On Sat, 26 Nov 2016 22:26:35 -0500, Rob Davenport
<xxxxxx@gmail.com> wrote:

>So a J-6 ship could "double back" cheaply?  Jump out 1pc (40 hours, 12
>fuel) and jump back (another 40 hours 12 fuel) - 80 hours total, much
>faster than normal (300 hours).   Could be useful to get out of a jam
>and come back on scene after 80 hours of repairs, rest, healing, to
>"try again".

Yes - but remember that when you built that ship, you paid for the
full jump-6 capability, which means that the drive is large. You also
have to pay for the extra fuel capacity.

As an example, let's take a Marava hull (hull rate 200) and refit it
for a J6 drive but max distance J2 (it will be able to do out-and-back
J1 without refueling):

Using the CE SRD for reference (because I have it handy), in a Marava
hull (hull rate 200), you'd have a 35-dton drive (Drive Code F for J6)
instead of a 15-dton drive (Drive Code B for J2). Right there, you're
losing 20 tons of cargo capacity, per jump.

Remember that the fuel usage is percent of hull rate, not dtons of
fuel. For a Marava hull with a standard J2 drive, you need to allocate
20% to fuel, or 40 dtons. To do the J2 with the J6 drive, you need to
allocate 24% to fuel, or 48 dtons. That represents an additional loss
of 8 tons of cargo capacity, per jump.

So, your modded Marava has only 54 dtons of cargo capacity, vs the 82
of a 'stock' Marava. That's going to play hell with the commercial
viability, whether you use Ken Pick's Commercial Efficiency Rating to
calculate it, or work up the financial profile the way I did.

For military ships, the loss of mission space may be considered
worthwhile, especially if your enemy doesn't know you have this drive.
I don't think you'll see this among independent traders, and even
large-corporation rosters are unlikely to have a large proportion of
ships with uprated drives.

>On Sat, Nov 26, 2016 at 6:58 PM, Jeff Zeitlin <xxxxxx@freelancetraveller.com> wrote:

>>Depending on the reaction here, I may work this up into a full
>>alternate-tech article for Freelance Traveller, much like I did with
>>the Lyman Drive
>>(http://www.freelancetraveller.com/features/rules/tech/lymanjd.html,
>>reprinted from 2002 in the April 2013 issue).
>>
>>The core of the idea: A Jump-n drive can (usually) do Jump-m (m<n) in
>>less time than it takes to do a Jump-n. However, it takes more fuel to
>>do so than it would take a Jump-m drive, and the time required to do
>>multiple jumps, even if there is zero delay between emergence from one
>>jump and entry into the next, is greater than the single jump would
>>be.
>>
>>A first cut at numbers:
>>
>>Assume that N is the rated jump capability of the drive, and that it
>>uses fuel according to the standard rules (e.g., for CT, 10% of hull
>>rate * N). Then, for a Jump M where M is strictly less than N,...
>>
>>   Fuel usage = M/(N-1) times the standard fuel usage.
>>
>>Time: 168 hours is annoying to work with - it just happens to match up
>>nicely with "a week". Let's tweak that: A basic jump isn't "a week",
>>though it's generally treated as such on the calendar; it's 150 hours.
>>Given that Jump N takes 150 hours, for a Jump M, where M is strictly
>>less than N,...
>>
>>   Time required for jump = M/(N-1) times 200 hours.
>>
>>In tabular form:
>>
>>Fuel (% of hull rate)
>>
>>Drive     Jump 6   Jump 5   Jump 4   Jump 3   Jump 2   Jump 1
>>J6 Drive  60       60       48       36       24       12
>>J5 Drive  --       50       50       38       25       13
>>J4 Drive  --       --       40       40       27       13
>>J3 Drive  --       --       --       30       30       15
>>J2 Drive  --       --       --       --       20       20
>>J1 Drive  --       --       --       --       --       10
>>
>>
>>Time (hours)
>>
>>Drive     Jump 6   Jump 5   Jump 4   Jump 3   Jump 2   Jump 1
>>J6 Drive  150      200      160      120       80       40
>>J5 Drive  ---      150      200      150      100       50
>>J4 Drive  ---      ---      150      200      133       67
>>J3 Drive  ---      ---      ---      150      200      100
>>J2 Drive  ---      ---      ---      ---      150      200
>>J1 Drive  ---      ---      ---      ---      ---      150
>>
>>Note that using a Jump N drive for Jump N-1 is really not something
>>you want to do unless it's unavoidable; it takes *longer* without
>>saving you any fuel. Also, the drive that uses the _least_ fuel for
>>jump N is the Jump N drive - but at the cost of being the
>>second-slowest.
>>
>>Comments/Discussion?