Drop Tanks

[Carlobrand]

I can easily ignore that particular error introduced by errata then.
That refers to civilian use of drop tanks and first appeared in JTAS 2 TAS News The military have been using them for much longer.
As has already been mentioned the Gazelle contradicts this bit of silliness.

I think the JTAS mention was regarding their use in the Marches. I expect they'd have already been in use elsewhere in the Imperium.

The T4 bit is certainly different from the CT approach. What's being described there would be more akin to CT exterior demountable tanks, which you'd carry with you through jump for increased fuel but reduced your performance since they increased your volume. CT drop tanks would be dropped just prior to jump so you didn't need to deal with their volume. Then they'd be recovered by the starport or by supporting ships and reconditioned for their next use.

 

[warwizard]

If I can hazard a guess, this is about reverse engineering the displacement per jump number from the tonnage used for the jump drive.

Thank you for the answer. I am a gearhead, and enjoy the more detailed design sequenses. It has been decades since I designed my last book 2 or book 5 ship. But yeah, but taking it to the extreme of recalculating the fuel needed to do the full J-# solving for the changes in jump drive and power plants and loss of internal volume due to increased crew, jump drive and power plant size and power plant fuel, and the required grapples while sacrifacing internal tankage in favor of the external tankage. The design goal is to provide full J-# using external fuel only while retaining the tanks and being able to retain enough internal fuel to perform another jump with the internal tankage.

 

[McPerth]

No, just the jump drives are imported from TL 15 yards. I used this in the islands campaign to give some better legs to the lower tech squadrons.

That would make them TL15 ships, as, per HG (page 20):

Equipment and components of a starship may always be equal to or less than the ship's tech level.

The Gazelle is a HG design, so it fully applies.

 

[Rupert]

I think the JTAS mention was regarding their use in the Marches. I expect they'd have already been in use elsewhere in the Imperium.

The T4 bit is certainly different from the CT approach. What's being described there would be more akin to CT exterior demountable tanks, which you'd carry with you through jump for increased fuel but reduced your performance since they increased your volume. CT drop tanks would be dropped just prior to jump so you didn't need to deal with their volume. Then they'd be recovered by the starport or by supporting ships and reconditioned for their next use.

You would do either of these in CT, depending on mission requirements. A Gazelle would normally be a '400-DTon' J-4 ship, carrying the drop tanks. If they really needed J-5 they'd jettison the tanks just before the jump to get the extra range, and then they'd have to live with J-2 for the future until they could source some replacement tanks.

I don't recall exterior demountables in CT (or elsewhere, for that matter). There were drop tanks (external), and demountable and collapsible internal tanks. Demountables worked just like standard tanks when set up, while collapsible tanks' fuel had to be pumped into the ships normal tanks before use.

 

[Carlobrand]

You would do either of these in CT, depending on mission requirements. A Gazelle would normally be a '400-DTon' J-4 ship, carrying the drop tanks. If they really needed J-5 they'd jettison the tanks just before the jump to get the extra range, and then they'd have to live with J-2 for the future until they could source some replacement tanks.

I don't recall exterior demountables in CT (or elsewhere, for that matter). There were drop tanks (external), and demountable and collapsible internal tanks. Demountables worked just like standard tanks when set up, while collapsible tanks' fuel had to be pumped into the ships normal tanks before use.

Trillion Credit Squadron, Rules and Rulings, Fuel Tankage: "There are four varieties of fuel tankage which are not integral to a ship, each with its advantages and disadvantages. These are collapsible tanks, demountable tanks, exterior demountable tanks, and drop tanks."

 

[Rupert]

Trillion Credit Squadron, Rules and Rulings, Fuel Tankage: "There are four varieties of fuel tankage which are not integral to a ship, each with its advantages and disadvantages. These are collapsible tanks, demountable tanks, exterior demountable tanks, and drop tanks."

Further proof of my advancing years...

 

[warwizard]

OK my three hull design is done in the https://www.travellerrpg.com/threads/t4-merchant-design-challenge.44372/ thread. Took me two days to do it without a spreadsheet. Man was I rusty. But there are interesting things that I did there and would like comments here as the things could be unusual.

 

[Badenov]

So, after having technically analyzed everything to death, I have finally run the numbers. Adding drop tanks to warships can be done and it's not ruinously expensive (about 1-2% of final ship cost) nor does it affect much firepower-wise. But the cost of a drop tank bag is huge: .1MCR/50T (in Mg1), about 4-5 times the ship's normal operating expenses based on the two sample ships I statted out at 5000 tons and 250,000 tons. For normal operating expenses, I only considered Life Support, Maintenance, and Salaries.

My 5000-ton ship needed 1500 tons of jump fuel, and the tanks cost .1MCr/50T x 1500T, so 3MCr. In contrast, Life support+maintenance+salaries came to just under .8MCr.

My 250,000-ton ship needed 75,000 tons of jump fuel, so a bag was 150MCr. Operating costs were only 30MCr.

So, while drop tanks are super-important tactically, they might be reserved for wartime use because it -is- a huge expense. At TL14+, they survive jumping out, but you would be well served to keep a drop tank tender (Proposed here: https://www.travellerrpg.com/threads/drop-tank-tender.44001/) around jump out points to collect them after people jump. Since you're presumably jumping out from friendly systems, that shouldn't be too bad of a problem, and would certainly go a very long way to mitigating that expense.

 

[warwizard]

As you can actually armor a drop tank as much as you want, imangine if wou would, a battlerider with external slabs of armor that are designed to look exactly like that light crusier's drop tanks, with the light crusier and the battlerider being the same configurations and perhaps volume. Is that actually a LCr or is it a battlerider (monitor)?. For this usage the "drop tanks" would have a pilot aboard, some power and maneuver abilities and oversized universal grapples, allowing them to attach to each other or to abandon a destroyed ship and reattach to another ship. After the battle they could help in retrieving the scattered battle crippled hulks.

Designing ships that typically do J-1+n on internal tankage to carry into battle an 11%+ J-n% of hull size of extra fuel in a drop tank so the ship can retreat J-1 as needed.
So it is a calculation: for example the Gazelle class does J-5 on internal tankage (150DT fuel needed) J1 is 30DT. However 33.3DT of fuel needed to J-1 at 333DT plus you are short 16.66 DT when you retained the tank through the J-5, so we add 70DT of fuel to the drop tank and recalculate J-5 now needs 200DT J-1 needs 40, have 100DT tank so we are 10DT over need. Move 10DT from internal tankage to the drop tank to make room for the jump drive to oversize, J-5 needs Jump drive 18DT@300, 24DT@400, so 6DT more Jump drive needed at 24MCr, the remaining 4DT is for 2DT of power plant and one half stateroom for the additional engineering crewman. Now the Gazelle(mod) spent some 30MCr for the drop tank grapel, increase in engineering and crew to obtain the following: Can J-5 +J1 while retaining the drop tank. Losing the drop tank makes internal fuel only jumps now at J-4. Hmm that seems to make sense why the Gazelle retains a 100DT drop tank.

 

[Carlobrand]

So, after having technically analyzed everything to death, I have finally run the numbers. Adding drop tanks to warships can be done and it's not ruinously expensive (about 1-2% of final ship cost) nor does it affect much firepower-wise. But the cost of a drop tank bag is huge: .1MCR/50T (in Mg1), about 4-5 times the ship's normal operating expenses based on the two sample ships I statted out at 5000 tons and 250,000 tons. For normal operating expenses, I only considered Life Support, Maintenance, and Salaries.

My 5000-ton ship needed 1500 tons of jump fuel, and the tanks cost .1MCr/50T x 1500T, so 3MCr. In contrast, Life support+maintenance+salaries came to just under .8MCr.

My 250,000-ton ship needed 75,000 tons of jump fuel, so a bag was 150MCr. Operating costs were only 30MCr.

So, while drop tanks are super-important tactically, they might be reserved for wartime use because it -is- a huge expense. At TL14+, they survive jumping out, but you would be well served to keep a drop tank tender (Proposed here: https://www.travellerrpg.com/threads/drop-tank-tender.44001/) around jump out points to collect them after people jump. Since you're presumably jumping out from friendly systems, that shouldn't be too bad of a problem, and would certainly go a very long way to mitigating that expense.

What is a drop tank bag?

 

[Grav_Moped]

As you can actually armor a drop tank as much as you want, imangine if wou would, a battlerider with external slabs of armor that are designed to look exactly like that light crusier's drop tanks, with the light crusier and the battlerider being the same configurations and perhaps volume. Is that actually a LCr or is it a battlerider (monitor)?. For this usage the "drop tanks" would have a pilot aboard, some power and maneuver abilities and oversized universal grapples, allowing them to attach to each other or to abandon a destroyed ship and reattach to another ship. After the battle they could help in retrieving the scattered battle crippled hulks.

Designing ships that typically do J-1+n on internal tankage to carry into battle an 11%+ J-n% of hull size of extra fuel in a drop tank so the ship can retreat J-1 as needed.
So it is a calculation: for example the Gazelle class does J-5 on internal tankage (150DT fuel needed) J1 is 30DT. However 33.3DT of fuel needed to J-1 at 333DT plus you are short 16.66 DT when you retained the tank through the J-5, so we add 70DT of fuel to the drop tank and recalculate J-5 now needs 200DT J-1 needs 40, have 100DT tank so we are 10DT over need. Move 10DT from internal tankage to the drop tank to make room for the jump drive to oversize, J-5 needs Jump drive 18DT@300, 24DT@400, so 6DT more Jump drive needed at 24MCr, the remaining 4DT is for 2DT of power plant and one half stateroom for the additional engineering crewman. Now the Gazelle(mod) spent some 30MCr for the drop tank grapel, increase in engineering and crew to obtain the following: Can J-5 +J1 while retaining the drop tank. Losing the drop tank makes internal fuel only jumps now at J-4. Hmm that seems to make sense why the Gazelle retains a 100DT drop tank.

Per Wiki, Gazelles have 81Td internal fuel, and 100Td of drop tanks.

Jump 4 at 400Td (performance with the tanks retained through Jump) takes 160Td fuel (plus 5Td for a week of Pn-5). Ship exits jump with 16Td fuel (3.2 weeks at Pn-5).
Jump 5 at 300Td (performance with the tanks ejectead as emptied, immediately prior to Jump) takes 150Td fuel (plus 5.25Td for a week of Pn-7). Ship exits jump with 26Td fuel (4.95 weeks at Pn-7).

Jump-1 without tanks takes 30Td plus power plant fuel. The ship therefore cannot make a second jump after a full-distance jump either with or without the tanks. If it starts with full internal tankage (81Td) and no drop tanks it can do a J-2 and have 3 weeks of power plant fuel remaing upon jump exit,

Note that if throttling its power plant to Pn-1 drops fuel consumption accordingly (TCS/JTAS #14 power-down rule, broken into weeks) and all 6Td of cargo space is used for demountable fuel tanks, it can accomplish a J-1 after a burn&drop J-5, with 1.67 weeks of Pn-1 fuel remaining on exit from the second jump.

In all of these cases, I've assumed jump entry with entirely full tankage (no fuel used to get to 100D limit). Knock off about a day of power plant fuel for the outbound trip.

 

[warwizard]

@Grav_Moped General traveller discussions, rules may vary T4, T5 rulesets may not require nearly as much LHd as you expect for power plant fuel.
From my merchant design challenge under T4 FF&S rules for TL-15: "Power plant 15.4m3 30.8mt .098m2 98.04MW 4.62 MCr... 9.8m3/year fuel" or about 3/4 of a m3 per month for 100MW output, make it an EP and your monthly fuel is near 2m3/month. But yeah doing the Gazelle in T4 FF&S lets you do some interesting things. Let's see for TL 14 that is 3mt/m3 and 4MW per m3 instead of 6, and the scaleing does not show up due to minimum size being larger so around 24 or 25m3 for the TL14 power plant addition to J-5@400DT. I'd need to recalculate the entire Gazelle under T4, and I'm not doing that in the 9 minutes till bed time and without a spreadsheet. I think I could get a T4 TL-14 Gazelle to J-5 then J-1 while retaining the drop tank, but I would have to crunch the numbers.

 

[Grav_Moped]

@warwizard

Yep, the chosen rule set matters. I generally work in CT 2nd Ed. (B2, B5), as you may have guessed.

 

[Badenov]

What is a drop tank bag?

In Mongoose 2008, a drop tank has 2 components, the part that's permanently mounted to the ship, and the part of the drop tank that holds fuel and gets jettisoned when its empty. That second part is what I was calling the bag. Though given that it's just flexible and has no space or weight of its own (in Mg1 anyhow) it seems like you could just recollapse it and retain it rather than tossing it away.

 

[Condottiere]

That's not quite how I recall drop tank rules.

 

[AnotherDilbert]

In Mongoose 2008, a drop tank has 2 components, the part that's permanently mounted to the ship, and the part of the drop tank that holds fuel and gets jettisoned when its empty. That second part is what I was calling the bag. Though given that it's just flexible and has no space or weight of its own (in Mg1 anyhow) it seems like you could just recollapse it and retain it rather than tossing it away.

I believe you are conflating Drop tanks and Collapsible tanks.

Drop tanks are hard, not flexible.

MgT1 HG, p43:
Jump tanks come in two parts. Firstly, there are the docking ports, fuel injectors and explosive collars that allow the spacecraft to mount drop tanks and to jettison them quickly. Secondly, there are the physical tanks themselves.

A drop tank mount costs MCr1 per 50 tons of fuel in the drop tank, and takes up two tons of space per 50 tons of fuel for the fittings to transfer fuel.
A drop tank itself costs MCr 0.1 per 50 tons of fuel space.

For example, a mercenary cruiser wants to mount a 250 ton drop tank. This would cost MCr5 and take up ten tons of internal space. The tank itself would cost MCr 0.5.

Collapsible tanks are soft bags:

MgT1 TCS, p23:
Collapsible Tanks: Collapsible fuel tanks (also called fuel bladder) are large flexible bladders which expand when filled with the hydrogen fuel vital to starships. They take up cargo spaces in a ship and are used to extend range without the work of mounting demountable tanks. Fuel cannot be pumped directly from these tanks to the J-drive, and so a ship must finish a jump before it can use the fuel stored in collapsible tanks. When empty these tanks take up 1% of the tonnage they use when full. Collapsible tanks cost Cr. 500 per ton.

Note that it does not matter if the drop tanks are full or not, they still load the drives:

MgT1 HG, p44:
A ship’s M–Drive rating must be recalculated when carrying a drop tank. For example, a 200–ton ship with a 150–ton drop tank counts as being a 400–ton ship for the purposes of determining its effective M–Drive rating. Round the tank’s tonnage up to the nearest hull size. Drop tanks can also be used to store fuel for other purposes, such as reaction drive propellant.

The jump performance for the ship is calculated assuming that the drop tanks are not attached unless the jump is to be carried out without jettisoning the drop tanks. In this case, the jump performance should be calculated in a similar manner to the effective M–Drive rating.

 

[Badenov]

I believe you are conflating Drop tanks and Collapsible tanks.

No, I was just misunderstanding drop tanks. I had imagined they were flexible, weightless bags and imagined them as inflatable when you want to fill them and deflatable when empty because the drop tank itself has no mass.

Drop tanks are hard, not flexible.

That is not obvious from the description text, but not contrary to the description.

Collapsible tanks are soft bags:

I don't own whatever book TCS is, so I have zero clue what collapsible tanks are.

Note that it does not matter if the drop tanks are full or not, they still load the drives:

That is my understanding also: if not jettisoned, they count against you full or empty, as all editions of Traveller seems to Jump and Thrust by volume rather than mass. This hurts my TL7 brain, but is how the game world is described.

 

[Marcatlas]

Traveller seems to Jump and Thrust by volume rather than mass. This hurts my TL7 brain, but is how the game world is described.

that's because Jump has nothing to do with accelerating your ship. It has to do with a Jump drive opening and placing the ship in a different dimension so volume would be the logical measure. The real space thrust is all over the map. In some versions it makes sense in most versions it should be by mass as you envision

 

[Carlobrand]

...

I don't own whatever book TCS is, so I have zero clue what collapsible tanks are.

...

TCS = Trillion Credit Squadron, a CT adventure. A collapsible tank is a "(l)arge fuel bladders ... to hold additional fuel; the
collapsible tanks are filled with fuel and take up space in the ship's main cargo hold. ... When not in use, collapsible tanks collapse and are stored in the cargo hold; they take up 1% of their filled tonnage." They can't pump directly into the drive - you pump them into the main fuel tanks - so they're intended to allow freighters to make two consecutive jumps to clear a gap between systems.

They're potentially quite frightening when you consider they're holding something that boils at -252.8°C under normal pressure and has a reputation for finding ways to leak out of almost anything. There's a lot of opportunity there for a crafty gamemaster to make trouble for a hapless subsidized merchant owner.

 

[Marcatlas]

They're potentially quite frightening when you consider they're holding something that boils at -252.8°C under normal pressure and has a reputation for finding ways to leak out of almost anything. There's a lot of opportunity there for a crafty gamemaster to make trouble for a hapless subsidized merchant owner.

IMTU they can only hold water, ammonia or the like. J-fuel is then refined from them during jump so it is in the main tanks by the time you arrive in system.