"Near" Future/"hard-sci fi", low infrastructure, SSTOs

[KerikBalm]

18 hours ago, RCgothic said:

I don't think sci-fi shuttles generally are SSTOs in the conventional sense. When you have enough power to maintain 1g indefinitely you don't have to maintain orbital velocities. Re-entries also don't have to be heat shielded because you don't come in that hot. 

This also explains why sci-fi ships tend to fall out of orbit when they're damaged/lose power. It's because they were actively maintaining their altitude rather than falling around the planet and missing.

If that were the case, the dV requirements would be even worse...

 

18 hours ago, SinBad said:

why not stage it... in reverse.

assuming a mothership, how about a delivery/retrieval vehicle serviced by whatever fueling infrastructure the mother ship depends on? so the d/r vehicle grabs the lander, heads to to low orbit. then it does the re-entry burn, drops the lander and boosts back up to low orbit. the lander goes into a semi-ballistic glide until its slow enough for a nuclear thermal turbo jet (no fuel required, just air) to work, then it can hug the ground to its destination for a landing. once there, it starts compressing/liquefying air (worst case scenario, the hot exhaust from the nuke jet can be routed into a compressed air powered reciprocating engine coupled to an air compressor). lift off on the nuke jets again, then up into a a sub orbital flight with an apoapsis at low orbit (compressed/liquid air sent through the nukes at high altitude to maintain thrust.) timed to meet DR vehicle. DR vehicle does a retrograde burn, catches the lander around apoapsis, then boosts them both back to the mothership.

no fuel is used in atmosphere, allowing long in air loiter times and very high cross range capability. that would help a lot when trying to match a suborbital hop with a low orbit rendezvous. because the only time the lander needs vacuum propellent is on its way up, and the excess compressed/liquid air can be discarded before docking, the mass the DR vehicle needs to brake and boost is very predictable and aside from payload (which should be known by the time it hits space) is always the same.

this removes the need for efficient engines in both vacuum and atmosphere on the same craft by splitting it into two vehicles, each in its element so to speak.

This would place massive propellant requirements on the mothership, and would limit them to very few trips before the mothership has to pack up and leave.

11 hours ago, sevenperforce said:

Honestly the most far-fetched part of this is the whole "they still speak English after thousands of years" thing.

Ok, we'll go with just plain old Earth then. Something like a comic book where a single genius like Tony Stark can invent toys that put the combined works of the rest of Earth's scientists and engineers to shame... or maybe its the result of the creation of the first AI.

Or maybe the new arrivals just learn the native's language really fast so they can do business with commoners on the surface.

1 hour ago, Hypercosmic said:

Are you going to radiate the surface of the planet with radioactive stuffs? 

No, closed cycle NTR designs don't expel radioctive particles. EM and neutron radiation, sure - but when the ship is gone, the radiation is gone.

Of course if the reactor core is damaged and starts shedding pieces.... or if it sits in one place for a long time, that neutron radiation is going to cause some weak induced radioactivity of the surroundings... but these are relatively minor effects.

This is not nearly as bad (by orders of magntiude) as the nuclear weapons testing that was performed on Earth.

[Wjolcz]

I don't know about all this atomic stuff. I still think a conventional approach with a base/spaceport on the surface would be better. Politically problematic when it comes to relations of the two factions but at least the planet inhabitants wouldn't have to deal with fallout and radiation if something goes wrong with the SSTO.

What about an SSTO with SABRE-like engines? Or even a whole fleet cycling there and back all the time? I guess this approach makes landing anywhere on the planet out of the question but then I'd imagine these spacecraft would be used to transpprt important people and secret equipment instead of troops. Besides, drones can do the job of troops when it comes to warfare.

[sevenperforce]

8 hours ago, Hypercosmic said:

I kinda want to ask about how did the radiation get out of the NTR, but let's just get-

Hydrolox is the WORST? Isn't water one of the more common substances in the universe? YOU'VE GOT TO BE KIDDING ME.

As @KerikBalm notes, a closed-cycle NTR doesn't leak significant radiation. The propellant flow is irradiated enough that some traces of radioactivity escape -- somewhere in the "one additional case of cancer worldwide for every 1000 hours of runtime at sea level" area -- which is enough to make it a legal/political no-no, but it's really not that bad.

Hydrolox is great if you have a permanent station, plenty of time, and a source of constant power. But for an SSTO with onboard ISRU that needs to be able to land and relaunch in a reasonably short period of time? Definitely bad. Hydrolox is a poor choice for SSTO to begin with, given its poor impulse density and heavy tankage requirements, but adding the onboard ISRU requirement makes the whole affair even worse. Finally, splitting water into hydrogen and oxygen is slow, and condensing them into liquids is even slower and requires ridiculously large refrigeration capabilities.

A pebble-bed water NTR (which has the advantage of being able to readily dump the nuclear fuel and recharge without removing the entire engine) has a specific impulse of 485.2 seconds and a T/W ratio of 30-40.

7 hours ago, KerikBalm said:

Quote

Honestly the most far-fetched part of this is the whole "they still speak English after thousands of years" thing.

Ok, we'll go with just plain old Earth then. Something like a comic book where a single genius like Tony Stark can invent toys that put the combined works of the rest of Earth's scientists and engineers to shame... or maybe its the result of the creation of the first AI.

Or maybe the new arrivals just learn the native's language really fast so they can do business with commoners on the surface.

Ah, yeah, that's not a problem. If the inhabitants and the newcomers speak two different derivatives of the same base language, translation between the two will be a cinch in a few weeks...months at most. I thought you wanted them to each speak the same exact language without any differences or translation difficulty.

[sevenperforce]

4 hours ago, Veeltch said:

I don't know about all this atomic stuff. I still think a conventional approach with a base/spaceport on the surface would be better. Politically problematic when it comes to relations of the two factions but at least the planet inhabitants wouldn't have to deal with fallout and radiation if something goes wrong with the SSTO.

Yeah, I suggested that the mothership do one-way drops of self-contained ISRU units around the planet to enable refueling, but the OP insisted this wouldn't be possible due to aggression from the planet inhabitants.

Which, in a sense, is a self-solving problem. If the natives have a nasty habit of attacking and wrecking anything you own, you won't worry so much about the risk of irradiating them. And fallout wouldn't actually be a concern, anyway; fallout only happens with actual nuclear detonations. The worst that would happen with a nuclear SSTO RUD would basically be a large radiological spill. The impact site would be an exclusion zone but it wouldn't spread. An in-air breakup would spread radioactive debris over a larger area, but the contamination would be less severe.

Fallout is the result of large amounts of non-radioactive material from the environment (soil, rock, other debris) being pulled up into the radioactive fireball and turned into lethal dust. The rising cloud then spreads the fallout over a wide area. In this way, a nuclear ground blast can produce many hundreds of times more radioactive material than the bomb itself ever contained.

4 hours ago, Veeltch said:

What about an SSTO with SABRE-like engines? Or even a whole fleet cycling there and back all the time? I guess this approach makes landing anywhere on the planet out of the question but then I'd imagine these spacecraft would be used to transpprt important people and secret equipment instead of troops. Besides, drones can do the job of troops when it comes to warfare.

Nuclear SABRE is basically what I'm suggesting, minus the aggressive precooler. SABRE has to precool air dramatically in order to efficiently compress, inject, and combust it. In contrast, a nuclear thermal rocket doesn't have any chemical combustion happening at all, so it can theoretically operate in airbreathing mode at very high speeds without needing to precool the airstream.

One thing I like about the OP's scenario is that the entire military campaign is completely improvised. The colony ship was not expecting to meet resistance, so it didn't bring drones or heavily armored vehicles or orbital bombardment munitions. The SSTO dropships would have been built back on Earth with no intended purpose other than acting as orbital shuttles, so military use has to be worked in on the fly.

[DDE]

On ‎05‎.‎04‎.‎2017 at 7:40 AM, Hypercosmic said:

Nobody in our forums use them for their ships. They are pathetically weak (both likely less than 3. Water tested, CO2 not yet).

Actually, the four things we use are hydrogen deuteride, methane, decane and RP-1. Hydrogen deuteride performs slightly better than hydrogen.

Off-topic question: what's your opinion on ammonia? High boiling point, not as stupidly scarce as RP-1, decomposes unlike water.

Edited April 6, 2017 by DDE

[sevenperforce]

21 minutes ago, DDE said:

Off-topic question: what's your opinion on ammonia?

Anhydrous ammonia is probably the ideal NTR propellant. Cheap and readily available in massive quantities, disassociates readily, and has a specific impulse nearly as good as liquid methane without any coking risk. It can be burned with any oxidizer for RCS, it can be used as a very efficient coolant despite requiring only mild refrigeration, and it can be mixed with water or liquid nitrogen for more thrust or with liquid hydrogen for more specific impulse.

EDIT: It also plays very well with a LANTR design, especially if you plan on having LOX on hand to burn with the ammonia for, well, any other purposes.

Edited April 6, 2017 by sevenperforce

[Hypercosmic]

May I post a link to this thread on the Children of a Dead Earth forums? I'm curious what the others will say.

Note 1: The only computer that can play CDE is broken, so i'm pretty blind here. Guess that water NTR thingy was mistaken by me, but nobody really use them. RP-1 can be synthesized, though.

Edited April 6, 2017 by Hypercosmic

[Hypercosmic]

The first opinion incoming...

"A Fluorine - RP-1 chemical engine seems better. For SSTO, you want to pack as much delta-v in as little volume as possible."

"If so, Decane seems the way to go. It got an exit velocity of roughly 5.4km/s but it's amazingly dense."

Edited April 6, 2017 by Hypercosmic

[sevenperforce]

10 minutes ago, Hypercosmic said:

The first opinion incoming...

"A Fluorine - RP-1 chemical engine seems better. For SSTO, you want to pack as much delta-v in as little volume as possible."

There really aren't any fluorine-RP1 engines out there; fluorine is just too difficult to handle. FLOX, or a 70:30 mixture of fluorine and liquid oxygen, can get a SL specific impulse of 316 seconds and a net propellant density of 1.2 g/cc. I'm estimating vacuum specific impulse around 380 seconds, for an impulse density of 4.5 million kg/s*m².

In contrast, a pebble-bed water NTR has an impulse density about 6% higher even though its T/W ratio won't be quite as good. Of course, if you need ISRU, then simply pumping water up into your tanks is a great deal easier than trying to find a source of fluorine and then synthesizing kerosene.

[Hypercosmic]

18 minutes ago, sevenperforce said:

There really aren't any fluorine-RP1 engines out there; fluorine is just too difficult to handle. FLOX, or a 70:30 mixture of fluorine and liquid oxygen, can get a SL specific impulse of 316 seconds and a net propellant density of 1.2 g/cc. I'm estimating vacuum specific impulse around 380 seconds, for an impulse density of 4.5 million kg/s*m².

In contrast, a pebble-bed water NTR has an impulse density about 6% higher even though its T/W ratio won't be quite as good. Of course, if you need ISRU, then simply pumping water up into your tanks is a great deal easier than trying to find a source of fluorine and then synthesizing kerosene.

My apologies, I didn't clarify those stuffs. I'll add those limitations into the CDE forums. Thanks.

"Water seems to have bad exit velocity, about 3.8km/s." I assume this is enough, right? (the original comment is much more entertaining, if you ask me)

"Ammonia seems to have an exit velocity of about 4.3km/s." - Note, the exhaust velocity is highly dependent on nozzle configurations, which is what I am asking the tester.

Anyways, what are the remasses (reaction masses) in your choices? I'll ask my friends to test the delta-v. I'll look into thrust later.

Edited April 6, 2017 by Hypercosmic

[sevenperforce]

18 minutes ago, Hypercosmic said:

My apologies, I didn't clarify those stuffs. I'll add those limitations into the CDE forums. Thanks.

"Water seems to have bad exit velocity, about 3.8km/s." I assume this is enough, right? (the original comment is much more entertaining, if you ask me)

Anyways, what are the remasses (reaction masses) in your choices? I'll ask my friends to test the delta-v.

Yes, the original comment is entertaining.

3.8 km/s is still a specific impulse of 387 seconds, which is a much better impulse density than any chemical propellant combination. Propane+HTP might be in the ballpark, but I'm not sure.

Remass is simply water, with disassociation commensurate to a pebble-bed reactor operating at 3200 K or a bit higher. Project rho gives the exhaust velocity for a solid-core LH2 NTR as 8,093 m/s and 4,042 m/s for a solid-core water NTR. Exhaust velocities for a pebble-bed NTR are about 18% higher. 

[Hypercosmic]

1 minute ago, sevenperforce said:

Yes, the original comment is entertaining.

3.8 km/s is still a specific impulse of 387 seconds, which is a much better impulse density than any chemical propellant combination. Propane+HTP might be in the ballpark, but I'm not sure.

Remass is simply water, with disassociation commensurate to a pebble-bed reactor operating at 3200 K or a bit higher. Project rho gives the exhaust velocity for a solid-core LH2 NTR as 8,093 m/s and 4,042 m/s for a solid-core water NTR. Exhaust velocities for a pebble-bed NTR are about 18% higher. 

Nice! I need to ask the dev to add this pebble-bed NTR into the game.

However, for us CDE players, whose spacecraft nuclear reactors run at 1 K lower than the meltdown point and have no radiation shieldings, our hydrogen NTRs have exhaust velocity of about 9,140 m/s, and hydrogen deuteride NTRs can have as high as 9,200 m/s, when the nozzle expansion ratio and angle were 200 and 10 deg (my standard), respectively.

The reactor fuel is U-233 dioxide.

[sevenperforce]

5 minutes ago, Hypercosmic said:

However, for us CDE players, whose spacecraft nuclear reactors run at 1 K lower than the meltdown point and have no radiation shieldings, our hydrogen NTRs have exhaust velocity of about 9,140 m/s, and hydrogen deuteride NTRs can have as high as 9,200 m/s, when the nozzle expansion ratio and angle were 200 and 10 deg (my standard), respectively.

Worrying with nozzle expansion angles is a bit premature; we can assume that this will be optimized for the given application.

On a prior note...if you burn pure hydrazine and high-test peroxide (which, for the record, is a ridiculously horrible idea second only to working with stuff like fluorine, chlorine, and pentaborane), you get an impulse density of around 4.1 million kg/s*m², which is starting to approach the impulse density of a water NTR. Of course, if you're willing to play games with hydrazine, you can just do a hydrazine NTR, which will blow the water out of pretty much every other combination...no pun intended.

[Hypercosmic]

UPDATE!

"fyi, with some more fillding, I got water's exit v to 4.15 km/s."

Some pics

Spoiler

"P.S. even tho Ammonia has a better exit v, when I was messing with to optimize it, it was giving me problems, and its thrust-mass ratio is horrific. water is better in that regard."

Edited April 6, 2017 by Hypercosmic
Pictures are fixed. Hope you can view them!

[sevenperforce]

8 minutes ago, Hypercosmic said:

UPDATE!

"fyi, with some more fillding, I got water's exit v to 4.15 km/s."

Some pics....

(snip)

That's more like it! A little disassociation never hurt anyone. That exhaust velocity corresponds to an impulse density of about 4.2 million kg/s*m^2.

FYI, pics won't come through for non-forum members.

Edited April 6, 2017 by sevenperforce

[DDE]

1 hour ago, sevenperforce said:

There really aren't any fluorine-RP1 engines out there; fluorine is just too difficult to handle. FLOX, or a 70:30 mixture of fluorine and liquid oxygen, can get a SL specific impulse of 316 seconds and a net propellant density of 1.2 g/cc. I'm estimating vacuum specific impulse around 380 seconds, for an impulse density of 4.5 million kg/s*m².

That's because carbon fuels are best burned with an oxygen carrier; FLOX has a wonderful tendency to produce FOOF, which makes your rockets go BOOM, hence fluorine monoxide may be a superior alternative. Its a cryogen... so you might want to roll out some mix based around ClF5 instead if you want to avoid cooling issues.

I also happen to have a manual on ClF3 handling, and some papers on FLOX spills. Then there's the RD-301, a flight-rated fluorine-ammonia engine. Just because you're afraid of fluorine, doesn't mean more desperate people would be.

Quote

And here are my guesses as to which liquid propellants are going to be used during the next few years, and possibly for the rest of the century, although here I'm sticking my neck out a long way. For short-range tactical missiles, with a range up to 500 km or so, it will be RFNA-UDMH, gradually shifting over to something like ClF5 and a hydrazine-type fuel. Monopropellants are unlikely to be used for main propulsion, and the problems with gels and slurries are so great that it is unlikely that the benefits to be derived from them can outweigh the difficulty of developing them to operational status.

For the upper stages, the hydrogen-oxygen combination of the J-2 is very satisfactory, and will probably be used for a long time. Later, as more energy is needed, there may be a shift, for the final stage, to hydrogen-fluorine or hydrogen-lithium-fluorine. The nuclear rocket will take over there.
For lunar landers, service modules, and the like, N2O4 and a hydrazine fuel seem likely to remain useful for a long time. I can't think of any combination likely to displace them in the foreseeable future.
Deep space probes, working at low temperatures, will probably use methane, ethane, and diborane for fuels, although propane is a possibility. The oxidizers will be OF2, and possibly ONF3 and NO2F, while perchloryl fluoride, ClO3F, would be useful as far out as Jupiter.

 

[monstah]

Pebble-bed spits out radioactive death exhaust, right? Nyrath doesn't spell it out on Atomic Rockets, but it seems to me so.

[sevenperforce]

17 minutes ago, monstah said:

Pebble-bed spits out radioactive death exhaust, right? Nyrath doesn't spell it out on Atomic Rockets, but it seems to me so.

Not necessarily. The pebbles are tiny spheres of fissile material wrapped in porous carbon wrapped in hardened carbon wrapped in Zirconium carbide ceramic. Even at 3200K+, they should maintain their integrity.

Of course, depending on your propellant, the intense neutron radiation may or may not induce radioactivity in the exhaust stream. There will be leakage, but it's not "radioactive death exhaust". You're probably thinking of an open-cycle vapor-core NTR, which is a particularly nasty beast, though not as nasty as its dragonish cousin, the Nuclear Saltwater Rocket.

[KerikBalm]

23 minutes ago, sevenperforce said:

Of course, depending on your propellant, the intense neutron radiation may or may not induce radioactivity in the exhaust stream. There will be leakage, but it's not "radioactive death exhaust". You're probably thinking of an open-cycle vapor-core NTR, which is a particularly nasty beast, though not as nasty as its dragonish cousin, the Nuclear Saltwater Rocket.

Well, assuming we're going with NH3 or OH2: Most hydrogen if transmuted, will simply transmute to deuterium... which is stable and non-radioactive. Tritium production should be rare in the exhaust - but much higher in H2O/NH3 that circulates in a loop to cool the reactor when not thrusting (and to generate power if its a bimodal design)

Even then, Tritium's half-life is only 12 years, so we're not really going to worry about induced radioactivity or spoiling the world for generations to come with hydrogen in our propellant.

Oxygen: Most common isotope is 16. Oxygen 17 (1 neutron capture) is stable and non-radioactive. Oxygen 18 (a 2nd neutron capture) is stable and non-radioactive. Oxygen 19 is radioactive and decays by beta particle emission to become stable Flourine-19. The halflife of Oxygen 19 is 26 days. Even if an oxygen atom somehow managed to have 3 neutron capture events as it passes through the reactor elements, that's not going to generate much long lasting radioactivity. This is less of a problem than the tritium production

Nitrogen: 14 is by far the most common. If it absorbs a Neutron, Nitrogen 15 is also stable. If it absorbs a 2nd neutron, it becomes the unstable Nitrogen 16 isotope. Nitrogen 16 has a halflife of 7 seconds. It decays into stable, non-radioactive isotopes. Even in recirculating NH3, radioactive nitrogen isn't going to be a problem. If a nitrogen 16 atom manages to absorb a 3rd neutron before decaying, it becomes nitrogen 17, which has a half life of 4 seconds. It decays into stable elements as well. Induced radioactivity is not going to be a problem with the nitrogen.

So, Tritium is pretty much the worst thing that may be produced, and that is going to be very very low levels of production. There will be a small amount of naturally occurring Deuterium in the water that passes through the reactor, so for a small percent of your hydrogen, a single neutron capture event will make H-3. Recirculating water will cause deuterium to build up, which will then cause tritium to build up- but you can keep your coolant water separate from propellant water.

Even so:

https://en.wikipedia.org/wiki/Tritium#Health_risks

The beta particle from Tritium is very low energy and won't penetrate your skin. As with many low energy/penetration radioactive substances, its only dangerous if you ingest it. Unlike heavy metals, hydrogen has a pretty short biological halflife (as you constantly drink and urinate), and it won't stay in your body very long, or stay in one location in your body. 

Induced radioactivity isn't going to be a problem. If you sat your reactor in one location/recirculate stuff so that there are chances for repeated neutron captures, then it becomes a problem. A SSTO expelling propellant is not going to be a problem.

 

4 hours ago, sevenperforce said:

Which, in a sense, is a self-solving problem. If the natives have a nasty habit of attacking and wrecking anything you own, you won't worry so much about the risk of irradiating them. And fallout wouldn't actually be a concern, anyway; fallout only happens with actual nuclear detonations. The worst that would happen with a nuclear SSTO RUD would basically be a large radiological spill. The impact site would be an exclusion zone but it wouldn't spread. An in-air breakup would spread radioactive debris over a larger area, but the contamination would be less severe.

Well, that depends on the scenario. Perhaps the world has fallen under the control of a future-space Hitler. The new arrivals/super advanced rebels don't seek to exterminate the populace, but rather to topple the government.

Or... even if you do want to exterminate the populace... presumably you're doing it so that you can colonize the planet they currently occupy. You don't want to be expelling radioactive saltwater high into the atmosphere on a weekly basis. There would be fallout from an open-cycle design where the fision products are expelled with the propellant. Just as in bombs, that is putting fission products way up high in the atmosphere.

For closed cycle designs, its much less of a threat unless the surfacedwellers manage to take out your SSTO when its at a high altitude (unlikely, that's going to be as hard or harder than intercepting an ICBM)

[sevenperforce]

25 minutes ago, KerikBalm said:

Well, assuming we're going with NH3 or OH2: Most hydrogen if transmuted, will simply transmute to deuterium... which is stable and non-radioactive. Tritium production should be rare in the exhaust - but much higher in H2O/NH3 that circulates in a loop to cool the reactor when not thrusting (and to generate power if its a bimodal design)

Induced radioactivity isn't going to be a problem. If you sat your reactor in one location/recirculate stuff so that there are chances for repeated neutron captures, then it becomes a problem. A SSTO expelling propellant is not going to be a problem.

In a working NTR, there is always going to be a non-negligible chance that slight materials imperfections cause tiny leaks of fissile salts into the propellant stream, plus the aforementioned tritium production by double hydrogen neutron capture. In the real world, that's enough to keep NTR launches thoroughly shelved. But for a lander intended for temporary use on a distant world, it's not going to be a problem for anybody.

25 minutes ago, KerikBalm said:

Perhaps the world has fallen under the control of a future-space Hitler. The new arrivals/super advanced rebels don't seek to exterminate the populace, but rather to topple the government.

Or... even if you do want to exterminate the populace... presumably you're doing it so that you can colonize the planet they currently occupy. You don't want to be expelling radioactive saltwater high into the atmosphere on a weekly basis. There would be fallout from an open-cycle design where the fision products are expelled with the propellant. Just as in bombs, that is putting fission products way up high in the atmosphere.

For closed cycle designs, its much less of a threat unless the surfacedwellers manage to take out your SSTO when its at a high altitude (unlikely, that's going to be as hard or harder than intercepting an ICBM)

This is definitely a reason to avoid a nuclear saltwater rocket -- which, for the record, is not necessarily near-future.

Even if the surfacedwellers do manage to intercept one of your SSTOs with a (comparatively) primitive missile, there still won't be fallout in the strict sense. Just radiological contamination. And the higher the SSTO is when it breaks up, the less lethal the contamination will be, even though it will be dispersed over a wider area.

Edited April 6, 2017 by sevenperforce

[monstah]

1 hour ago, sevenperforce said:

though not as nasty as its dragonish cousin, the Nuclear Saltwater Rocket.

I was thinking exactly this, thanks for clarifying.

[KerikBalm]

22 minutes ago, sevenperforce said:

In a working NTR, there is always going to be a non-negligible chance that slight materials imperfections cause tiny leaks of fissile salts into the propellant stream, plus the aforementioned tritium production by double hydrogen neutron capture. In the real world, that's enough to keep NTR launches thoroughly shelved. But for a lander intended for temporary use on a distant world, it's not going to be a problem for anybody.

well, I think as the tech matures, there's going to be less chance of fissile salts leaking. However, the material ship around the reactor will become radioactive (but not fissile). After a bit of use, you don't want anything flaking/eroding off. Although... its not a *major* issue, its still not advisable.

I'll remind you that people even protest when RTGs are launched on one way trips away from Earth. No radioactivity at all gets released into the atmosphere.

However, the public knows that rockets still have a tendency to RUD - and if that happens the plutonium could potentially be released. I think the main reason NTR isn't allowed is because a RUD would likely be much worse than a RUD with some RTGs in the payload section.

In my future scenario, we can assume improvements int the design and operation of spacecraft has lead to a much better safety record - so that RUDs with nuclear reactors are less of a concern.

Also - the new arrivals don't answer to the surface-dwellers (they may or may not want good relations with them, but they don't need to win an election or sit through city councils), and haven't even tell them how their SSTO shuttle's engine works - surface dweller scientists could probably deduce it is using nuclear reactions, but if the SSTO is never captured, and stays in remote locations, it will be hard to generate a good guess as to what it does.

[sevenperforce]

18 minutes ago, monstah said:

I was thinking exactly this, thanks for clarifying.

A pebble-bed rocket uses a cylindrical "combustion" chamber filled with small protected spheres of fissile material. Propellant is pumped in, and the entire chamber is spun magnetically, centrifuging the fissile material against the sides of the cylinder, which is made of a neutron reflector. This produces criticality around the perimeter of the chamber, heating the propellant and forcing it toward the center and out the nozzle. This has the advantage of controlling criticality by altering the rotation speed. If you lose power, the bed slows and criticality drops off naturally; it can't melt down.

The only real problem is that you must be damn sure your bearings don't seize up, because that's catastrophic.

A vapor-core closed-cycle rocket, or nuclear lightbulb, induces criticality in a magnetically-suspended radioactive gas; temperature is transferred through quartz or another very heat-resistant crystal. Not as great as it sounds; the core temperature is much higher but the temperature of the propellant isn't much higher.

A vapor-core open-cycle rocket is very exciting, because the propellant is allowed to mix directly with the fissioning vapor. This does, sadly, involve significant losses of fissile material with a resultant lower fission efficiency, but it more than makes up for the losses in specific impulse. Just don't run it anywhere below the Karman Line.

A nuclear saltwater rocket is essentially liquid-core open-cycle: nuclear dragon drive.

10 minutes ago, KerikBalm said:

well, I think as the tech matures, there's going to be less chance of fissile salts leaking. However, the material ship around the reactor will become radioactive (but not fissile). After a bit of use, you don't want anything flaking/eroding off. Although... its not a *major* issue, its still not advisable.

In my future scenario, we can assume improvements int the design and operation of spacecraft has lead to a much better safety record - so that RUDs with nuclear reactors are less of a concern.

Also - the new arrivals don't answer to the surface-dwellers (they may or may not want good relations with them, but they don't need to win an election or sit through city councils), and haven't even tell them how their SSTO shuttle's engine works - surface dweller scientists could probably deduce it is using nuclear reactions, but if the SSTO is never captured, and stays in remote locations, it will be hard to generate a good guess as to what it does.

Defnitely.

By the way, you never did specify downmass requirements. What should the shuttle be able to carry? People? Cargo? People and cargo? If it needs to take cargo to the surface, will it also need to return cargo to orbit, or can we assume the takeoff is with an empty cargo bay?

[monstah]

1 hour ago, sevenperforce said:

A pebble-bed rocket uses a cylindrical "combustion" chamber filled with small protected spheres of fissile material. (...)

That underlined part is the most important thing I had missed so far; your explanation is more in-depth than what I knew, tho!  

temperature is transferred through quartz or another very heat-resistant crystal. Not as great as it sounds; the core temperature is much higher but the temperature of the propellant isn't much higher.

And the damned thing weighs like a... well, a huge chunk of quartz.

A nuclear saltwater rocket is essentially liquid-core open-cycle: nuclear dragon drive.

Hahaha! Made my day. Tsoukalos was right, after all, Chinese dragon myths were actually about alien spacecraft!

 

Well, by reading this thread, one important thing I've learned is that NTRs (solid-core, at least) have great Isp mostly because of the ability to use H2 for propellant rather than being confined by whatever results from a combustion (H2O at best?), not because of its inherent engineering or chamber temperature. Kinda puts some brakes on my RPG idea of using CO2 NTRs extensively on Mars.

But then, if enriched fissile material becomes more cheaply available after the next step in space colonization, perhaps using CO2 NTRs has economic benefits? The propellant is basically just atmosphere, or even better, lying around frozen at the poles.

[KerikBalm]

1 hour ago, sevenperforce said:

By the way, you never did specify downmass requirements. What should the shuttle be able to carry? People? Cargo? People and cargo? If it needs to take cargo to the surface, will it also need to return cargo to orbit, or can we assume the takeoff is with an empty cargo bay?

Intentionally left unspecified. First something needs to be found that can get a payload fraction >0. From there it is a matter of optimization and scaling to needs.