Fuel Expenditure from Surface to Highport

Discuss the Traveller RPG and its many settings
bklokis
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Fuel Expenditure from Surface to Highport

Postby bklokis » Wed Jul 24, 2019 5:49 pm

It just occurred to me recently that the majority of the fuel in a modern-day shuttle is used breaking the planet's gravity well, which is why Highports are way more efficient than Downports for point-to-point destinations.

There is a time table for surface-to-orbit and orbit-to-surface travel in the PoD Book 1, however, I don't recall seeing anything about fuel expenditure.

From a Traveller perspective, wouldn't there be an extra expenditure of fuel over and above that used in a typical 'X-weeks of operation' sense if the Travellers are landing at system Downports rather than Highports, or scooping fuel from within the gravity well of a gas giant? Or is this still negligible in regard to the amount of fuel on hand for basic operations?
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AnotherDilbert
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Re: Fuel Expenditure from Surface to Highport

Postby AnotherDilbert » Wed Jul 24, 2019 6:11 pm

"Using more fuel exiting the gravity well" presumes that spaceflight is mostly coasting with short bursts of acceleration (like today). Traveller spacecraft generally does not work like that; they accelerate fully from take-off to jump.

Traveller M-drives does not use fuel, just power from the PP; the PP uses fuel to produce power.

So, yes, the extra fuel used to run the PP slightly hotter for acceleration is negligible.
bklokis
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Re: Fuel Expenditure from Surface to Highport

Postby bklokis » Wed Jul 24, 2019 6:21 pm

AnotherDilbert wrote:
Wed Jul 24, 2019 6:11 pm
So, yes, the extra fuel used to run the PP slightly hotter for acceleration is negligible.
Makes sense, thank you!
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Moppy
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Re: Fuel Expenditure from Surface to Highport

Postby Moppy » Wed Jul 24, 2019 8:26 pm

Maneuver drive doesn't use fuel in most Traveller editions. It's powered by the powerplant (this is a science fantasy game).

As of 5.10, T5 ships also now have anti-grav lifters as standard because 1G ships unassisted lack the acceleration to get off a >1G planet's surface. So even less maneuvering energy required.
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Re: Fuel Expenditure from Surface to Highport

Postby bklokis » Wed Jul 24, 2019 9:40 pm

Moppy wrote:
Wed Jul 24, 2019 8:26 pm
(this is a science fantasy game).
But I always thought it was a Hard Sci-Fi game...:)
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Juums
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Re: Fuel Expenditure from Surface to Highport

Postby Juums » Wed Jul 24, 2019 9:52 pm

bklokis wrote:
Wed Jul 24, 2019 9:40 pm
But I always thought it was a Hard Sci-Fi game...:)
It is!

Very hard.

To ignore all of the magitech and call it "hard sci-fi".
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Re: Fuel Expenditure from Surface to Highport

Postby Pyromancer » Wed Jul 24, 2019 10:19 pm

bklokis wrote:
Wed Jul 24, 2019 5:49 pm
It just occurred to me recently that the majority of the fuel in a modern-day shuttle is used breaking the planet's gravity well, which is why Highports are way more efficient than Downports for point-to-point destinations.

There is a time table for surface-to-orbit and orbit-to-surface travel in the PoD Book 1, however, I don't recall seeing anything about fuel expenditure.

From a Traveller perspective, wouldn't there be an extra expenditure of fuel over and above that used in a typical 'X-weeks of operation' sense if the Travellers are landing at system Downports rather than Highports, or scooping fuel from within the gravity well of a gas giant? Or is this still negligible in regard to the amount of fuel on hand for basic operations?
The big difference is that a modern-day rocket has to carry reaction mass, and with that, you run into the exponential growth of the Tsiolkovsky rocket equation. Traveller ships use reactionless drives, avoiding this whole problem.
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Re: Fuel Expenditure from Surface to Highport

Postby Moppy » Wed Jul 24, 2019 11:57 pm

Juums wrote:
Wed Jul 24, 2019 9:52 pm
bklokis wrote:
Wed Jul 24, 2019 9:40 pm
But I always thought it was a Hard Sci-Fi game...:)
It is!

Very hard.

To ignore all of the magitech and call it "hard sci-fi".
Zhodani are like jedi, only jedi can't teleport - apart from one that Darth.

edit: I guess there might be some others, but it's really rare.
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Re: Fuel Expenditure from Surface to Highport

Postby Reynard » Thu Jul 25, 2019 1:47 am

Use the High Guard rules to build reaction rockets rather than reactionless maneuver drives and integral lifter technology. Have the reaction drives for leaving an atmosphere and assume a few piloting rolls to safely guide back to the Downport without power. I'm thinking those craft will regularly feature aerofins. Very quickly you see why most civilizations are so glad for gravitic technology.

A really big advantage for Highports is ship capacity and the ability to serve huge ships with partial and no streamlining who save a fortune just in hull costs.
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Re: Fuel Expenditure from Surface to Highport

Postby Condottiere » Thu Jul 25, 2019 8:13 pm

Ignoring the gravitic motors and drives, I believe a two gee reactionary rocket should place you in orbit in one turn, which is what, half to a fiftieth of a percent in fuel?
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Re: Fuel Expenditure from Surface to Highport

Postby Reynard » Thu Jul 25, 2019 10:22 pm

Surface to orbit at 2g with a reaction drive takes 24 minutes. It burns 5% of the hull size in fuel per hour, in this case 2% for a 24 minutes. For a 100 ton ship, that's 2 tons fuel just to get off world. Same amount to land. 2g maneuver for a 100 ton ship uses 20 power (2 tons fusion 8 powerplant) which uses a fraction of ton of fuel out of the power total for at least 4 weeks continuous maneuver use.

If we ever were to discover gravitic technology, it will set us free upon the our local system backyard to explore, expand and exploit.
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Re: Fuel Expenditure from Surface to Highport

Postby Moppy » Fri Jul 26, 2019 12:30 am

On Earth, Delta-V for the ISS's orbit is 9.4 km/s including air resistance (wikipedia). Assuming g is 10 m/s-2 that's 940 seconds at 1g or about 15 minutes. Half that for 2g.

Mongoose 2 High Guard wants 2.5% volume for 1g for one hour. We're using 15 minutes, so it's about a quarter of that, or 0.625 dtons per 100 tons of ship.

Not much fuel required. You can save more by doing a glider landing like the space shuttle did, but then you need runway and it might not be worth it for the fuel saving.

edit: Surface to geostationary orbit (about 40,000 km) is a total delta-V of 13.63 km/s or 23 minutes at 1g. I see you've used the Traveller distance formula for a distance of 10K km, which doesn't compute the same thing. But I can see why one might do that in the Traveller game.
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Re: Fuel Expenditure from Surface to Highport

Postby Reynard » Fri Jul 26, 2019 12:50 am

If you're using a real life rocket motor, how much dton equivalent fuel does it take to achieve orbit? From what I remember, it's immense.

I was using Traveller orbit distance of 10,000 km and a 2g Traveller reaction motor taking 1414 seconds (24 minutes or 4 rounds).
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Re: Fuel Expenditure from Surface to Highport

Postby Moppy » Fri Jul 26, 2019 1:23 am

Reynard wrote:
Fri Jul 26, 2019 12:50 am
If you're using a real life rocket motor, how much dton equivalent fuel does it take to achieve orbit? From what I remember, it's immense.
By mass, around 20:1 for LEO (whole rocket including fuel, vs payload)

I'm not sure why you can't mix the numbers though. The maneuver drive obeys newton's laws for velocity addition.
Sigtrygg
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Re: Fuel Expenditure from Surface to Highport

Postby Sigtrygg » Fri Jul 26, 2019 8:38 am

Rocket science is pretty well understood.

The MgT reaction drive fuel use rates are too small by an order of magnitude at least.

You can swap the numbers around, you just have to make sure you are using the correct equations.

delta V = exhaust velocity x ln (total mass/mass without fuel)

or you can use delta V = specific impulse x g x ln (total mass/mass without fuel)

specific impulse can be usefully calculated using

specific impulse = force of thrust / fuel mass flow rate.

Note the reason why you can not just 'move the numbers around' - a natural logarithmic function and or a rate of change...
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Re: Fuel Expenditure from Surface to Highport

Postby WingedCat » Fri Jul 26, 2019 6:03 pm

bklokis wrote:
Wed Jul 24, 2019 5:49 pm
It just occurred to me recently that the majority of the fuel in a modern-day shuttle is used breaking the planet's gravity well, which is why Highports are way more efficient than Downports for point-to-point destinations.
For modern tech - in Traveller parlance, pre-gravitic low-TL low-efficiency reaction drives - yes. This is a large part of why space access is so expensive today, and why NASA et al are talking about an orbital fuel depot (which would be the barest beginning of a highport).

Once you have maneuver drive and all the other trappings of TL 9+, the concern is more about aerodynamics: large ships made cheaply are often not very compatible with atmospheres, and thus need a way to unload their cargo in orbit (unless they only ever visit asteroid belt and vacuum worlds). Atmo-capable streamlined smallcraft are one answer; highports are a more convenient answer.
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Re: Fuel Expenditure from Surface to Highport

Postby ShawnDriscoll » Fri Jul 26, 2019 6:08 pm

bklokis wrote:
Wed Jul 24, 2019 5:49 pm
It just occurred to me recently that the majority of the fuel in a modern-day shuttle is used breaking the planet's gravity well, which is why Highports are way more efficient than Downports for point-to-point destinations.

There is a time table for surface-to-orbit and orbit-to-surface travel in the PoD Book 1, however, I don't recall seeing anything about fuel expenditure.

From a Traveller perspective, wouldn't there be an extra expenditure of fuel over and above that used in a typical 'X-weeks of operation' sense if the Travellers are landing at system Downports rather than Highports, or scooping fuel from within the gravity well of a gas giant? Or is this still negligible in regard to the amount of fuel on hand for basic operations?
See 57th century travel.
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Re: Fuel Expenditure from Surface to Highport

Postby Reynard » Fri Jul 26, 2019 8:16 pm

Maneuver drives use 'power' produced by the power plant. That power is normally allocated in the PP's design so it's available any time as long as the plant works. No one thinks about it but the High Efficiency Batteries can also power the maneuver drive in 1 round (6 minute) increments at the power level the drive uses. Whether maneuvering in combat, a straight flight from Earth to Mars or landing/taking off a world it's the same power from the plant for the operational duration of the fuel load often measured in weeks or more.

Question can be if I shut down the maneuver drive do I save fuel proportional to the amount of power it uses? As far I know the rules don't make such an allowances possibly because the power produced by the fuel over weeks to operate the maneuver drive is insignificant to bean count. I do consider shutting down the maneuver allows its power to be reallocated to other systems as needed. Very often my designs require the maneuver drive power be allocated to power the jump which only runs just before jumping. Other power might also be rerouted to the jump which is what jump dimming is.

Reaction drives have the problem of burning fuel measured in hours rather than days, weeks or months and it burns a lot for what it produces and only for the reaction engine. All other operations are coming from the power plant whether the rocket is on or off.
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Re: Fuel Expenditure from Surface to Highport

Postby AndrewW » Fri Jul 26, 2019 9:57 pm

Reynard wrote:
Fri Jul 26, 2019 8:16 pm
Question can be if I shut down the maneuver drive do I save fuel proportional to the amount of power it uses? As far I know the rules don't make such an allowances possibly because the power produced by the fuel over weeks to operate the maneuver drive is insignificant to bean count. I do consider shutting down the maneuver allows its power to be reallocated to other systems as needed. Very often my designs require the maneuver drive power be allocated to power the jump which only runs just before jumping. Other power might also be rerouted to the jump which is what jump dimming is.
Yes, the maneuver drive only needs power if it is operating, if it isn't feel free to allocate it's share of the power plants output elsewhere.
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Re: Fuel Expenditure from Surface to Highport

Postby Condottiere » Sat Jul 27, 2019 7:52 pm

1. Medium earth orbit is between two and thirty six kay klicks, then you have to add in decreasing gravitational pull from the planet, but amend for breaking manoeuvre if you want to dock at the space station and not just have a fly by; yeah, gravitational motors and thrusters are easier to deal with.

2. I'm not a nuclear engineer, so I don't know if you can regulate output by limiting input of hydrogen fuel; but if you can, you can certainly save gas.

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