

Right away the problem I see is weight.

But if the ship had enough thrust out of the aerospike/pusher plate it could at least get clear of the ground before igniting the nukes/pure fusion bombs if they have AM.

The othet problem I see is I am not sure the rocket nozzled used to spray the pusher plate won't be damaged during nuke pusher plate blasts.

 

Yet there is a way to shield them. Plasma windws can block air, and I grant that that woukd require same major scifi shenanigans in the power ratios,probably magnetic fields too. But that's why they call it scifi.

To imagine what could be. Not what is.

[magnemoe]

13 hours ago, Dragon01 said:

You can have a nuclear SSTO the size of a 747. It's just a matter of making it big. Of course, it would be a lot bigger than a 747, but that's a given. Check out Skylon, and then imagine you can replace the engines with nuclear ones. The real Skylon concept could carry 11T, with airbreathing nuclear engines it would be higher. Now, for a 747-class payload (100T), you would need to build bigger, but it's far from impossible. 

Oh, and you can use nuclear engines even from standstill. Leaving aside the fact that anything flying wouldn't weigh as much as a floating ship (because the latter doesn't need to worry about weight that much), it's just a matter of having a big enough runway. In fact, the propellant mode on most nuclear rockets will have less thrust than air mode, because air is quite heavy. Nitrogen is 28 g/mol, methane is 16 g/mol.

Yes, main problem with an nuclear engine is that it will be radioactive after use. This is an problem for crew and more so for maintenance personnel. 
You might want some scifi radiation shielding material. 
Or an good fusion reactor might do the tricks. 

[Guest]

27 minutes ago, magnemoe said:

Yes, main problem with an nuclear engine is that it will be radioactive after use. 

Not really, at least if it's designed right. A nuclear reactor is radioactive while in use, which can cause problems, but nuclear reactors themselves don't produce much radiation when inactive. The biggest problem is neutron radiation, because neutrons can transmute things they hit, and make them radioactive. However, materials that have a very high neutron capture cross section, such as lithium-6 or boron-10, are lightweight and relatively cheap. If a reactor is shielded with a material like this, neutrons won't be a problem except for anything directly behind the nozzle (which is a bad place to be with any jet engine, TBH).

Gamma radiation is a concern, especially for maintenance personnel (a shadow shield can protect the crew and passengers completely), but that's pretty much only a problem while the reactor is in use. For a vertical launch SSTO, it could be as simple as making landing pads that, after landing or launch, would be lowered into the ground, to minimize the spread of radiation while the reactor is spooling up and down, and also make access to the hull easy. For a horizontal launch SSTO, engines could be started on the runway, and taxiing done with a tug or a built-in electric motor). Unless this startup would go very fast (which may be possible with some reactor designs), this wouldn't quite permit airliner levels of traffic, but it should work well enough when there's enough time.

[sevenperforce]

Building an SSTO is straightforward enough, and nukes (esp. airbreathing nukes) makes it almost trivial, once you have the powerplant. The better your propulsion package is (smaller, thrustier, higher-efficiency), the more margin you have for other parts of your spaceship. But the problem is not getting into space; it's getting out of space. EDL -- entry, descent, and landing. Bucknell's airbreathing SSTO is lovely but it has no good way of getting back down to the ground. 

[Spacescifi]

5 hours ago, sevenperforce said:

Building an SSTO is straightforward enough, and nukes (esp. airbreathing nukes) makes it almost trivial, once you have the powerplant. The better your propulsion package is (smaller, thrustier, higher-efficiency), the more margin you have for other parts of your spaceship. But the problem is not getting into space; it's getting out of space. EDL -- entry, descent, and landing. Bucknell's airbreathing SSTO is lovely but it has no good way of getting back down to the ground. 

 

With some modifications I don't see why not.

Use several reactors for different modes.

High thrust low ISP: Use a solid reactor or reactors (however many it takes) encased in quartz, let UV rays heat the solid propellant for max thrust. Useful for boosting. Drop used tanks.

 

Reentry: Plain nuclear airbreathing should work great, and if not good enough, augment it with chemical nuclear propelled propellant at the same time.

If reactors can be made small enough we can use several. At once for different engine modes.

 

The possibilities are several.

[sevenperforce]

On 3/10/2020 at 3:44 PM, Spacescifi said:

On 3/10/2020 at 10:40 AM, sevenperforce said:

Building an SSTO is straightforward enough, and nukes (esp. airbreathing nukes) make it almost trivial, once you have the powerplant. The better your propulsion package is (smaller, thrustier, higher-efficiency), the more margin you have for other parts of your spaceship. But the problem is not getting into space; it's getting out of space. EDL -- entry, descent, and landing. Bucknell's airbreathing SSTO is lovely but it has no good way of getting back down to the ground. 

With some modifications I don't see why not.

Use several reactors for different modes.

High thrust low ISP: Use a solid reactor or reactors (however many it takes) encased in quartz, let UV rays heat the solid propellant for max thrust. Useful for boosting. Drop used tanks.

Reentry: Plain nuclear airbreathing should work great, and if not good enough, augment it with chemical nuclear propelled propellant at the same time.

If reactors can be made small enough we can use several. At once for different engine modes.

The possibilities are several.

Wait, what?

In what universe can you have a nuclear thermal reactor with solid propellant? The whole basic, underlying concept of a NTR is that the liquid propellant flows around the reactor, cooling it while absorbing as much thermal energy as possible. NTRs (and, for that matter, antimatter reactors) are not magic; it's a matter of how fast the propellant leaves the engine bell. You can get better specific impulse by pushing hydrogen through a solid NTR than by pushing methane through an antimatter or fusion reactor. I don't know what you think is so amazing about antimatter. It has hella specific energy, but specific energy is not the same as specific impulse.

You cannot "drop used tanks" if your propellant is solid fuel. It makes no sense.

"Nuclear airbreathing" does not help you on re-entry. Re-entry is a function of cross-section, control surfaces, and thermal properties. 

[Spacescifi]

9 hours ago, sevenperforce said:

You can get better specific impulse by pushing hydrogen through a solid NTR than by pushing methane through an antimatter or fusion reactor.

 

I doubt that, since specific impulse in simple terms is a measure of how efficient a rocket is at converting it's propellant into forward thrust. Shooting out several tons of propellant per second is less efficient than a rocket that could shoot out a ton of propellant per second for the same amount of thrust.

The difference is energy. The more energy one has at their disposal to heat up the propellant, the more efficient the rocket becomes... so long the rocket won't melt.

Fusion and antimatter both could easily far yield far greater yields than fission, which means it is possible to use less propellant going where one wants to go.

Realistically though, barring some science discovery breakthrough, the equipnent needed for both fusion and antimatter to work would weigh down a vessel too much, leaving project Orion the easiest way to lift it to space, which again makes both kind of unnecessary.

Regarding reentry,  remember that I am a writer. Main characters hsve to survive reentry so the ship surviving is a given.

Nuclear airbreathing rocketry is useful for landing.

And if that alone is not enough, add chemical fuel into the airbreathing nuclear mix for extra thrust.

Either way saves propellant for landing on Earth worlds.

 

EDIT: Are not solid propellants powders? 

If so then they could go in a tank and once exhausted the tank could be dropped.

That is merely an engineering challenge, not impossible

I based my idea on the nuclear lightbulb, so provided one could keep a gas core nuclear reaction ongoing inside a quartz chamber (hard if not currently impossible), that could heat the solid powder propellant which surrounds the quartz container.

There are ways to move powders from storage tanks to a nuclear chamber too.

Mechanically. Air blowing. Other ways too.

Drop tanks after if no longer needed.

 

 

 

Still... in scifi, far less plausible is routinely accepted.

I tend to think we may not have an answer for how yet, there IS an answer for how. And someone WILL find it.

Someday.

Almost nothing is impossible. What is possible can be done a variety of ways, not an infinite number of ways.

 

Edited March 12, 2020 by Spacescifi
Science

[kerbiloid]

2 hours ago, Spacescifi said:

Regarding reentry,  remember that I am a writer. Main characters hsve to survive reentry so the ship surviving is a given.

Then the nuclear airbreathing is excessive.

Spoiler

... Senior Subleutenant J'Hoongr opened shis^*) eyes and looked around. The huge transgalactic starship was almost crashed but happily the captain's bridge survived.
"What happened?"
Subleutenant gave both three hands to the 2nd Thermospiritual Psychoengineer who was trying to get out of the heap of electronic junk which was a control panel just several minutes before.
"Antimatter. Again that broken antimatter power source in the office cooler. I told not once that at last it will blast."
"Do you think we can repair the ship?"
"Of course. But it will take a year or two. And some farmer's garage with tools and farm machinery."

_{*) The character is still undecided, is it he or she in this decade. It's an extraterrestrial species, and it's in trend. So +5 to relevance on publication, +10% to sells.}

 

[Spacescifi]

3 hours ago, kerbiloid said:

Then the nuclear airbreathing is excessive.

  Hide contents

... Senior Subleutenant J'Hoongr opened shis^*) eyes and looked around. The huge transgalactic starship was almost crashed but happily the captain's bridge survived.
"What happened?"
Subleutenant gave both three hands to the 2nd Thermospiritual Psychoengineer who was trying to get out of the heap of electronic junk which was a control panel just several minutes before.
"Antimatter. Again that broken antimatter power source in the office cooler. I told not once that at last it will blast."
"Do you think we can repair the ship?"
"Of course. But it will take a year or two. And some farmer's garage with tools and farm machinery."

_{*) The character is still undecided, is it he or she in this decade. It's an extraterrestrial species, and it's in trend. So +5 to relevance on publication, +10% to sells.}

 

 

SF aliens are not a concept I like because it's a trend.

I like them because it allows a bit of constrained creativity.

I don't view scifi aliens as reality at all.  I would go so far as to admit it in the foreword and tell the readers to please not take that concept as prophetic. Only some of the technology if similar enough IRL. Like nuclear technology for example. Alien technology will be like human but more advanced based on size and energy.

In other words.... they have mini nuclear reactors the size of a softball.

Plausible?  Have no clue... so i leave that to the future to figure out how.

 

Nonetheless, given that nuclear power is more prolific with the aliens nuclear powered personal jetpacks would also be a reality.

Is making books about scifi aliens essentially promoting fantasy to me?

Absolutely. But I will do my best to at least make it entertaining to read to the average reader.

Edited March 12, 2020 by Spacescifi

[sevenperforce]

11 hours ago, Spacescifi said:

14 hours ago, sevenperforce said:

You can get better specific impulse by pushing hydrogen through a solid NTR than by pushing methane through an antimatter or fusion reactor.

I doubt that, since specific impulse in simple terms is a measure of how efficient a rocket is at converting it's propellant into forward thrust. Shooting out several tons of propellant per second is less efficient than a rocket that could shoot out a ton of propellant per second for the same amount of thrust.

That is my point. An antimatter or fusion reactor rocket using methane as its propellant will use more for the same amount of thrust than a solid NTR using hydrogen.

If you're pouring antimatter directly into your propellant flow as it enters the combustion chamber (or annihilation chamber, in this case), then you can gin up the chamber temperature higher, but you'd have to really get the temperature ridiculously high before the propellant performance of methane would exceed the propellant performance of hydrogen. You would be forced to use methane to regeneratively cool the chamber, of course, but at some point the heat capacity of the methane simply isn't enough.

11 hours ago, Spacescifi said:

The difference is energy. The more energy one has at their disposal to heat up the propellant, the more efficient the rocket becomes... so long the rocket won't melt.

Specific energy is the measure of how much energy your energy source has per unit of energy source mass; specific impulse is the measure of how much thrust your rocket can produce using each unit of propellant mass. They are linked when you're talking about chemical propellants, but not directly linked when you move away from chemical propellants.

11 hours ago, Spacescifi said:

Fusion and antimatter both could easily far yield far greater yields than fission, which means it is possible to use less propellant going where one wants to go.

Well, it means that the dry mass of the energy source is lower. Which technically means it is possible too use less propellant, but not for the reason you're thinking.

11 hours ago, Spacescifi said:

Regarding reentry,  remember that I am a writer. Main characters hsve to survive reentry so the ship surviving is a given.

Nuclear airbreathing rocketry is useful for landing.

And if that alone is not enough, add chemical fuel into the airbreathing nuclear mix for extra thrust.

Either way saves propellant for landing on Earth worlds.

Entry and descent do not require engines. Landing is about TWR, not specific impulse, since you by definition need only a tiny amount of dV. The best way to land is chemical airbreathing without nukes at all; save your nuclear cycle for where specific impulse is needed.

[kerbiloid]

At what temperature do the methane molecules turn into hydrogen polluted by soot?

[sevenperforce]

16 minutes ago, kerbiloid said:

At what temperature do the methane molecules turn into hydrogen polluted by soot?

Methane disassociates at 1000-1200 C, so well below rocket engine operating temperatures. This helps the specific impulse a little. 

[kerbiloid]

9 minutes ago, sevenperforce said:

Methane disassociates at 1000-1200 C, so well below rocket engine operating temperatures. This helps the specific impulse a little. 

So, a methanuke with working temperature 2+ kK turns into a sooty hydrogen nuke. An antimatter one especially.

[sevenperforce]

1 hour ago, kerbiloid said:

So, a methanuke with working temperature 2+ kK turns into a sooty hydrogen nuke. An antimatter one especially.

It comes down to atomic mass units. Diatomic hydrogen is 2.01 amu; a methane molecule is 16.04 amu; diatomic carbon is 24.02 amu. Diatomic hydrogen is constant at 2.01 g/mol. When a pair of methane molecules disassociate, they release four molecules of diatomic hydrogen and one molecule of diatomic carbon, going from 16.04 g/mol to 6.41 g/mol. So hydrogen still has 3.2 less atomic mass. Molecule velocity scales inversely with the square root of atomic mass so pure hydrogen will be pushing about 79% more Isp. 

Exhaust velocity squares directly with the square root of temperature so your methanuke would need to have a combustion temperature 3.2x higher than a comparable hydrogen nuke, just to break even.

You'd need to get over 10 kK to get diatomic hydrogen disassociation.

[Spacescifi]

2 hours ago, sevenperforce said:

It comes down to atomic mass units. Diatomic hydrogen is 2.01 amu; a methane molecule is 16.04 amu; diatomic carbon is 24.02 amu. Diatomic hydrogen is constant at 2.01 g/mol. When a pair of methane molecules disassociate, they release four molecules of diatomic hydrogen and one molecule of diatomic carbon, going from 16.04 g/mol to 6.41 g/mol. So hydrogen still has 3.2 less atomic mass. Molecule velocity scales inversely with the square root of atomic mass so pure hydrogen will be pushing about 79% more Isp. 

Exhaust velocity squares directly with the square root of temperature so your methanuke would need to have a combustion temperature 3.2x higher than a comparable hydrogen nuke, just to break even.

You'd need to get over 10 kK to get diatomic hydrogen disassociation.

 

The calculations are off... at least as far as I am concerned for the OP.

It is not a methnuclear rocket.

 

It is a methnuclear AIR BREATHING rocket.

So on liftoff and landing on any Earth clone world as seen in scifi the engine is taking air AND methane both, flowing it all through the reactor at once for max thrust.

 

Once the air thins out then we can talk about methnuclear.

 

Until then it has plenty of help with thrust and cooling.

Plug in your numbers now if you want and they will actually be correct.