Ship Design Philosophy

[Condottiere]

Spaceships: Power Plants and Alchemy

So, we got a tank full of hydrogen, and another one of carbon.

Process them simultaneously through a variant fuel processor, we should have a synthetic hydrocarbon admixture, suitable for the onboard chemical plant.

 

[baithammer]

There is no advantage to a chemical plant using carbon based fuel, honestly hydrogen is much more prevalent.

 

[Condottiere]

Maybe as a form of stealth movement.

No stray neutrons giving away the location of a fusion reactor.

 

[baithammer]

It was in reference to the use of hydrogen or petro chem, the petro chem has no advantages in a universe with a well developed hydrogen infrastructure.

 

[AnotherDilbert]

There is no advantage to a chemical plant using carbon based fuel, honestly hydrogen is much more prevalent.

Hydrogen has an energy density of ~140 MJ/kg or 10 GJ/Dt.
Petrol has an energy density of 47 MJ/kg or 33 GJ/Dt.

But you also have to carry the needed oxygen to burn the fuel.
Each kg of hydrogen requires about 8 kg of oxygen or ~0.45 Dt LOX for each Dt LHyd.
Each kg of petrol requires about 3 kg of oxygen or ~1.6 Dt LOX for each Dt petrol.

So totally we get:
Hydrogen: 10 GJ for 1 + 0.45 = 1.45 Dt, or 6.9 GJ/Dt.
Petrol: 33 GJ for 1 + 1.6 = 2.6 Dt, or 12.7 GJ/Dt.

So, if we use hydrogen we need about twice as much fuel as if we use hydrocarbon fuel.

 

[baithammer]

There is an error in the math shown.

140 MJ/kg = 140 GJ/dt as the conversion between MJ -GJ is 1/1000 whereas kg to dt is 1,000 to 1 therefore negating the difference between units.

While petrol is 47 MJ/kg = 47 GJ/dt.

Further, you need 2.98 kg of Petrol per 1 kg of Hydrogen to match energy potential, which still gives a lead to hydrogen when factoring in LOX as hydrogen only requires 2.67 kg to 1 kg for petrol.

 

[AnotherDilbert]

There is an error in the math shown.

140 MJ/kg = 140 GJ/dt as the conversion between MJ -GJ is 1/1000 whereas kg to dt is 1,000 to 1 therefore negating the difference between units.

Yes, that is correct, thank you.

Let me start over:

Hydrogen has an energy density of ~140 MJ/kg. 1 Dt = 14 m³ contains 1 tonne LHyd, so 1 Dt contains 140 MJ/kg × 1000 kg = 140000 MJ = 140 GJ.
Petrol has an energy density of 47 MJ/kg. 1 Dt = 14 m³ contains 14 × 0.7 ≈ 9.8 tonnes of petrol, so 1 Dt contains 47 MJ/kg × 9800 kg ≈ 460000 MJ = 460 GJ.

But you also have to carry the needed oxygen to burn the fuel.
Each kg of hydrogen requires about 8 kg of oxygen. 1 Dt LHyd = 1000 kg requires ~8000 kg LOX which is 8 / 1.3 ≈ 6 m³ or 0.45 Dt LOX.
Each kg of petrol requires about 3 kg of oxygen. 1 Dt petrol = 9800 kg requires 9800 × 3 = 29.4 tonnes of LOX, which at ~1.3 tonne/m³ takes 22.6 m³ or 1.6 Dt.

So totally we get:
Hydrogen: 140 GJ for 1 + 0.45 = 1.45 Dt, or 96 GJ/Dt.
Petrol: 460 GJ for 1 + 1.6 = 2.6 Dt, or 176 GJ/Dt.

I seem to have made the same mistake for both hydrogen and petrol, so the conclusion stands:
If we use hydrogen we need about twice as much fuel as if we use hydrocarbon fuel.

 

[Condottiere]

Spaceships: Bridges to Nowhere

Now that spaceship bridges are capped at a default sixty tonnes for circa two kay tonnes and upwards, you have the discrepancy that at two kay tonnes, a bridge costs you ten megaschmuckers, and one for a hundred kay tonne spaceship costs a cool half a gigaschmuckers.

So what accounts for this discrepancy, since you can't really cram in more machinery?

While the capability to control ever larger ships might require more expensive machinery, I tend to think that the input output side of the equation remains the same, you just need better programmes and bridge personnel to handle the increased information inflow, and coordinate other ship departments.

The increased cost reflects the need to set up a widespread network that connects the bridge workstations to other ship components, and could take control of them directly, as well as the usual remote monitoring.

That means, that bridges could be interchangeable amongst most spaceships, if they are designed to strict common standards.

The price would remain, for the sixty tonne bridge, at ten megaschmuckers, and the remainder would be vested with the respective hull.

 

[baithammer]

Petrol has an energy density of 47 MJ/kg. 1 Dt = 14 m³ contains 14 × 0.7 ≈ 9.8 tonnes of petrol, so 1 Dt contains 47 MJ/kg × 9800 kg ≈ 460000 MJ = 460 GJ.

Can't use the 9,800 kg compared to 1,000 kg on the hydrogen side as the originating unit is MJ/kg (Both sides are required to use the same unit) , also where are is the 0.7 coming from?

Now that spaceship bridges are capped at a default sixty tonnes for circa two kay tonnes and upwards, you have the discrepancy that at two kay tonnes, a bridge costs you ten megaschmuckers, and one for a hundred kay tonne spaceship costs a cool half a gigaschmuckers.

So what accounts for this discrepancy, since you can't really cram in more machinery?

Probably the same as the 1t taken up by a turret, the 60t is the connection within the hull. All the rest of the bridge is an external volume.

 

[AnotherDilbert]

Petrol has an energy density of 47 MJ/kg. 1 Dt = 14 m³ contains 14 × 0.7 ≈ 9.8 tonnes of petrol, so 1 Dt contains 47 MJ/kg × 9800 kg ≈ 460000 MJ = 460 GJ.

Can't use the 9,800 kg compared to 1,000 kg on the hydrogen side as the originating unit is MJ/kg (Both sides are required to use the same unit) , also where are is the 0.7 coming from?

0.7 tonnes/m³ is roughly the mass density of petrol.

I'm comparing the same volume of fuel, since Traveller design systems generally use that as basic unit, 1 Dt ≈ 14 m³ (volume).

1 Dt = 14 m³ LHyd has a density of ~0.07 tonne/m³, so a mass of 14 m³ × 0.07 tonnes/m³ ≈ 1 tonne = 1000 kg.
1 Dt = 14 m³ petrol has a density of ~0.7 tonne/m³, so a mass of 14 m³ × 0.7 tonnes/m³ ≈ 9.8 tonnes = 9800 kg.

Once we have the mass of the fuel we can calculate the energy content as above.

 

[baithammer]

Figured out the missing piece of the puzzle, petrol isn't used in 1950s+ rocket fuel instead kerosene ( Refined from petroleum) is used in the mixture RP1 with LOX as the oxidizer but also requires an additional pressurization system using an inert gas such as nitrogen or helium. ( To displace kerosene.)

 

[AnotherDilbert]

Figured out the missing piece of the puzzle, petrol isn't used in 1950s+ rocket fuel ...

I thought we were discussing fuel for chemical power plants, not rockets.

But, OK, petrol is perhaps not the best fuel available, I just used that since data was easily available, to demonstrate the disadvantage with hydrogen.

 

[Condottiere]

Now that spaceship bridges are capped at a default sixty tonnes for circa two kay tonnes and upwards, you have the discrepancy that at two kay tonnes, a bridge costs you ten megaschmuckers, and one for a hundred kay tonne spaceship costs a cool half a gigaschmuckers.

So what accounts for this discrepancy, since you can't really cram in more machinery?

Probably the same as the 1t taken up by a turret, the 60t is the connection within the hull. All the rest of the bridge is an external volume.

Sixty tonnes is a constant, unless you upgrade to a command bridge

It's the cost that you have to reconcile, and that would have to due to the networking of the hull itself.

There's no option given for a cheaper commercial variant, only that you could use a forty tonne bridge instead, and it would cost you only half as much but at a minus one penalty for most, if not all, tasks.

 

[baithammer]

Figured out the missing piece of the puzzle, petrol isn't used in 1950s+ rocket fuel ...

I thought we were discussing fuel for chemical power plants, not rockets.

But, OK, petrol is perhaps not the best fuel available, I just used that since data was easily available, to demonstrate the disadvantage with hydrogen.

You mentioned needing an oxidizer, which also requires in gasoline / kerosene fuel a pressurized system to fill the displacement of said gasoline/kerosene which isn't required for a hydrogen based system. ( Apparently hydrogen / LOX generate this sort of pressure when mixed.)

Petroleum has other uses that can't be replicated through hydrogen so it still has uses in the face of large scale hydrogen infrastructure.

Sixty tonnes is a constant, unless you upgrade to a command bridge

It's the cost that you have to reconcile, and that would have to due to the networking of the hull itself.

That space is the internal volume displaced by the bridge, but most likely like a turret it displaces further volume outside the hull which would lead to a higher cost for a larger ship.

 

[Condottiere]

There is no additional volume requirement.

The Death Star could be controlled from a sixty tonne bridge.

 

[Sigtrygg]

Don't confuse bridge with the CIC - the latter is usually much larger on a warship with the former being relatively small.

 

[Condottiere]

In Traveller, there would be no difference, therefore, it weighs the same and costs the same, being a clone of the bridge, not counting if sensor and weapon system workstations are placed adjacent.

 

[Sigtrygg]

Of course there will be a difference, it's just that in the past Traveller has always lumped the CIC into the bridge tonnage. A warship will need a much bigger bridge/CIC set up than a civilian ship. You may also want to have a couple of back up bridges in a warship to ensure redundancy.

A bridge is for controlling the maneuvering of the ship - its secondary role is sensor and communication related, although both of those can be allocated elsewhere on the ship.

A CIC is for coordinating the weapons, defences, sensors, comms of the ship and any vessels sub-ordinate to it.

During combat where is the Captain of the ship to be found, the bridge or the CIC?

Now on a submarine the bridge and CIC are one and the same - usually. So if starships are closer to submarines the bridge/CIC will be one and the same and buried as deeply inside the vessel as you can design it to be.

The Star Wars and Star Trek bridge location set ups are just laughable.

 

[Condottiere]

Let's scale down from the Death Star to something more manageable, the Tigress superdreadnought.

It has only one bridge, but with holographic controls.

In which case, you would be correct: warships are designed like submarines.

 

[Condottiere]

So, out of curiosity, I checked out Space Station construction.

Bridge installation doesn't seem to align with it's spaceship counterpart.

But I didn't go away empty handed.

While space stations might move like lame ducks, nothing says they can't jump like kangaroos down the rabbit hole.

Space Stations: Engineering and Jump Drives