Drive rating matters less when there's fuel constraints

[whartung]

I was spreadsheeting some travel numbers for fun, expanding on a recent post in the misjump thread.

What was enlightening when it comes to intersystem travel, is how little the drive rating impacts overall travel times when you're limiting your burns because of fuel.

When you have a generic Traveller M-Drive, with "infinite" fuel, then drive rating has a great impact, because you can burn, baby, burn all the way to halfway before you decelerate.

Consider simple Jupiter to Earth scenario. Jupiter (in this case) is 588Mkm from Earth.

With a 1G drive, full burn/decel, it's about 5.6 days of travel. 2G, it's 3.8 days, almost 2 days difference. 5G, 2.4 days. Over 3 days faster.

A generic M1,J1 TNE Free Trader has 28 G-Hours of fuel. 1G for 28 hours.

If they burn 1G for 10 hours to start, needing 10 more to decel (handwaving gravity parts away), the trip takes 19.3 days.

If the ship had, say, a 5G drive, it could burn that much fuel in 2hrs, instead of 10 hrs.

In the case, the trip would take 18.98 days.

"Big deal"

The size of the drive is proportional to the G rating, so a 5G drive is 5x the volume/mass of a 1G drive.

Now, all that said, flying from Earth to 100D, a 2G drive saves about an hour over the trip (2hrs if you count the trip coming back in once out of jump). A 3G drive save about 2hr each leg.

You're still stuck in a week of jump, so it's not really clear how important save those few hours in normal space really matter (considering hours are likely consumed just getting in the pattern, docking, etc. which a drive can really help with).

In the end, I can see a lot of commercial ships having, at best, a 2G drive routinely.

Only combat vessels would likely have higher G drives, and that solely for combat maneuver, not even cruising. Why stress the ship and crew with a 5G burn if it doesn't really matter.

So, just a curious bit of insight how little the G rating would impact travel in a HEPlaR galaxy vs an M-Drive one.

[Condottiere]

Wouldn't it be a case of opportunity cost, between how much extra tonnage the larger manoeuvre drive takes up, against what could occupy that space, like more fuel and/or cargo?

[Grav_Moped]

Originally Posted by whartung

I was spreadsheeting some travel numbers for fun, expanding on a recent post in the misjump thread.

What was enlightening when it comes to intersystem travel, is how little the drive rating impacts overall travel times when you're limiting your burns because of fuel.

When you have a generic Traveller M-Drive, with "infinite" fuel, then drive rating has a great impact, because you can burn, baby, burn all the way to halfway before you decelerate.

Consider simple Jupiter to Earth scenario. Jupiter (in this case) is 588Mkm from Earth.

With a 1G drive, full burn/decel, it's about 5.6 days of travel. 2G, it's 3.8 days, almost 2 days difference. 5G, 2.4 days. Over 3 days faster.

A generic M1,J1 TNE Free Trader has 28 G-Hours of fuel. 1G for 28 hours.

If they burn 1G for 10 hours to start, needing 10 more to decel (handwaving gravity parts away), the trip takes 19.3 days.

If the ship had, say, a 5G drive, it could burn that much fuel in 2hrs, instead of 10 hrs.

In the case, the trip would take 18.98 days.

"Big deal"

The size of the drive is proportional to the G rating, so a 5G drive is 5x the volume/mass of a 1G drive.

Now, all that said, flying from Earth to 100D, a 2G drive saves about an hour over the trip (2hrs if you count the trip coming back in once out of jump). A 3G drive save about 2hr each leg.

You're still stuck in a week of jump, so it's not really clear how important save those few hours in normal space really matter (considering hours are likely consumed just getting in the pattern, docking, etc. which a drive can really help with).

In the end, I can see a lot of commercial ships having, at best, a 2G drive routinely.

Only combat vessels would likely have higher G drives, and that solely for combat maneuver, not even cruising. Why stress the ship and crew with a 5G burn if it doesn't really matter.

So, just a curious bit of insight how little the G rating would impact travel in a HEPlaR galaxy vs an M-Drive one.

Interesting.

Drop tanks might help on a "running Jump" mission profile. Burn all the way to 100D, burn the Jump fuel and then kick the tanks as you go into the hole. Both Jump and destination-side deceleration would be at lower tonnage.

[Timerover51]

Originally Posted by Grav_Moped

Interesting.

Drop tanks might help on a "running Jump" mission profile. Burn all the way to 100D, burn the Jump fuel and then kick the tanks as you go into the hole. Both Jump and destination-side deceleration would be at lower tonnage.

If you do that, when you drop the external drop tanks, you then have them sailing off into some form of orbit around the system sun, unless you reach stellar system escape velocity. Either way, you have some large objects moving at a high velocity in the system.

Personally, I use a boost-cruise profile, as I do not see a need for continuous boost and then continuous breaking. You are burning fuel that costs you credits, and the disposable tanks also cost you credits.

[whartung]

Originally Posted by Grav_Moped

Interesting.

Drop tanks might help on a "running Jump" mission profile. Burn all the way to 100D, burn the Jump fuel and then kick the tanks as you go into the hole. Both Jump and destination-side deceleration would be at lower tonnage.

What you end up with, I think, is simply a lot more intrasystem jumps. Not only is a jump to Jupiter faster, it uses less fuel. Jump-1 on the 200 dTon ship is 20 tons of fuel, 20 G-Hours is 35 tons.

On the Free Trader, 70 tons of the ship is fuel. 35%. Without maneuver (or, at least, very little), ship ship has 3 Jumps worth of fuel on board. 5.6+ G-Hours after 3 Jumps.

And, in truth, if all you're doing is going to 100D limits on "normal" planets, you can "do it" with those 5.6 hours of fuel. It's just you don't want to run the tanks dry.

But you can see a ship, on a full tank, easily transit from Earth to Jupiter and back, via Jump, faster than it can make the trip one way, with quite a bit of reserve fuel left.

This also puts a damper on Pirate activity, since ships will spend more time in Jump than not.

It would also, I think, be a curiosity as to the impact of ship design for system patrol ships.

I'm curious if they can put a large enough tank on one to make a difference in order for it to rapidly respond to concerns in system. Large enough to give them long, sustained burns to decrease response time. The stock designs cap out at around 50 G-Hours, certainly enough for local (100D) work, but near system stuff, maybe not so much as to make a real difference.

In that case, you just make more ships.

Inner system work, Earth to Mars, there's enough fuel on a trader to make the trip sub light faster than Jump, 4.5 days with 20 G-Hours (40 total) of acceleration. That would be a call on how much the fuel is worth.

[whartung]

Originally Posted by Condottiere

Wouldn't it be a case of opportunity cost, between how much extra tonnage the larger manoeuvre drive takes up, against what could occupy that space, like more fuel and/or cargo?

For sure.

On the 200t free trader, an additional G rating on the drive costs 15 tons. That's not nothing, to be sure. As is, the trader has 54 tons of cargo. Adding a G to the drive is almost 30% of that, definitely not worth the trade off.