Stealth Space Carrier thoughts

I was pondering a Star Raiders type game for the VIC-20, when my thoughts became less arcade-like and more simulator-like. Less air combat inspired, and more submarine combat inspired. When I started crunching numbers, I was shocked to find stealth space carriers seem viable. Some relevant numbers:

100 watts - the amount of heat a pilot generates. This boils about 13 grams of liquid hydrogen per minute, or about 19kg per day.

50 watts - the amount of solar heating a 99% reflective 2m diameter sun-facing mirror system suffers. This boils about 7 grams of hydrogen per minute, or about 10kg per day. Adding this to a human pilot, this translates to around 30kg per day.

44 kilowatts - the amount of solar power a 2m diameter concentrator mirror could give to a solar thermal thruster. Divide that by .5*9000m/s*9000m/s, and we get...

60kg/day - the hydrogen consumption of a solar thermal thruster using a 2m diameter concentrator, assuming the hydrogen is boiled by some other heat source. Note that this exceeds the 30kg/day of hydrogen boiled by the human pilot plus heating of the solar concentrator itself, but I figure various system inefficiencies can make up the difference. In particular, any heating of radar shrouds and a tail boom will boil additional hydrogen. The thruster can be a stealthy pulsed thruster, flash heating a drop of almost boiling hydrogen by squirting it through the heating element, which then expands into a large exhaust cone. Performance is quite good, with an exhaust velocity around 9km/s.

2m x 200m - The dimensions of a stealthy space fighter shaped like a spike. There is sufficient internal volume for up to 14,800kg of hydrogen, or about 8 months of continuous thrusting flight. Some internal volume will be used for other equipment, of course, but the bulk of volume will be hydrogen due to its low density.

The interesting thing is that the presence of a human pilot does not directly impact how stealthy this space fighter is. The waste heat generated by the pilot merely preheats the hydrogen propellant a bit before solar heating does most of the work of boiling and accelerating the propellant.

If we scale things up to a space carrier, things get even more stealthy. Let's consider a 10m x 1000m carrier. This boosts solar heating by 25x, but it also boosts internal volume by 125x. The bottom line is that it could perform 40 months of continuous thrusting flight, plenty of time to go on an interplanetary patrol and return for resupply.

Scaling things down, we can see that a stealth missile still can have significant endurance. A 20cm x 20m missile only has internal volume for around 14kg of hydrogen, but that's still good enough for 3 weeks of continuously thrusting flight.

The interesting challenge is designing a solar thermal thruster which can take in light from most of the front profile and also be stealthy to radar. This is fundamentally similar to the way stealth aircraft use serpentine intakes to prevent the jet turbine to be visible to radar.

Pictured is one idea I have. There are two sets of mirror slats, each covering half of the nose "intake". The angle of these slats deflect the incoming sunlight toward two parabolic concentrators, which each focus light onto the heating element. The heating element is surrounded by photon absorbing walls, such that any incoming photons will either hit the heating element or a wall. One issue with this design is that it has a square profile rather than a circular profile (which is what's most efficient for the LH2 tanks). But I'm sure an arrangement of mirror slats could work for a circular profile. This is just an example design to show the basic principles involved.

This solar thermal thruster is basically at the nose tip, so it requires an auxiliary tail thruster to counteract torque. The ship is very nose heavy, so the tail thruster doesn't need much thrust.

So how does this space carrier work? Well, attacking a non-stealthy target is dreadfully easy. The carrier can be stationed in a co-orbit nearby any planetary system, sending a fighter toward the target with the target none the wiser. Depending on the mission, the fighter might attack with stealth missiles or it could simply shoot some sort of gun from point blank range before slipping away.

Fighting other stealthy forces, though, is a game of hide and seek. Long range detection is sporadic and a matter of luck. As a carrier passes between a sensor and a star, it may see the star wink out or see a tell-tale diffraction pattern. Over time, this may give strategic information about where a carrier is lurking. This carrier likely has a CAP of stealth missiles constantly roving around looking for any incoming approaching carriers, so a fighter may have a better chance of getting close without detection. The fighter itself looses missiles to hunt for the target carrier. Basically, the more eyes in the area, the better the chances of occultation events detecting the enemy carrier. The fighter then remote commands the missiles closer and closer to the carrier based on those detections, until one actually directly hits it. Conversely, the enemy carrier may notice an attack based on occultation events; it may try to counterattack with its own missiles while trying to get away. Note that a carrier has a much lower acceleration than a fighter.

Another idea is to try and attack the logistics, but a carrier has an endurance of perhaps 3 years. It could take a long time to have a decisive effect even if you take out all of the enemy resupply bases.

Now, I don't actually see any compelling reason to have human crew on board. I'm just noting that the numbers for human heat generation are not such a big deal.

Either way, I think this qualifies as "stealth in space", according to popular imagining. The stealthy carriers roam interplanetary space like submarines, performing significant maneuvers continuously rather than just coasting on ballistic trajectories. When they attack "surface vessels", the results are one-sided surprise attacks. A non-stealthy spacecraft just gets "blown out of the water" without warning. When they fight each other, it's a game of hide and seek, using lucky sporadic detections to hunt the enemy and/or trying to slink away with the occasional turn to try and throw off the pursuit.
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