Author Topic: How badly do BT fusion rockets break physics?  (Read 10045 times)

GurlPower

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How badly do BT fusion rockets break physics?
« on: 09 December 2017, 07:42:55 »
So I've been wanting to get back into the Aerospace side of battletech and I've been on a realism kick these past few years. I know jump drives aren't realistic and the armor is magic foil, it's called suspension of disbelief so quiet you! But what I do remember was Cray saying way *way* back in the day that aerospace fusion engines are *actual* magic, being that they are so efficient that their drives expel hydrogen at velocities faster than the speed of light, which is just bad form for a space nerd like me.

So I wanna home brew a blanket nerf to drives by making them get less fuel points per ton of fuel. And the question is: how inefficient do I need to make the drives for them to be physically possible? Double the fuel tons per fuel points? Triple? Quintuple? An inquiring mind would like to know.

Daryk

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Re: How badly do BT fusion rockets break physics?
« Reply #1 on: 09 December 2017, 09:13:23 »
Cray was partially right... the exhaust wouldn't be super-luminal, but the energy efficiency required to obtain those thrusts with that amount of reaction mass exceeds what could be generated by pure matter-antimatter annihilation.

Personally, I've always favored cutting "strategic" thrust down by a factor of 10 (leaving fuel consumption in this mode where it is), and leaving tactical thrust alone (since it has a much higher fuel consumption).

cray

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Re: How badly do BT fusion rockets break physics?
« Reply #2 on: 09 December 2017, 15:49:31 »
Personally, I've always favored cutting "strategic" thrust down by a factor of 10 (leaving fuel consumption in this mode where it is), and leaving tactical thrust alone (since it has a much higher fuel consumption).

That's not bad for adding some crunchiness back to the numbers.

An alternative is to eliminate strategic fuel mode and only use tactical fuel consumption, but that complicates navigation and travel times.
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Daryk

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Re: How badly do BT fusion rockets break physics?
« Reply #3 on: 09 December 2017, 16:10:26 »
I think tactical fuel consumption on the strategic scale would result in fuel tanks being too small across the board, at least if people continued to drive around at a full 1G acceleration.  Of course, that might be the underlying reason to drop strategic thrust by a factor of 10, so maybe it could work...  Thanks Cray!

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Re: How badly do BT fusion rockets break physics?
« Reply #4 on: 10 December 2017, 17:04:58 »
I think tactical fuel consumption on the strategic scale would result in fuel tanks being too small across the board

Oh, sorry, I was working with the assumption that fuel tanks would be re-sized. Most WarShips have plenty of cargo room for the change. DropShips might need a big redesign. JumpShips...well, if they could work at an extreme fractional thrust would be fine for stationkeeping.
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**"A man walks down the street in that hat, people know he's not afraid of anything." --Wash, Firefly.
**"Well, the first class name [for pocket WarShips]: 'Ship with delusions of grandeur that is going to evaporate 3.1 seconds after coming into NPPC range' tended to cause morale problems...." --Korzon77
**"Describe the Clans." "Imagine an entire civilization built out of 80’s Ric Flairs, Hulk Hogans, & Macho Man Randy Savages ruling over an entire labor force with Einstein Level Intelligence." --Jake Mikolaitis


Disclaimer: Anything stated in this post is unofficial and non-canon unless directly quoted from a published book. Random internet musings of a BattleTech writer are not canon.

Daryk

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Re: How badly do BT fusion rockets break physics?
« Reply #5 on: 10 December 2017, 17:11:05 »
Driving around at 0.1G constant acceleration is still relatively fast for in system transit, so DropShips wouldn't need too much redesign.  And JumpShips don't really need too much for station keeping, either.  As I recall, gravitational fields near a typical zenith or nadir jump point are on the order of 10-6...

idea weenie

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Re: How badly do BT fusion rockets break physics?
« Reply #6 on: 13 December 2017, 19:12:25 »
Oh, sorry, I was working with the assumption that fuel tanks would be re-sized. Most WarShips have plenty of cargo room for the change. DropShips might need a big redesign. JumpShips...well, if they could work at an extreme fractional thrust would be fine for stationkeeping.

You'd have to make a new rule for station-keeping where vehicles at the jump zones (i.e. not near a gravity source) can spend any amount of time they wish at that location, with no short-term noticeable fuel use.  (10^-6 Gs means that for ~400 days you'd only burn 4 * 10^-4 burn-days of fuel).  You'd burn fuel to maneuver into the jump limit, decelerate, then later thrust out of the jump zone, but that might only be .1 Burn-day.

This way you can have Jumpships hanging out just inside the Zenith/Nadir jump zones, slowly absorbing solar energy, and when it is time for them to leave they thrust outside the jump limit, attach Dropships, and jump out.

Phrase maneuvers in terms of Burn-days, and that way players can determine which one to do.

Driving around at 0.1G constant acceleration is still relatively fast for in system transit, so DropShips wouldn't need too much redesign.  And JumpShips don't really need too much for station keeping, either.  As I recall, gravitational fields near a typical zenith or nadir jump point are on the order of 10-6...

Going at .1G means it will take ~3 times as long as going there at 1G (going meaning accelerating towards then decelerating towards the destination), while burning ~1/3 the fuel.  The specific multiplier/divisor values are:
k = [1G / (your acceleration in G)] ^(1/2)

Transit time = (Time @ 1G) * k
Fuel consumption = (Fuel consumption @ 1G) / k

To make the math easy, we could prefer to use 1/n times G, where n is a square value (4, 9, 16, 25, aso).
« Last Edit: 06 January 2018, 22:03:18 by idea weenie »

Daryk

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Re: How badly do BT fusion rockets break physics?
« Reply #7 on: 13 December 2017, 19:15:59 »
I figured people either already knew it was a factor of the square root of 10, or didn't care to know... I see you're in the first group... :)

SCC

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Re: How badly do BT fusion rockets break physics?
« Reply #8 on: 16 December 2017, 03:24:04 »
The idea that JumpShips need to burn fuel for station keeping really should be dropped. After all who keeps Rest Stop supplied with fuel? (Yes I know that a space station, but the point still stands)

HobbesHurlbut

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Re: How badly do BT fusion rockets break physics?
« Reply #9 on: 16 December 2017, 09:14:48 »
The idea that JumpShips need to burn fuel for station keeping really should be dropped. After all who keeps Rest Stop supplied with fuel? (Yes I know that a space station, but the point still stands)
Ermm.....it was explained in the books why they had to burn fuel to maintain their position.
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Re: How badly do BT fusion rockets break physics?
« Reply #10 on: 16 December 2017, 14:21:44 »
Rest Stop is a system in the deep periphery with a recharge station, the problem is the write up doesn't say who's been refueling it since the SL went kaput.

Liam's Ghost

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Re: How badly do BT fusion rockets break physics?
« Reply #11 on: 17 December 2017, 17:22:13 »
Rest Stop is a system in the deep periphery with a recharge station, the problem is the write up doesn't say who's been refueling it since the SL went kaput.

All they need is to harvest ice from a passing comet. They can crack the water for hydrogen fuel, as well as oxygen if the air is getting stale.

Depending on the area it's located in, they could also probably dip into the edge of the atmosphere of a gas giant and collect hydrogen from there. There's lots of options, and if you're going to the expense and effort to establish a recharge station, presumably you'd do it somewhere where fuel isn't a problem.
Good news is the lab boys say the symptoms of asbestos poisoning show an immediate latency of 44.6 years. So if you're thirty or over you're laughing. Worst case scenario you miss out on a few rounds of canasta, plus you've forwarded the cause of science by three centuries. I punch those numbers into my calculator, it makes a happy face.

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SCC

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Re: How badly do BT fusion rockets break physics?
« Reply #12 on: 18 December 2017, 01:52:38 »
That's not really the problem, the problem is the question of who's doing it.

Liam's Ghost

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Re: How badly do BT fusion rockets break physics?
« Reply #13 on: 18 December 2017, 02:22:32 »
That's not really the problem, the problem is the question of who's doing it.

The people who live there, obviously. It's had a lot of owners over the generations, and the planetary system provides all the resources they need.
Good news is the lab boys say the symptoms of asbestos poisoning show an immediate latency of 44.6 years. So if you're thirty or over you're laughing. Worst case scenario you miss out on a few rounds of canasta, plus you've forwarded the cause of science by three centuries. I punch those numbers into my calculator, it makes a happy face.

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SCC

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Re: How badly do BT fusion rockets break physics?
« Reply #14 on: 18 December 2017, 03:43:06 »
I'm going to chalk this one up to FASAnomics because there's no way such an isolated outpost would be otherwise able to support itself.

I'm also going to try and not wonder about what they charged passing JS and DS in fees.

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Re: How badly do BT fusion rockets break physics?
« Reply #15 on: 18 December 2017, 05:08:09 »
I'm going to chalk this one up to FASAnomics because there's no way such an isolated outpost would be otherwise able to support itself.

I'm also going to try and not wonder about what they charged passing JS and DS in fees.

The system has two gas giants and multiple asteroid belts, so water ice would be reasonably plentiful for water, fuel and oxygen needs. They got out there in the first place, so they have spacefaring capability to allow them to harvest these resources, their population is small enough that hydroponics is a reasonable solution for most of their food needs, and they have sufficient traffic to and from the station to justify the presence of a bar and a brothel. I don't see the problem.
Good news is the lab boys say the symptoms of asbestos poisoning show an immediate latency of 44.6 years. So if you're thirty or over you're laughing. Worst case scenario you miss out on a few rounds of canasta, plus you've forwarded the cause of science by three centuries. I punch those numbers into my calculator, it makes a happy face.

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pheonixstorm

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Re: How badly do BT fusion rockets break physics?
« Reply #16 on: 18 December 2017, 06:35:51 »
Driving around at 0.1G constant acceleration is still relatively fast for in system transit, so DropShips wouldn't need too much redesign.  And JumpShips don't really need too much for station keeping, either.  As I recall, gravitational fields near a typical zenith or nadir jump point are on the order of 10-6...

Not really fast enough, not for insystem transit anyway. Remember, some systems have a 30 day transit time from the jump point. How long would that take at .1G??

BirdofPrey

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Re: How badly do BT fusion rockets break physics?
« Reply #17 on: 18 December 2017, 06:57:38 »
The idea that JumpShips need to burn fuel for station keeping really should be dropped. After all who keeps Rest Stop supplied with fuel? (Yes I know that a space station, but the point still stands)
Why wouldn't they need to burn fuel to maintain their position?  They aren't actually in orbit, so they are going to start falling.
That said, given how long that would actually take, there's not a real pertinent reason not to just let yourself fall towards the star.

Stations on the other hand, should probably remain stationary.

Daryk

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Re: How badly do BT fusion rockets break physics?
« Reply #18 on: 18 December 2017, 07:59:40 »
Not really fast enough, not for insystem transit anyway. Remember, some systems have a 30 day transit time from the jump point. How long would that take at .1G??
And those systems routinely use pirate points, but to answer your question, if a 1G transit takes 30 days, a 0.1G transit would take around 100 days (you just multiply the first result by the square root of ten, as was mentioned up thread).

SCC

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Re: How badly do BT fusion rockets break physics?
« Reply #19 on: 18 December 2017, 13:39:24 »
Why wouldn't they need to burn fuel to maintain their position?  They aren't actually in orbit, so they are going to start falling.
That said, given how long that would actually take, there's not a real pertinent reason not to just let yourself fall towards the star.

Stations on the other hand, should probably remain stationary.
Because that far out acceleration towards the star would only be like 1/1000 of a meter per second, relatively simple to jump in far enough out that it's not a problem.

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Re: How badly do BT fusion rockets break physics?
« Reply #20 on: 18 December 2017, 14:11:50 »
Because that far out acceleration towards the star would only be like 1/1000 of a meter per second, relatively simple to jump in far enough out that it's not a problem.

A meter per second is speed, not acceleration.
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Re: How badly do BT fusion rockets break physics?
« Reply #21 on: 18 December 2017, 14:17:51 »
Rest Stop is a system in the deep periphery with a recharge station, the problem is the write up doesn't say who's been refueling it since the SL went kaput.

If JumpShips are using it, then it's being refueled by JumpShips as they stop by. If it isn't in use, it can be in a parking orbit instead of thrusting to stay near the zenith/nadir point.
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idea weenie

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Re: How badly do BT fusion rockets break physics?
« Reply #22 on: 20 December 2017, 19:09:55 »
Why wouldn't they need to burn fuel to maintain their position?  They aren't actually in orbit, so they are going to start falling.
That said, given how long that would actually take, there's not a real pertinent reason not to just let yourself fall towards the star.

Too true.  A Jumpship should only need to burn fuel twice.  Once to thrust inside the jump limit so it can park and recharge, and a second time to get outside the limit so it can jump.  The rest of the time it uses internal gyros to keep itself oriented towards the star.

Stations on the other hand, should probably remain stationary.

Stations just have to offset the gravity at the zenith/nadir point.  From Daryk's post, the acceleration is ~10^-6 Gs, That means a station would only need 1 Burn-Day of fuel for every (1/(10^-6)) million days it was on station (or about 2700 years).  Even if you multiplied that by 10 to allow for various maneuvers, losses, and similar stunts, that means if the station started thrusting at the fall of the Star League, it would have only used up 1 Burn-day of fuel by the time the Clans invaded.

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Re: How badly do BT fusion rockets break physics?
« Reply #23 on: 20 December 2017, 20:31:44 »
The fuel used for thrust wouldn't include life support and such, though.  I'm willing to cut the writers some slack on that score.

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Re: How badly do BT fusion rockets break physics?
« Reply #24 on: 20 December 2017, 22:00:22 »
The fuel used for thrust wouldn't include life support and such, though.  I'm willing to cut the writers some slack on that score.
Given that charge limits for JumpShips recharging their K-F Drives has to do with the K-F Drives themselves and not the supply of electricity I'm prepared to say that there's enough electricity generated by the sails to both charge the K-F Drives AND supply the ships general systems.

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Re: How badly do BT fusion rockets break physics?
« Reply #25 on: 20 December 2017, 22:04:58 »
The numbers are certainly fuzzy enough to support that position too.

cray

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Re: How badly do BT fusion rockets break physics?
« Reply #26 on: 22 December 2017, 16:06:32 »
I'm going to chalk this one up to FASAnomics because there's no way such an isolated outpost would be otherwise able to support itself.

You say "isolated outpost" I say "important transportation infrastructure speeding interstellar commerce." Recharge stations are in the middle of cargo transfer from JumpShips to DropShips to the local habitable planet. Any of the next dozen passing DropShips could drop off some spare hydrogen.

See p. 138 Strategic Operations for a discussion of the utility of Recharge stations.

That's not really the problem, the problem is the question of who's doing it.

The local planetary government's Department of Transportation, who wants the benefits of recharge stations on interstellar shipping? Like StratOps points out, besides recharging, "Recharge stations almost always have additional duties. They act as refueling platforms, cargo transshipment points, entrepôts and customs checkpoints." They're busy, lucrative space ports.
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Disclaimer: Anything stated in this post is unofficial and non-canon unless directly quoted from a published book. Random internet musings of a BattleTech writer are not canon.

idea weenie

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Re: How badly do BT fusion rockets break physics?
« Reply #27 on: 23 December 2017, 21:11:35 »
The fuel used for thrust wouldn't include life support and such, though.  I'm willing to cut the writers some slack on that score.

You have a multi-kiloton fusion power plant when only a half ton plant is needed to power an energy weapon.  Humans might need 10 kW each for life support (assuming 10:1 for energy conversion between trophic layers, and assuming there is a trophic layer between the fusion plant and human consumption.  Figure electricity is produced, it is used to power algae reclamation (with mussels and fish for extra protein/flavors), and humans eat the algae.

The nice part is you don't have to worry about keeping the ship warm, as space is a very effective vacuum.  You just have to keep the people inside sufficiently fed and oxygenated (plus water filtration/purification, sewage, etc).

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Re: How badly do BT fusion rockets break physics?
« Reply #28 on: 23 December 2017, 21:15:02 »
And all the tons and tons of helium and hydrogen at cryogenic temperatures in the same ship.  Vacuum helps, but there's still some work to be done there...

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Re: How badly do BT fusion rockets break physics?
« Reply #29 on: 25 December 2017, 06:28:45 »
You say "isolated outpost" I say "important transportation infrastructure speeding interstellar commerce."
I've constructed a simplistic business case around a BT water-based fuel tanker dropshuttle in the 22nd century (in the sense of those mentioned in SB House Kurita) before, and realistically it doesn't bring in that much money - basically the resulting profit is on a scale of 10-12% of the value of sold hydrogen while servicing interest in the 10% range on upfront cash investment, and even that requires stable trade routes yielding regular customers for your hydrogen.

The concept doesn't use a - manpower-intensive - permanent space station but instead a dropshuttle waiting for passing jumpships to refuel; one could see it as a precursor operation to a permanent space station which would need augmentation once the host planet also wants some cargo shipped in or out. Overall it makes for a viable enterprise on its own, but the financial impact on the host colony is rather small and at this level probably at best about sufficient to keep a small spaceport as ground support operational and staffed. The meta-impact of the system remaining tied into the interstellar network beyond just message exchanges with passing jumpships is probably worth it.