So one of the things I keep seeing is that ships in 2300 AD have thier thrusters rated for a world size. Does that mean that specific interface craft are rated for specific words and how does that translate to rules for leaving worlds. I'm very confused but that's cause I'm trying to read a book on a phone.
2300 AD Thrusters
- Thread starter videopete
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bytedruid
Mongoose
(An old question, but worth answering)
An interface craft has to exhaust enough reaction mass at a high enough velocity to overcome a world's surface gravity. Since the exhaust velocity is taken to be a constant for any given reaction drive type, it takes bigger thrusters to generate more thrust. It also takes more fuel to, but that's handled by bigger worlds requiring more "Burns" of fuel.
If your interface craft suffers from the same problems as NASA's current SLS core stage (aka can't lift it's own weight) then you'll have to help it get off the ground as stated in the AEH (Aerospace Engineer's Handbook), page 14:
Boosters are described on page 59 of the AEH, but that section has a logical hole. It's basically written as if the only problem an interface craft can have is that it doesn't carry enough fuel to get into orbit, so extra tankage can supply the rest, but as you pointed out, even a craft with enough "burns" might not be able to make it into orbit due to undersized thrusters.
If it were my game, I'd skip doing a careful booster calculation and just tell players that their craft is under powered and will need to attach boosters to get back into orbit. Further detail can be ignored unless they are really curious.
So in short, if your group goes into orbit about a world with little infrastructure and it's size rating is higher then the interface craft's thruster rating, landing becomes a one-way trip. If the local colony has boosters on hand, then these could be strapped to the craft (for a fee of course) to get back into orbit.
An interface craft has to exhaust enough reaction mass at a high enough velocity to overcome a world's surface gravity. Since the exhaust velocity is taken to be a constant for any given reaction drive type, it takes bigger thrusters to generate more thrust. It also takes more fuel to, but that's handled by bigger worlds requiring more "Burns" of fuel.
If your interface craft suffers from the same problems as NASA's current SLS core stage (aka can't lift it's own weight) then you'll have to help it get off the ground as stated in the AEH (Aerospace Engineer's Handbook), page 14:
Boosters are described on page 59 of the AEH, but that section has a logical hole. It's basically written as if the only problem an interface craft can have is that it doesn't carry enough fuel to get into orbit, so extra tankage can supply the rest, but as you pointed out, even a craft with enough "burns" might not be able to make it into orbit due to undersized thrusters.
If it were my game, I'd skip doing a careful booster calculation and just tell players that their craft is under powered and will need to attach boosters to get back into orbit. Further detail can be ignored unless they are really curious.
So in short, if your group goes into orbit about a world with little infrastructure and it's size rating is higher then the interface craft's thruster rating, landing becomes a one-way trip. If the local colony has boosters on hand, then these could be strapped to the craft (for a fee of course) to get back into orbit.
Bryn the 2300AD guy
Banded Mongoose
Firstly, let me note that 2k3 thrusters are "magic" in terms of performance. Any thruster with the implied exhaust velocity would melt like a candle in a blowtorch.
However, given the calculation that one "burn" is about a dV of 1.4 kps, then for a planet of density 1, 1 burn per world size is the mathematically correct dV to lift off. If the world isn't the same density of Earth, the number of burns should scale linearly. So to lift to orbit from a size 10, 1.2 Earth density world should take 12 burns.
If you have a spaceplane type design, with an airbreathing engine, then you basically get a "free" burn of dV in the early stages.
Realistically, SSTO from an Earth size world is darned difficult.
Using the real equations, in round figures, each burn, using a hydrogen/oxygen rocket, is about 27% of the mass of the vessel. However, as Traveller is a volume based system, the first burn is 59% of the volume of the vehicle (assuming the rest is sg1). Say 60%. If you have a 1,000 dTon starship, the first burn costs 600 dTons of fuel. Things pretty quickly get impossible.
However, given the calculation that one "burn" is about a dV of 1.4 kps, then for a planet of density 1, 1 burn per world size is the mathematically correct dV to lift off. If the world isn't the same density of Earth, the number of burns should scale linearly. So to lift to orbit from a size 10, 1.2 Earth density world should take 12 burns.
If you have a spaceplane type design, with an airbreathing engine, then you basically get a "free" burn of dV in the early stages.
Realistically, SSTO from an Earth size world is darned difficult.
Using the real equations, in round figures, each burn, using a hydrogen/oxygen rocket, is about 27% of the mass of the vessel. However, as Traveller is a volume based system, the first burn is 59% of the volume of the vehicle (assuming the rest is sg1). Say 60%. If you have a 1,000 dTon starship, the first burn costs 600 dTons of fuel. Things pretty quickly get impossible.
Geir
Emperor Mongoose
Yes... working a a different ruleset right now, and with 'spreadsheet spacecraft' it does appear that SSTO with hydrolox or methalox (liquid hydrogen ends up being a better fuel, even with the low density - which surprised me -> fuel/oxidizer mix is likely why) is possible with near-current tech (the 90's X-33/VentureStar NASA project was probably a road too far then, but maybe not now), BUT, you'll have no capability beyond LEO; the payload mass has to be negative to even reach GTO (uh, geosynchronous transfer orbit, not the car...). I'm assuming VTOL with landing legs only good for, er, landing, so you'd need dedicated launch facilities. Maybe adding a set of airbreather engines and full load landing gear would allow for HTOL, but those things add mass too, more than landing fuel and legs for an empty VTOL. In a spreadsheet, anyway. As SpaceX and Blue Origin have already figured, VTOLs scale better and two stage rockets scale WAY better than single stages.
bytedruid
Mongoose
Can you break that down a bit? Knowing how to do real mass to dTon conversions would be helpful for me, and potentially for others.
Also what the heck is a Traveller displacement ton? I know one of the source books hand-waved an explanation, but it didn't stick because there was no equation. It can't be the same as displacement tonnage used to describe ocean going vessels, can it? If a boat has a mass of 1 ton then it has to displace 1 ton of water, since the local gravitational acceleration is the same for both the water and the boat... right?
Condottiere
Emperor Mongoose
Synonym for volume, in this case about fourteen cubic metres per.
Bryn the 2300AD guy
Banded Mongoose
1 dTon is approximately the volume occupied by 1 ton of liquid hydrogen. 14 m3
Hydrogen/oxygen fuel is denser, because liquid O2 is denser. GDW made a mistake in the stoichiometry of the H2/O2 fuel. An exact 2:1 mole ratio fuel (i.e. perfect balance) has a density of 0.43 (which is what I used above), but real rockets run H2 rich to ensure complete burning and to allow the excess fuel to create a boundary layer on the thruster and prevent the thruster melting - something the liquid methane of the SpaceX Raptor is notoriously not doing leading to the very low burn life of the thruster. Real, H2 rich, fuel is about 0.36 density.
1 dTon of pure LHyd = 1 metric ton
1 dTon of perfectly balanced LHyd/LOx = 6 metric tons
1 dTon of real LHyd/LOx = 5 metric tons
Hydrogen/oxygen fuel is denser, because liquid O2 is denser. GDW made a mistake in the stoichiometry of the H2/O2 fuel. An exact 2:1 mole ratio fuel (i.e. perfect balance) has a density of 0.43 (which is what I used above), but real rockets run H2 rich to ensure complete burning and to allow the excess fuel to create a boundary layer on the thruster and prevent the thruster melting - something the liquid methane of the SpaceX Raptor is notoriously not doing leading to the very low burn life of the thruster. Real, H2 rich, fuel is about 0.36 density.
1 dTon of pure LHyd = 1 metric ton
1 dTon of perfectly balanced LHyd/LOx = 6 metric tons
1 dTon of real LHyd/LOx = 5 metric tons
