That reads mostly right to me, and would float in most games.
I've moved into a T5 rules mindset though.

In T5, you can get a Modified or Improved J-drive that's more
efficient, if your build TL is a couple levels higher than the
introduction TL of the Jump drive.
According to  "T5.10 Book 2 p76" , if you're building a TL 13+ ship,
you can put in a Modified jump drive:  110% jump efficiency, 90% fuel
usage. ...and 50% the volume of the original drive, so you get back
another 5 tons. On the other hand, T5 makes you pay a 1 ton for fuel
scoops and such, so that 5 tons is going to get used up on other
things. ...
Including 1 ton for the grapple for holding an external pod of up to 35 tons.

The +10% on jump covers the volume of the drop tanks, so you're jump-2
even with the extra tonnage.
The 90% fuel usage is ... well.

Jump with fuel tanks: =24 tons * 0.9 = 21.6 tons
Jump without tanks: 18 tons

So my T5 Type-S-with-jump-tanks build would be :
Modified TL-11 drive, get 5 tons back in drive, 2 tons in fuel back
(18 tons of jump fuel in the ship) with the jump configured for
"Bubble" rather than "Plates"
 Spend 2 tons on fuel scoop, intake,..
1 ton on fuel transfer pump, 1Mcr.
2 grapples (Port & Starboard underbody), +2 Mcr,  2 tons
Pod Options:
10 ton fuel pods -- hold 9 tons each, 1 ton taken up by grapples.
Gives a jump 2 when installed. Note that these pods include Z-drive
lifters, so inside the 1D limit they have really crummy performance
but can be moved on their own without much hassle. You could design
them as remote-operated fueling drones...
20 ton fuel pods: Hold 19 tons. Could put one on one grapple and have
Jump-2+Jump-2
30 ton... oh. Grapples hold up to 35 tons. You could stick standard
cutter modules under the wings.... You drop to jump-1, but a 30 ton
fuel module would be 29 tons of fuel. Add in 18 in the inboard tank,
and that's 47 tons. That's enough for Jump-1 out, drop the pod(s), and
jump back home.

On Wed, Nov 27, 2019 at 11:21 AM Thomas RUX <xxxxxx@comcast.net> wrote:
>
> Morning from the Pacific Northwest,
>
> I'm working on trying to get the use of drop tanks right and I'm running into a snag.
>
> My understanding is that the jump drive number or rating is the maximum range in parsecs that a ship can travel using the entire jump fuel load out.
>
> The information I've gleaned indicates that the fuel in drop tanks is used before the fuel carried in the ship's internal fuel tankage and that with the tanks mounted increases the hull tonnage until they are jettisoned.
>
> I have an 100 ton ton hull with a J-2 drive that requires 20 tons of fuel to make one 2 parsec jump.
>
> By installing 2 drop tanks each carrying 10 tons of fuel for a total of 20 tons which would increase hull to 120 tons. Using the jump fuel fuel formula a 120 ton hull needs 0.1 x 120 x J2 = 120 x J2 = 24 tons of fuel to make one 2 parsec jump and 12 tons for an one parsec jump.
>
> When retaining the drop tanks the 20 tons in the tanks are funneled to the jump and another 4 tons comes from the internal fuel tank, leaving 16 tons, allowing the ship to make a 2 parsec jump lasting between 6.3 and 7.7 days. With remaining 16 tons the ship could make a single one parsec jump consuming 12 tons of fuel leaving 4 tons. In this case the ship could cove 3 parsecs in 12.6 to 15.4 days plus the time needed to calculate the second jump.
>
> When jettisoning the drop tanks just before jump space entry the ship would make a 2 parsec jump and when arriving at the destination could make a second 2 parsec jump.
>
> How far off track am I with the concept of drop tanks and jump drive number/rating?
>
> Tom Rux
>
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