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Old 07-31-2018, 10:01 AM   #21
SteveS
 
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Default Re: Collapsibles and Drop Tanks

I don't like the drop tank rules much, at least not the warm up for jump, jettison tanks, and jump rules. But a warship that wants a little more acceleration might benefit from drop tanks that come through jump, though maybe not enough.
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Old 07-31-2018, 05:19 PM   #22
Anaraxes
 
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Default Re: Collapsibles and Drop Tanks

Thanks for the example. It's a good illustration of how your system works.

Quote:
Originally Posted by cptbutton View Post
(I assume jump drives can be run at lower settings with lower fuel use, which may not be clear canonically.)
That's the way we played it with the LBBs.
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Old 07-31-2018, 07:14 PM   #23
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Default Re: Collapsibles and Drop Tanks

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Originally Posted by Anaraxes View Post
So what makes them "drop" tanks, as opposed to a ship design that for whatever reason chooses to put the fuel tanks on the exterior of another "main" hull? What's droppable?

The "overly large jump drive" suggests that you have to equip the ship as if the tankage were part of the hull. So why not just build the ship that way with the regular design sequence, and say there are no such things as "drop tanks"? There has to be some practical difference for the naval architects to have a different bit of jargon.

Do you ignore the volume of drop tanks for purposes of ship displacement for performance? "Overly large drives" suggests not, but perhaps you have lesser fuel use, even though the drive is sized for the full displacement including the drop tanks?
Canonically:
you wind up doing 3 parallel ratings...
Core ship only. (Bridge, weapons, and crew based upon this rating)
Ship + Tanks retained after use.
Ship + Tanks dropped with use.

The drive needs to work (ie, rating 1+) only in core and tanks dropped. The ratings are identical between them, except for the additional range and reduced maneuver.

If the drive can still work retaining the tanks, a second set of ratings is generated, for the drives on the combined dTonnage (and in TNE, T4, GT, the mass).

Note that T5 makes clear that tanks dropped are usually destroyed by the jump-wake (My term for Marc's concept). (This also is true for smaller ships, small craft, small rocks, people, etc, within 100 diameters of a jumping ship...)

T5 doesn't change the roughly 15-20 minute run-up for jump.... but the fuel is expended during the run-up, and the tanks can refill the mains, and then be jettisoned; at TL 14+, tanks can be cleared of the ship's jump-wake.

Let's take an example.
A type S uses 20 Td of jump fuel, and has drives rated for J2, M2, and P2 at 100 Td.
If I put extra tanks on the outside, it's a 120 Td hull for rating purposes only.
Which is J1, M1, P1.
If it jumps with tanks retained, it needs 12 tons of fuel for the J1, and can do 2J1, but it cannot do 2J2
If it jumps while dropping tanks, it can do 2J2 - one off the tank fuel and one off the internal. (Technically, both off the internal, but replacing the internal from the external as the drive is drawing it off.)

Now, the thing is, if I design for a J2 with tanks, I build it as a 100 Td ship for everything but fuel and drives; fuel and drives are for a roughly 126 ton ship... because it needs 25.2 Td (round up to 26) of fuel for J2. The (Bk 5) JDrive goes from 3 to 3.6 Td (rounding up to 4), likewise the PP and MD go up. It jumps J2 on the tank fuel, and again on internal.

If, however, we up the internal fuel from 26 to 30, when it drops the tanks, the now 4% drive and 30% fuel can support a full J3. So, if you need 5 PC, you drop the externals, using 26 tons fuel for J2, then jump again on internal 30 Td for J3. Total 5 Pc in 2 weeks.
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Old 08-01-2018, 12:35 AM   #24
jeff_wilson
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Default Re: Collapsibles and Drop Tanks

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Originally Posted by tanksoldier View Post
...everybody realizes recovering the dropped tanks is the norm, right?
How is this done? Wouldn't it introduce an inefficient delay to include a turnover and deceleration to below solar escape velocity to prevent the dropped tanks from vanishing into interstellar space? Or would you include something like a small m-drive to decel the tanks and return them to some local Lagrange point?
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Old 08-01-2018, 02:26 AM   #25
Mike Wightman
 
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Default Re: Collapsibles and Drop Tanks

Civilian ships are usually not moving when they jump, so the remains of the drop tanks are just floating there in normal space, with whatever minor vector they pick up from being jettisoned.

And edited to clarify - the JTAS news item says the drop tanks are destroyed when jettisoned.

Last edited by Mike Wightman; 09-04-2018 at 11:51 AM.
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Old 08-01-2018, 02:30 AM   #26
Mike Wightman
 
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Default Re: Collapsibles and Drop Tanks

Quote:
Originally Posted by ak_aramis View Post
Let's take an example.
A type S uses 20 Td of jump fuel, and has drives rated for J2, M2, and P2 at 100 Td.
If I put extra tanks on the outside, it's a 120 Td hull for rating purposes only.
Which is J1, M1, P1.
If it jumps with tanks retained, it needs 12 tons of fuel for the J1, and can do 2J1, but it cannot do 2J2
Shouldn't that be 3J1? It has 40t of tankage in total and uses 12t per jump 1
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Old 08-01-2018, 10:02 AM   #27
Fred Brackin
 
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Default Re: Collapsibles and Drop Tanks

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Originally Posted by Mike Wightman View Post
Civilian ships are usually not moving when they jump, so the drop tanks are just floating there in normal space, with whatever minor vector they pick up from being jettisoned.
The tanks would need enough impetus to get clear of the ship that's jumping. That's 100 of their own diameters or there's no Jump.

The4n there's whatever chaos jumping wreaks on the close vicinity. Even if it's sjt the jump capcitors that's a lot of energy and I beleive that canon makes Jump entrance pretty highly detectable.
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Old 08-02-2018, 10:26 AM   #28
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Default Re: Collapsibles and Drop Tanks

Quote:
Originally Posted by Mike Wightman View Post
Civilian ships are usually not moving when they jump, so the drop tanks are just floating there in normal space, with whatever minor vector they pick up from being jettisoned.
Where is this writ, please?
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Old 08-02-2018, 10:56 AM   #29
Fred Brackin
 
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Default Re: Collapsibles and Drop Tanks

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Originally Posted by jeff_wilson View Post
Where is this writ, please?
Gurps Traveller (1st ed) p.120 discusses "standing jump" v. "running jump" and indicates that a standing jump is definitely safer.

However, even a standing jump is calculated to give you near zero relative velocity upon exit. This could still be several miles per second relative to the departure point.

I thought the necessity of getting clear of the drop tanks 100D limit was obvious but I have no data on how short the time betwen dropping and jumping has to be. If your tanks were 10 yards long you'd need to be 1000 yards away when you tried to jump. The issue is whether they need to reach that distance in something like 1 minute or 1 second.

If it had to be 1 second you'd blow the tanks into confetti so the 100D would be a very small number rather than trying to leave them in recoverable condition.

<shrug>As I said I have no data on the time frame but it's just one more complication that makes drop tanks a bad idea IMTU.
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Old 08-02-2018, 11:06 AM   #30
Mike Wightman
 
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Default Re: Collapsibles and Drop Tanks

Too many places to list them all, but here is a selection:
Quote:
Although jumps are usually made at low velocities, the speed and direction which a ship held prior to jump is retained when it returns to normal space.
High Guard ('80)

Then there is MWM's definitive jumpspace article in JTAS 24:

Quote:
The typical jump begins on a world surface when a ship prepares to leave. Completely fuelled and crewed, the ship leaves the world and proceeds to a point more than I00 diameters out. Trips are planned so that the ship reaches the jump point with zero velocity.
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