Post by acatalepsy on Oct 4, 2016 4:00:29 GMT
I've been working on some new designs, recently, built around a new strategy - Hydrogen NTRs.
The advantage of methane NTRs is that they provide a good mix of capabilities; okay exhaust velocity, good density, good thrust-to-weight ratio. Methane is also pretty easy to find in the solar system, and where it can't be found, it can be easily produced - four parts hydrogen to one part carbon, and you can find hydrogen and carbon anyway. Thus, methane nuclear thermal rockets have been the gold standard for spacecraft primary drive systems, and all of the main "stock" ships use methane.
Comparatively, hydrogen NTRs have one advantage over methane, which is their higher exhaust velocity. Hydrogen NTRs have about 2 km/s higher exhaust velocity than ones using methane, but are worse in every other respect...their thrust to weight is lower, and the less dense hydrogen is expensive to armor. Only a few ships use it, notably the Privateer design, and aren't a "real" warship; it's there mostly for the situation where you absolutely need the maximum dV.
But I'm slowly coming around on this, for a number of reasons. One is that the tyranny of the rocket equation is boundless; the total amount of mass dedicated to doing things other than being shoved through a nuclear reactor at high velocity is determined by your mass ratio, and your mass ratio is determined by how much delta-V you need to get where you need to be and fight your enemy. The rocket equation is:
Δv = Exhaust Velocity * ln[Wet Mass / Dry Mass].
I find that more often, the mass limits are more limiting than the actual cost limits. To show how this works out, Let's assume we need a d/V of about 6 km/s to accomplish the mission, and have a mass budget of 20 kilotons. How much non-propellant stuff - crew, missiles, engines, armor, etc - can we actually pack? With methane NTRs with an exhaust velocity of 6.2 km/s, the answer is -
6 km/s = 6.2 km/s * ln(20 kt / DM)
2.632 = 20 kt / DM
Dry Mass = 7.6 kt
For a hydrogen NTR, with an exhaust velocity of 8.5 km/s, the same calculation yields -
6 km/s = 8.5 km/s * ln(20 kt / DM)
2.026= 20 kt / DM
Dry Mass = 9.87 kt.
That's a very solid increase in overall fighting capability. That's another two kilotons worth of extra missiles, lasers, reactors, and more that I can bring to a fight. That's very, very big. But it gets worse as you add delta-V requirements. Running the same calculation again; same mass limit, but now we need something closer to 10 km/s for our mission.
Methane
10 km/s = 6.2 km/s * ln(20 kt / DM)
5.017 = 20 kt / DM
Dry Mass = 3.98 kt
Hydrogen
10 km/s = 8.5 km/s * ln(20 kt / DM)
3.243= 20 kt / DM
Dry Mass = 6.17 kt
Up from a 30% increase in overall dry mass, that's now a 50% increase in total dry mass. That's a substantial advantage, if you're willing to press it. For fighting in the outer systems, where necessity may compel you to fight over multiple objectives across multiple moonlets, that matters.
In addition, bring more deltaV to a fight gives you more options. You can get to a fight quicker. You can boost your missiles and drones to higher intercept velocities and then back off, and give your opponents an uncomfortable choice between burning too much dV to effectively contest an objective, and surviving a storm of hypervelocity flak. You can afford to make "sloppy" moves that would otherwise require weeks or months of maneuvering. And strategically, of course, more delta-V allows your invasion-staged ships to get to the fight more quickly, cutting down on the amount of defensive preparations your opponents can manage in the mean time. That's a free hand any admiral will find valuable.
Still, the downsides. The stock hydrogen NTR is kind of pathetic. We can do better, of course -
- compared to the standard methane engine, this hydrogen NTR is superior in pretty much every way. We can build better methane engines, but by and large they don't seem to go much past 6.5 km/s exhaust velocity, which is the key advantage here.
Past a certain point, the better thrust to weight isn't terribly helpful; you are not going to be able to outrun a missile, and as long as your acceleration is sufficient, your delta-V matters more than your total acceleration.
What about armor? Well, it's true bigger, hydrogen NTR ships are more difficult to armor; but on the other hand the most effective armoring strategies seem to revolve around armoring as little as possible, on the theory that it's impossible to really armor any ship so extensively its propellant tanks are safe, so you might as well write them off. This isn't as big of an advantage as it looks like. And given the substantial amount of extra dry mass afforded to a hydrogen powered fleet, it's not unreasonable for them to invest in stronger defenses for what really matters, as well as active defenses that prevent the damage in the first place.
Another note: I've also been coming around to the power of tankers and tenders. I've been playing around with militarized tenders that enable their squadron to spend freely in maneuvers, and then rejoin (or be joined by) the supply ship when the mission is complete. When your fleet is mass limited, you can save a lot of total delta-V by, effectively, not having large parts of it engaging in unnecessary maneuvers. I've experimented with 3kt squadron tenders and 10kt fleet tenders, and so far both have been very effective force multipliers, and allowed me to build comparatively smaller, sturdier, faster, and all around more lethal gunships.
Thoughts? Are you ready to join the cult of Hydrogen? Does Methane still rein supreme?
The advantage of methane NTRs is that they provide a good mix of capabilities; okay exhaust velocity, good density, good thrust-to-weight ratio. Methane is also pretty easy to find in the solar system, and where it can't be found, it can be easily produced - four parts hydrogen to one part carbon, and you can find hydrogen and carbon anyway. Thus, methane nuclear thermal rockets have been the gold standard for spacecraft primary drive systems, and all of the main "stock" ships use methane.
Comparatively, hydrogen NTRs have one advantage over methane, which is their higher exhaust velocity. Hydrogen NTRs have about 2 km/s higher exhaust velocity than ones using methane, but are worse in every other respect...their thrust to weight is lower, and the less dense hydrogen is expensive to armor. Only a few ships use it, notably the Privateer design, and aren't a "real" warship; it's there mostly for the situation where you absolutely need the maximum dV.
But I'm slowly coming around on this, for a number of reasons. One is that the tyranny of the rocket equation is boundless; the total amount of mass dedicated to doing things other than being shoved through a nuclear reactor at high velocity is determined by your mass ratio, and your mass ratio is determined by how much delta-V you need to get where you need to be and fight your enemy. The rocket equation is:
Δv = Exhaust Velocity * ln[Wet Mass / Dry Mass].
I find that more often, the mass limits are more limiting than the actual cost limits. To show how this works out, Let's assume we need a d/V of about 6 km/s to accomplish the mission, and have a mass budget of 20 kilotons. How much non-propellant stuff - crew, missiles, engines, armor, etc - can we actually pack? With methane NTRs with an exhaust velocity of 6.2 km/s, the answer is -
6 km/s = 6.2 km/s * ln(20 kt / DM)
2.632 = 20 kt / DM
Dry Mass = 7.6 kt
For a hydrogen NTR, with an exhaust velocity of 8.5 km/s, the same calculation yields -
6 km/s = 8.5 km/s * ln(20 kt / DM)
2.026= 20 kt / DM
Dry Mass = 9.87 kt.
That's a very solid increase in overall fighting capability. That's another two kilotons worth of extra missiles, lasers, reactors, and more that I can bring to a fight. That's very, very big. But it gets worse as you add delta-V requirements. Running the same calculation again; same mass limit, but now we need something closer to 10 km/s for our mission.
Methane
10 km/s = 6.2 km/s * ln(20 kt / DM)
5.017 = 20 kt / DM
Dry Mass = 3.98 kt
Hydrogen
10 km/s = 8.5 km/s * ln(20 kt / DM)
3.243= 20 kt / DM
Dry Mass = 6.17 kt
Up from a 30% increase in overall dry mass, that's now a 50% increase in total dry mass. That's a substantial advantage, if you're willing to press it. For fighting in the outer systems, where necessity may compel you to fight over multiple objectives across multiple moonlets, that matters.
In addition, bring more deltaV to a fight gives you more options. You can get to a fight quicker. You can boost your missiles and drones to higher intercept velocities and then back off, and give your opponents an uncomfortable choice between burning too much dV to effectively contest an objective, and surviving a storm of hypervelocity flak. You can afford to make "sloppy" moves that would otherwise require weeks or months of maneuvering. And strategically, of course, more delta-V allows your invasion-staged ships to get to the fight more quickly, cutting down on the amount of defensive preparations your opponents can manage in the mean time. That's a free hand any admiral will find valuable.
Still, the downsides. The stock hydrogen NTR is kind of pathetic. We can do better, of course -
- compared to the standard methane engine, this hydrogen NTR is superior in pretty much every way. We can build better methane engines, but by and large they don't seem to go much past 6.5 km/s exhaust velocity, which is the key advantage here.
NuclearThermalRocketModule 8.56 km/s Hydrogen Gimballed Nuclear Thermal Rocket
ReactorCoreHeight_m 0.3
NuclearReactor
Coolant Hydrogen
Moderator Aluminum
ModeratorMass_kg 0
Fuel U-235 Dioxide
FuelMass_kg 150
FuelEnrichment_Percent 0.87
ControlRodComposition Boron Nitride
ControlRodMass_kg 110
NeutronReflector Boron Nitride
ReflectorThickness_m 0.4
AverageNeutronFlux__m2_s 1.9e+020
ThermalRocket
ChamberComposition Vanadium Chromium Steel
ThroatRadius_m 0.72
ChamberWallThickness_m 0.00094
ChamberContractionRatio 1
NozzleExpansionRatio 20
NozzleExpansionAngle_degrees 17
RegenerativeCooling_Percent 0.99
Injector
Composition Vanadium Chromium Steel
PumpRadius_m 0.47
RotationalSpeed_RPM 3400
Gimbal
InnerRadius_m 0.68
ArmorComposition Diamond
ArmorThickness_m 0.01
ReactionWheels
Composition UHMWPE
RotationalSpeed_RPM 43000
GimbalAngle_degrees 20Past a certain point, the better thrust to weight isn't terribly helpful; you are not going to be able to outrun a missile, and as long as your acceleration is sufficient, your delta-V matters more than your total acceleration.
What about armor? Well, it's true bigger, hydrogen NTR ships are more difficult to armor; but on the other hand the most effective armoring strategies seem to revolve around armoring as little as possible, on the theory that it's impossible to really armor any ship so extensively its propellant tanks are safe, so you might as well write them off. This isn't as big of an advantage as it looks like. And given the substantial amount of extra dry mass afforded to a hydrogen powered fleet, it's not unreasonable for them to invest in stronger defenses for what really matters, as well as active defenses that prevent the damage in the first place.
Another note: I've also been coming around to the power of tankers and tenders. I've been playing around with militarized tenders that enable their squadron to spend freely in maneuvers, and then rejoin (or be joined by) the supply ship when the mission is complete. When your fleet is mass limited, you can save a lot of total delta-V by, effectively, not having large parts of it engaging in unnecessary maneuvers. I've experimented with 3kt squadron tenders and 10kt fleet tenders, and so far both have been very effective force multipliers, and allowed me to build comparatively smaller, sturdier, faster, and all around more lethal gunships.
Thoughts? Are you ready to join the cult of Hydrogen? Does Methane still rein supreme?
