"Stealth Transit drive" Someone look into my logic?

[Liam's Ghost]

  So a ship under thrust is visible from all directions to other ships out to tens of millions of kilometers (out to a distance of 40,000,000 kilometers before the target number to detect becomes too high to achieve under standard rules. Maybe around 150,000,000 kilometers if the tracking unit has a large naval comm scanner)

  This seems to imply that the transit drive is not only very bright, but also spewing detectable charged particles in every direction, because... well.. you can see it from all directions. You can't see it if it's not sending something for you to see towards you.

  (this also means the fantastically powerful transit drives of large ships are even worse than typical estimates, because typical guesses of power output don't factor in that an unknown amount of that energy isn't even going to accelerating the ship)

  So my idea was a transit drive that could produce a more efficient, confined beam rather than the standard fusion torch. Closer to a particle beam and less like an omnidirectional continuous nuclear blast out the back of the ship. This "stealth mode" would function as an alternate mode to a drive that could also perform standard tactical or thermal expansion maneuvering.

 The limitation to this movement mode would be that it would be slow, maybe limited to what the rules laughably call "station keeping", or 0.1 gravities. It might also be less fuel efficient. I imagine you wouldn't be able to get the same exhaust velocity while keeping the output in a tight beam, so you'd have to burn more fuel to get any decent acceleration.

  The advantage would be that it would be hard to detect. Maybe impossible (or close to it) if the target is heading right towards you, and a drastically cut down detection range if the target is moving  on any course but directly away from you. Of course, if you are behind it,  the transit drive would be clearly visible, because the beam is spewing all its charged particles in your general direction.

  So does any of this make sense?



 

[Charistoph]

I'd probably start with being more massive, though being less efficient with it's Daily Burn would make sense, too.

With it taking more mass, as well as having to stock more fuel, you're already limiting the amount of acceleration you can perform with it.

Exact number to increase by will take some fiddling, but I'd probably start with 1.3x more massive to start and see how that floats.

Set it up as if it was on a Leopard for a comparison, as smaller Dropships like that would be used more often than would an Overlord or Fortress.

[Retry]

This seems to imply that the transit drive is not only very bright, but also spewing detectable charged particles in every direction, because... well.. you can see it from all directions. You can't see it if it's not sending something for you to see towards you.

  (this also means the fantastically powerful transit drives of large ships are even worse than typical estimates, because typical guesses of power output don't factor in that an unknown amount of that energy isn't even going to accelerating the ship)

They are efficient, it's just that there's an unbelievably insane amount of energy involved that even very high efficiencies produce an insane amount of waste energy (heat, presumably).

Battletech warships weirdly get more efficient as the ship gets bigger, with the exception of sharp drops after hitting certain tonnage milestones which reduces the thrust points you get per ton.

Let's take a really beefy bugger.  2,500,000 ton boat.  Every ton yields 2 'thrust' (really acceleration) points.  IIRC, an Aerospace turn is something like 1 whole minute.

Thrust is easy to calculate: Just multiply your acceleration times your mass, that gives you the amount of thrust you need to actually accelerate your massive hulk.

M*A=F
2,500,000,000 kg * 9.81 m/s^2 = 24,525,000,000 N = 24.5 GN

Delta-V is basically "fuel efficiency" for spacecraft: It's basically how much a unit can change their velocity vector before running out of fuel.  Chemical rockets have absolutely terrible Delta-V, but there's not much other choice to counteract gravity and get off the Earth at the moment.  Electric station-keeping drives ain't too bad with Delta-V


T(t) is thrust, a force.  BT's engines are already set for acceleration though, so it's the same as the acceleration you get times the mass of the ship, or A(t)m(t)

m(t) again is the mass of the ship, which leaves you with the following (May ludd forgive me for this notation)

dV = integral_t0^t1A(t)dt

For a test case of our 2,500,000 ton warship accelerating at a constant 1G (2 thrust points) for one aerospace turn (1 minute?):

dV = 9.81t m/s² [60s, 0]
dV = 589 m/s.

Thus, this warship can change its velocity vector by a bit over half a km/s (almost 2x the speed of sound if the Warship could enter Earth's atmosphere without disintegrating) by just one ton of fuel.  With BT's simplified fuel calculations, every extra ton of fuel added increases your delta-V by the same amount: 10 tons yields 5,890 m/s dV, 1,000 tons yields 589,000 m/s.  If you were feeling really ornery and filled that puppy up to 1,000,000 tons (almost all of its free weight), you can, on paper, hit a delta-V of 589,000,000 m/s.  (For reference, the speed of light is only about 300,000,000 m/s)

That's the delta-V of the warship though.  In order to move at all, that ship's gotta eject reaction mass out the back in the opposite direction.  Due to the conservation of momentum we can actually calculate the exhaust velocity fairly accurately.

m₁v₁=m₂v₂
m₁= warship mass=2,500,000 tons
m₂= exhaust mass=1 ton
v₁= warship velocity change=589 m/s
v₂= exhaust velocity=V

(2,500,000 ton) * (589 m/s) = (1 ton) * V
V = 1,470,000,000 m/s

Which is, umm, almost 5x that of the speed of light.  Whoops, did I say we could estimate it accurately?  Things get spooky when you get within a couple percentage points from the speed of light, let alone several times over.  Well, guess I'll pretend this value makes sense and continue with the Newtonian value for entertainment purposes until Cray smites me for high crimes against physics.

Going forward, we can also determine the ideal propulsive power, that is, the power required if there's exactly 0 waste anything: all of the energy to make the hydrogen exhaust go fast is solely devoted to make it go fast and not just make it hot (which is waste heat).  In general, that looks something like this:


P = Propulsive power (thrust)
T = Thrust
v = Exhaust Velocity

Hey would you look at that, we already have the latter two numbers.  Let's just plug in the thrust and the totally-not-meaninglessly-high exhaust values in to see what we get:

(24.5 GN) * (1,470,000,000 m/s) = 3.6e¹⁹ Watts = 36ish Exawatts?

Assuming I somehow did these calculations right while I'm half-asleep, that means one super-sized warship with a totally, 100% efficient drive, consumes more power than the current electrical output of the Earth (350 W per capita * population of 7.8 billion = 2.7 TW power output).  Like more than 10 million times more.  Honestly, I almost find this Battletech technology more impressive than the literal physics-breaking K-F drives.

Usually you have to balance between the two attributes, ridiculously high delta-V and ridiculously high thrust, specifically because the required propulsive power would just be insurmountable to get both.  Battletech warships don't really have that problem, and I can only assume that it is only thanks to the pure anger of the spirit of McKenna inside each transit drive system that provides the necessary power to actually reach the required power output.  Honestly, with this sort of energy behind them you could probably use these drives as weapons...

Even if that drive if 99% efficient, a 36 Exawatt level of effective propulsive power is going to result in several Petawatts of waste energy per second, and that's going to be distributed between not that much exhaust mass per second (16.7 kg/s).  That's going to be really really hot.  Please don't ask me how hot; I'm much too tired for that and it might make the exhaust hydrogen hotter than the center of the sun.  Plus neither my engineering training nor my two hours of dabbling in Terra Invicta have adequately prepared me for Battletech physics.


Yeah, guess I better circle back to the topic at hand...

Well, for a stealth mode, maybe add some equipment, "Reduced Exhaust Signature Module" or something that costs a fraction of the transit drive's weight (thus scaling to be heavier for larger and more powerful transit drives).  You could limit the exhaust velocity to station-keeping thrust, or alternatively to "just" safe thrust, but dramatically increase fuel consumption: For example, 1 "effective" thrust point costs 10 "real" fuel points.  The excuse being that you're pumping way more mass at a lower velocity to get your thrust, but in return that mass is far cooler than normal and harder to detect (Hence it has a reduced exhaust signature).

I didn't need to post a massive text wall to get to this paragraph-sized point, but there you go.

[Daryk]

Just marking this thread for later reading after work... :)

[Lone-Wolf]

While I understand the logic behind it, I would say that a more focused beam would be brighter and therefore would cancel out the advantage of a smaller exhaustplume.

On the other hand:
Why not use some materials (like the one used in solar sails), attach them to the bottom of your ship and cover the exhaustplume with it.
(You know, just like a tank where the upper part of the track is covered by metal plates.)

[Charistoph]

Yeah, the plume would have to be diffused far more, which would reduce its overall force capacity.

[Maingunnery]

Any high-energy particle in the exhaustplume will radiate, which is likely what is detected.
A pure EM exhaust would be far less visible as long as it does not hit any debris or gases. However such a drive will produce less thrust.

[Daryk]

The stealthiest way to approach is to accelerate beyond detection range, coast most of the way there, and burn only at the last minute.

[DevianID]

So the expanse did something like this.  More or less, firing the fusion engine would tell everyone where you are, so the pilot plotted a crazy multiple gravity slingshot maneuver to get to their destination without firing the engine in detection range.

Any kind of engine is gonna give you away, but drifting from far away and doing a high G burn at the last second/gravity slingshot would make you pretty stealthy upon approach.

[idea weenie]

So the expanse did something like this.  More or less, firing the fusion engine would tell everyone where you are, so the pilot plotted a crazy multiple gravity slingshot maneuver to get to their destination without firing the engine in detection range.

Any kind of engine is gonna give you away, but drifting from far away and doing a high G burn at the last second/gravity slingshot would make you pretty stealthy upon approach.

The Expanse did this with just the moons of Jupiter.  So you'd have to first get to the planetary body you want to move around.  Getting to the body means plenty of time drifting, aka time + Life Support used up.

The fun part of course, is waiting the entire time.  You have to wait for the planets to be in the right arrangement, you have to drift between them, and the entire time you have people using up life support.  It won't take much energy, but the time needed will be impressive.  Depending on the course, you may need to bring along a lot of re-enlistment papers for the crew.

[phoenixalpha]

So if there were stations at the nadir and zenith jump points, its very possible that their detection range wouldn't cover the entire planetary disk especially at the extreme edges. So make sure the jumpship has one or more planets between the target and beyond the last planet of the system. Fire up the engines and get as much initial inertia as possible and then shut down the drives, coast the rest of the way and then turn and burn at the last possible moment....

It's a lot of trouble to go to when pirate points exist

[worktroll]

Any high-energy particle in the exhaustplume will radiate, which is likely what is detected.

This is what I'd come here to say, but MG beat me to it. The thermal plume is going to be enormous, even if we assume not-quite-lightspeed velocity.

I did try riffing off drive baffles, which have a venerable ancestry back as far as the Lensman books, and currency in terms of helicopter stealthing. But helicopters don't use jet exhaust for thrust, and any sort of exhaust baffles would just convert to very strong IR sources, while reducing thrust. Daryk did spot the obvious workaround.

[Liam's Ghost]

Thanks all for your input and information.

[Demiurge]

Warship propulsion, such as it is described, is probably some sort of fusion rocketry. This jives pretty well with the rest of the setting; humanity in BT is very good at making fusion gizmos.

The way a rocket works is that it makes a bunch of compressed gas, and then trades that pressure for velocity by accelerating the gas through a nozzle. Replace "gas" for "extremely angry plasma" and replace "nozzle" with "magnetic bottle that works just like a nozzle because no actual solid material is going to tolerate that sort of abuse for more than a millisecond" and you have a plausible model of how BT spacecraft propel themselves.  Again, humanity in BT is very good at making all kinds of fusion gizmos of different scales so this is basically consistent with everything else.

Even though the propellant mass is so hot that the electrons and nuclei have long since become estranged, the high school level gas law physics provide a useful approximation.  Yes, the numbers that Retry crunched show that the reaction mass in question is very much a plasma, and not a gas.  Still, close enough for a basic description.

Bernoulli's principle shows that increasing the velocity of a gas decreases the pressure.  Or rather that velocity and pressure are two sides of the same coin; normal pressure is static pressure where all the constituent molecules of the gas are bouncing around in a completely random and isotropic manner, while gas velocity is dynamic pressure where the molecules are bopping around but there is a net direction to their movement.  A nozzle is a device which trades off static pressure for dynamic pressure, basically it takes the random bopping of the gas molecules (or in this case the extremely angry buzzing of a quasi-neutral soup of fundamental particles that are terrifyingly hot) and converts it into net directional flow.

So, as you might guess, a nozzle works better when the starting static pressure is higher.  How do we make the reaction mass higher pressure?  That's right, we make it hotter.  You may recall the ideal gas law which states that PV=NRT; that there's a basically linear relationship between pressure and temperature.

The extremely angry plasma will cool down as it is accelerated through the nozzle but... it's still going to be really angry cooking hot when it gets out there.  Someone with a better grasp of plasma physics could probably do a basic estimation based on the Stefan–Boltzmann law and Wien's displacement law of exactly how bright and in what frequency a warship's exhaust plume would shine, but I would guess it's on the order of crazy, unpleasantly bright and bring sunscreen because it's probably hot enough to be pumping out hard UV.  And actually some lead shielding might not be a bad idea either because it might be glowing hot enough to put out X-rays.

So the problem isn't so much that the transit drive is spewing out charged particles in all directions.  Indeed, if the magnetic nozzle is well-designed the flow of the plasma should be pretty well collimated into a parallel flow, as any radial spread represents inefficiency and loss of thrust.  The drive plume would tend to spread out in space due to some residual pressure and probably weird plasma physics I don't particularly understand, but the main issue vis a vis detectability will be the fact that the exhaust plume will be incredibly bright.

[glitterboy2098]

yeah, you're basically creating a hot plasma comet tail many many times your own size behind you. and that tail is going to not only glow in visible light and IR but also in UV and all sorts of radiation. directing that tail into a narrower path isn't going to help much.

what you'd need to do is use a different less detectable drive. like say a photon drive (basically using a laser as our rocket), which would get you the nearly invisible drive plume but also reduce your thrust by a huge degree. (thousands of times less power efficient than a direct fusion rocket, and pretty much useless for orbit to ground and back)

and the drawback that when you flip over at midpoint to slow down, you are now pointing an absurdly powerful laser right at your target. a laser with beyond capital level firepower and an effective beam range even without weaponized focusing gear measured in planetary diameters.

"hmm.. we seem to have a ship incoming"
"how can you tell sir?"
"half the southern continent just vaporized."

[Mechanis]

I thought about this too because my big AU project has the Dantes as space submarinesStealth Ships, and what I went with is a Cold-Gas thruster that gets 0.1/0.2 thrust points for the tonnage of a 1/2 fusion engine (and so on.)
It's nearly undetectable because it's outputting exhaust gas that's the same temperature as the local stellar background, and the thrust is shit because... it's outputting exhaust gas that's the same temperature as the local stellar background.
So perfect for doing Space Submarine things like sneaking up to a shipyard to put nuclear tipped torpedoes into it, but not super useful in a piched battle (though they've got a conventional 1/2 fusion engine too, for when they aren't sneaking around.)

[Daryk]

The thing is, even 0.1 thrust (continuous, with a reasonable fuel consumption) is amazing compared to what we know can be done.

[Mechanis]

Well yes

I just assume they're applying whatever Hyperspace Witchcraft lets the regular drives get those downright absurd exhaust velocity values (which as mentioned upthread are often well in excess of the speed of light) to the things in the interest of having something actually playable.

[Daryk]

To be fair, the exhaust velocities aren't supra-luminal.  The Lorentz transformations will give you the relativistic speeds, and the attendant apparent increase in mass.

[Demiurge]

An electrical propulsion system like an ion or Hall effect thruster might be able to get reasonable specific impulse without ultra-hot exhaust, since it's accelerating the reaction mass with electromagnetic forces rather than heating it and running it through a nozzle.

[Lycanphoenix]

I actually really love the idea of making The Expanse-style “stealth ships”, but… even if your ship is the size of the Voyager space probe and is propelled by an ion engine, it’d be impossible to actually fly undetected. Kyle Hill made a video about this. The laws of physics don’t allow stealth in space.

Then again, BattleTech does bend things quite a bit…

[Daryk]

Truly invisible is impossible, yes, but looking like "just another space rock" isn't.

[FastConcentrate8]

Another way to balance could be that a ship must devote a significant amount of its mass budget on Heat Sinks (half that for double sinks) since a ship must store its heat while in stealth mode and slowly radiate it passively rather than via active heat pumps. 

[Lycanphoenix]

You'd have to redshift your thermal emissions to look like the cosmic microwave background, or make it spread-spectrum, or both.

Maybe also use selective heat baffles so that only surfaces facing away from prying eyes will radiate heat.

[Mechanis]

I actually really love the idea of making The Expanse-style “stealth ships”, but… even if your ship is the size of the Voyager space probe and is propelled by an ion engine, it’d be impossible to actually fly undetected. Kyle Hill made a video about this. The laws of physics don’t allow stealth in space.

Then again, BattleTech does bend things quite a bit…

Stealth, in the context we're talking about here, is not "being invisible", it is making the enemy's computer ignore you. Which you do by not looking like a ship. And frankly, the whole "you can't have stealth in space" argument is fundamentally flawed anyway, because it assumes perfect 360° sensor coverage and perfect analysis of said sensor data, which is impossible without your ship being a perfect sphere whose entire surface is an uninterrupted Everything Sensor, connected to a strong AI with arbitrary amounts of processing power on tap to analyze the take, and I trust you can see the issue there. Frankly, the ability to actually detect ships in BattleTech is already pretty magic, because apparently even the most backwards backwaters have telescope networks that make modern earth look like a hobbiest four year old with a 10$ monoscope.

[Daryk]

Again, the trick is looking like "just another space rock"... That means coasting for a LONG time with all your heat radiated AWAY from the target.  It takes forever, but is more the surprise when you pull it off... ;)

[idea weenie]

I actually really love the idea of making The Expanse-style “stealth ships”, but… even if your ship is the size of the Voyager space probe and is propelled by an ion engine, it’d be impossible to actually fly undetected. Kyle Hill made a video about this. The laws of physics don’t allow stealth in space.

Then again, BattleTech does bend things quite a bit…

Batletech's sensor ranges are really short, compared to real life.

Again, the trick is looking like "just another space rock"... That means coasting for a LONG time with all your heat radiated AWAY from the target.  It takes forever, but is more the surprise when you pull it off... ;)

Make sure you have lots of Life Support, so your crew have something to eat and breathe the entire time.