Low tech ships - Need help

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Linwood
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Re: Low tech ships - Need help

Postby Linwood » Sat Dec 16, 2017 3:54 pm

Hydrogen can be stored under high pressure or possibly chilled to near-absolute zero to reduce the volume needed for storage. Although the way the collapsible bladders are described the pressures are likely limited - probably no more than 100 MPa, maybe only 50 MPa.
Sigtrygg
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Re: Low tech ships - Need help

Postby Sigtrygg » Sat Dec 16, 2017 4:13 pm

I know this is a low tech thread but - you just got me thinking. Perhaps the liquid hydrogen tanks on TL9+ships are not cryogenic but are a result of gravitics being used to liquefy the hydrogen? Another application of gravitics along with artificial gravity, acceleration compensation and waste heat management.

On low TL ships TL8- you are going to have to have cryogenically cooled liquid hydrogen, but it can be stored in a dewar vessel (vacuum flask) easily enough. Perhaps the fuel tankage on low TL ships need to have a dt surcharge - pumps etc for hydrocarbon fuels, dewar storage for liquid hydrogen and liquid oxygen.
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Re: Low tech ships - Need help

Postby Condottiere » Sat Dec 16, 2017 8:18 pm

Image

Vapour chambered hulls?
Linwood
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Re: Low tech ships - Need help

Postby Linwood » Sun Dec 17, 2017 1:27 am

Condottiere wrote:
Sat Dec 16, 2017 8:18 pm
Image

Vapour chambered hulls?
Sigtrygg wrote:
Sat Dec 16, 2017 4:13 pm
I know this is a low tech thread but - you just got me thinking. Perhaps the liquid hydrogen tanks on TL9+ships are not cryogenic but are a result of gravitics being used to liquefy the hydrogen? Another application of gravitics along with artificial gravity, acceleration compensation and waste heat management.

On low TL ships TL8- you are going to have to have cryogenically cooled liquid hydrogen, but it can be stored in a dewar vessel (vacuum flask) easily enough. Perhaps the fuel tankage on low TL ships need to have a dt surcharge - pumps etc for hydrocarbon fuels, dewar storage for liquid hydrogen and liquid oxygen.
Ooohhh, I like where this is going!

But - if we say the hydrogen is cryogenically cooled, do we then have a power requirement for fuel storage? I think it might also call the idea of collapsible bladders into question even more. Maybe “readily disassembled” would be more appropriate?
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Re: Low tech ships - Need help

Postby Skaran » Sun Dec 17, 2017 4:21 am

And flour.
And when one dreams dark dreams dark days shall follow
AnotherDilbert
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Re: Low tech ships - Need help

Postby AnotherDilbert » Sun Dec 17, 2017 7:30 am

Sigtrygg wrote: I know this is a low tech thread but - you just got me thinking. Perhaps the liquid hydrogen tanks on TL9+ships are not cryogenic but are a result of gravitics being used to liquefy the hydrogen?
As far as I understand H₂ cannot be liquified above 33.2 K.
https://en.wikipedia.org/wiki/Critical_ ... ynamics%29

Metallic hydrogen (H₁) might be a liquid at something like 400 GPa, but the gravity needed would collapse the ship before the hydrogen liquified.
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Re: Low tech ships - Need help

Postby Sigtrygg » Sun Dec 17, 2017 9:31 am

Traveller laser ranges are only possible because of the grav focussing elements that produce gravitic fields strong enough to focus the laser light - if they are strong enough to do that they are strong enough to pressurise hydrogen to enable it to take on its metallic phase.
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Re: Low tech ships - Need help

Postby Condottiere » Sun Dec 17, 2017 3:35 pm

Is the hydrogen in the fuel bladders under pressure?

Because capacity is comparable to actual tank capacity.
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Re: Low tech ships - Need help

Postby Linwood » Sun Dec 17, 2017 4:22 pm

Condottiere wrote:
Sun Dec 17, 2017 3:35 pm
Is the hydrogen in the fuel bladders under pressure?

Because capacity is comparable to actual tank capacity.
I’ve been assuming it must be. Otherwise it’s hard to see how a ship could carry enough hydrogen to meet its needs. Today’s vehicle applications typically use a pressure vessel in the <100 MPa pressure range. Using a storage medium (liquid or solid) or cracking it as you go from water or hydrocarbons probably reduces the pressure required but I believe the trade-off is less net hydrogen per unit volume. Been a while since I’ve worked in that industry so I don’t have the details memorized any more.
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Re: Low tech ships - Need help

Postby AnotherDilbert » Sun Dec 17, 2017 7:13 pm

Linwood wrote: I’ve been assuming it must be.
Hydrogen fuel is canonically stored as liquid hydrogen. Liquid hydrogen does not need to be put under pressure, just kept refrigerated.

I believe you can get slightly higher density with H₂ gas under very low temperatures and very high pressure.
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Re: Low tech ships - Need help

Postby baithammer » Tue Dec 19, 2017 2:35 am

At low temp and high pressure it appears to become a solid rather than a gas state.
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Re: Low tech ships - Need help

Postby Condottiere » Tue Dec 19, 2017 5:26 am

Probably not the ideal form to drive the jump turbines.
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Re: Low tech ships - Need help

Postby AnotherDilbert » Tue Dec 19, 2017 10:27 am

baithammer wrote: At low temp and high pressure it appears to become a solid rather than a gas state.
Don't even need much pressure, just cool it below 14 K. It changes to a different structure at very high pressure.

But the solid state is barely more dense then the liquid phase, 0.086 tonnes/m³ instead of 0.07 tonnes/m³ for liquid.
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Re: Low tech ships - Need help

Postby Condottiere » Tue Dec 19, 2017 11:28 pm

If you're not using it to power the jump drive, solid state is alright.
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Re: Low tech ships - Need help

Postby DivineWrath » Wed Dec 20, 2017 1:38 am

I've been working on small craft lately. I've been giving them 4 units of thrust. I picked that number so that delta v is 1 unit. I also give them enough chemicals for power to last little over a day.

I'm wondering if it is enough to for them to be able to travel between ships and stations orbiting the same planet? How much power and fuel would it really take to make such trips? I was aiming for it to be enough for several trips.

The next questions are for getting to orbit, to the moon, and landing. Obviously, lacking gravity drives, getting off planets and landing again are going to be trouble. Heat shielding covers landing and tells you how to do it. I decided that I would use reduced gravity on the homeworld, maybe 0.8 or 0.7, to make it easier to get off world or even make a space elevator possible at TL 7. I'm wondering if there are any resources for figuring this stuff out?
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Re: Low tech ships - Need help

Postby baithammer » Wed Dec 20, 2017 2:06 am

For non-gravitonic propulsion think in terms of limited bursts of thrust rather than constant thrust.

There are some basic formulas in the MGT main book to do calculations. ( Assumes 10 m/s^2 per point of thrust and g. )
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Re: Low tech ships - Need help

Postby AnotherDilbert » Wed Dec 20, 2017 3:31 am

DivineWrath wrote: I've been working on small craft lately. I've been giving them 4 units of thrust. I picked that number so that delta v is 1 unit. I also give them enough chemicals for power to last little over a day.
I don't understand. An hour or two of reaction fuel should get you anywhere in Earth orbit, including the Moon.

1 hour acceleration at 1 G gives you a speed of 1 h × 3600 s/h × 10 m/s² [~1 G] = 36000 m/s = 36 km/s ≈ 130 000 km/h. You'll need another burn of 1 G for 1 h to slow down again.
Just divide the distance you want to travel by 130 000 and add an hour for the accel and decel to get the travel time. It's not really that simple, but it's a usable first order approximation.

Liftoff from an Earth-sized planet should take 15 - 30 minutes, depending on effective acceleration (according to MT). That does not give you much usable speed, it just gets you off the planet and into orbit.


DivineWrath wrote: I'm wondering if there are any resources for figuring this stuff out?
MegaTraveller Imperial Encyclopedia has tables with suggested travel times to orbit.

I don't think there are any Traveller designs for Space Elevators, but it can be googled, there is quite a lot of thought put into that. I don't think TL7 has materials good enough.
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Re: Low tech ships - Need help

Postby DivineWrath » Wed Dec 20, 2017 5:13 am

AnotherDilbert wrote:
Wed Dec 20, 2017 3:31 am
I don't understand. An hour or two of reaction fuel should get you anywhere in Earth orbit, including the Moon.
I was listing some of the stats for my ships. I thought it was important information. Sorry about the confusion. I didn't ask questions yet. That said, its good to hear that 1g for hour will get me to the moon.
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Re: Low tech ships - Need help

Postby Condottiere » Wed Dec 20, 2017 7:36 am

It might happen in our lifetimes.

I suspect you need long runways at either end; VTOL would appear to slow the process down if non gravitic.
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Re: Low tech ships - Need help

Postby baithammer » Wed Dec 20, 2017 11:15 am

With aero-breaking and a little reserve fuel you can have a fairly contained landing.

Just don't try to launch interplanetary travel from the surface.

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