Reaction Drive questions

[zero]

This was mainly noticed by me from looking at the stat blocks for ships in Cstars.
I noticed the ships used Reaction Drives (fair enough for TL 8) but also it had two fuels stated; thrust hours fuel under the Reaction Drive and weeks of operation fuel under the P-plant.

I havent made a ship with a Reaction Drive yet, but is that normal? Or is the fuel bought for the ship just divided up between travel and operations?

I cant see in High Guard where this kind of thing is mentioned and I'm puzzled as I'd like to make my own custom Cstars ship, but would like to figure out this Thrust Hours Fuel and Operations Fuel first.

 

[far-trader]

Reaction drives are notoriously thirsty, and if you think the ones in sci-fi are bad you should check out real ones

So, reaction drives normally don't burn all the time when traveling. You'll boost an hour (or whatever half your fuel is, minus a safety margin), then coast at a constant speed the rest of the trip (taking days, weeks, or longer) until you use the remaining thrust fuel to stop as you near your destination. You spend most of your trip in zero-g unless you have artificial gravity/spin gravity.

Reactionless drives on the other hand burn continuous (as long as the power plant is running) so they are always under thrust (going faster or slowing down) and can make the same trip in a fraction of the time, even with the same (or less) maneuver thrust.

Meanwhile your power plant is running continuous (usually) so that fuel is rated longer because there is less needed.

 

[zero]

I understand the fuel needed for the Reaction Drive (though thanks for clarifying the accel/deccel thing) and I even see where in High Guard the rules are to see how much is needed.

Its more the Power plant thing. Usually ships have Jump fuel and the rest is used up in interstellar travel. Could it be that apart from the fuel needed to thrust (which has its own stat in Cstars) there is fuel for the Power-plant that is basically the fuel normally used for interplanetary operations in MGT?

So I see it being there is the fuel total for the Thrust, as seen on p. 42 of High Guard, then the fuel needed for the Power plant (like an A-class needing 2 tons of fuel for two weeks) as seen on p. 107 of the Core?

Tbh, I think that might be it, much thanks for letting me blabber until I fell on it!

 

[far-trader]

Yep, you've got it

 

[atpollard]

Depending somewhat on your technology assumptions, the Power Plant and Reaction Drive might also use different fuels. The reaction 'fuel' is really more just reaction mass - some mass to throw out the back to create an equal and opposite force (like accelerating the ship). The reaction fuel could be water or ammonia (both common in the solar system and easier to store than LH2).

Fusion, almost be definition, will probably require LH2 as its fuel (and perhaps even specific isotopes of hydrogen).

Another variable worth playing with is the question about whether it is better to accelerate/decelerate at 1-4G in short bursts with a long coast between them, or to accelerate/decelerate at a constant 0.01 - 0.1G for the entire trip. Ultimately, it will depend on the length of the trip and the mission goal (shortest time or largest payload/least fuel). As a rule of thumb, burst-coast is better for short trips and slow-and-steady is better for long trips, but trial and error are the only way to define long and short.

 

[zero]

Well in Cstars, replacing the J-Drive is the Transit Drive.

Its basically is a second M-Drive that can accel/deccel the ship to 0.005% the speed of light. Its exhaust is incredibly destructive so it must be 5000km away from anything else for safety purposes (the T-Drive version of the 100d rule).

So, Reaction Fuel is only used to get to the 5000km point (and from that point to their destination when the T-Drive is switched off), where fuel rods within the T-Drive (which get replaced as part of ship maintenance) take over. The fuel rods in the T-Drive are then included in the initial cost for the Drive in ship design, so they are noted in stat blocks.

Apparently for outer planet travel, the ship uses the Reaction Drive to get to the 5000km point, boosts up the T-Drive for a week, coasts at 0.005% light speed then deccels for a week to the 5000km point so the Reaction Drive can deccel to the reach the target world.
It'll be a different amount of time accelling/deccelling within the Inner planets because of that (they all take a fortnight give or take a day), but outer world journeys generally take over a month to complete.

Astrogation would therefore be important, not to judge Jumps but to judge the two stage deccel so accidents dont happen! :roll:

Then there is obviously the fuel needed for the Power-plant which fuels the amount of time the ship will work, so an inner planet ship needs at least a fortnight of operations fuel aside from Reaction fuel, whilst Outer world ships obviously need alot more P-p fuel.

 

[Captain Jonah]

The thrust drives use Xeon, the power plant uses some form of H.

So basicly two fuel tanks and no way to swap them. At a push you could use any gas in the thrusters with a decent engineer handy to tweak the performance and Ionising/charging grid.

With regard to how much power plant fuel you need. If you have a common snese house rule that you can run the power plants at lower levels then you only need those powerplants to run at 3/4/5 for a hour or two while the reaction drives are up. Otherwise you can coast on power plant 1 and have months of power plant fuel.

 

[DFW]

If you have a common snese house rule that you can run the power plants at lower levels then you only need those powerplants to run at 3/4/5 for a hour or two while the reaction drives are up. Otherwise you can coast on power plant 1 and have months of power plant fuel.

With reaction drives, you don't (shouldn't need) a PP that matches the reaction-D value as it isn't providing any power to it. You should be able to get away with a PP# that is the minimum for that hull size.

 

[Captain Jonah]

If you have a common snese house rule that you can run the power plants at lower levels then you only need those powerplants to run at 3/4/5 for a hour or two while the reaction drives are up. Otherwise you can coast on power plant 1 and have months of power plant fuel.

With reaction drives, you don't (shouldn't need) a PP that matches the reaction-D value as it isn't providing any power to it. You should be able to get away with a PP# that is the minimum for that hull size.

You would think so. Its not mentioned anywhere but every sample ship had a power plant rating to match the thrust of its reaction drive. The fluff does say the reaction drives are much more efficient that previous ones, presumably the idea is that the increase in thrust and drecrease in fuel use requires the massive power plants on the designs.

Still even if you need the high power output to energise those ionising grids its only for two hours at most since thats all the fuel you have, just dial down the power plant and streach your fuel a lot more

 

[DFW]

You would think so. Its not mentioned anywhere but every sample ship had a power plant rating to match the thrust of its reaction drive. The fluff does say the reaction drives are much more efficient that previous ones, presumably the idea is that the increase in thrust and drecrease in fuel use requires the massive power plants on the designs.

Still even if you need the high power output to energise those ionising grids its only for two hours at most since thats all the fuel you have, just dial down the power plant and streach your fuel a lot more

Found it pg. 58 HG - "The power plant rating must be at least equal to the manoeuvre drive rating for gravitic M–Drives. For reaction M–Drives, the power plant rating must be at least 1."

 

[Sturn]

If you are planning on a campaign with lots of reaction drive ships, I have a simple spreadsheet for trip times that could be useful:

Reaction Drive Trip Times

 

[zero]

Cool. I think I should be ok on the math for the journey (more or less, I have posited a similar point on the Ctech forum). And I didnt know the P-plant could be ran as the minimum, thats a nice help to freeing up those valuable dtons when planning a reaction-drive ship.

I'm going to build a 200 dton ship as my first Cstars ship (a TL 8 ship), its a nice number and I have a bit more choice when it comes to internal gubbins and even Thrust, whilst keeping the price of it quite cheap.

I'll also remember about the Xeon and Hydrogen fuel tanks, its a fair and excellent point and makes the ship actually more realistic.

I really like the Cstars setting and I think making a ship will have it more like the ones in Atomic Rocket's website which have a cool retro feel to them.

 

[GamerDude]

If you are planning on a campaign with lots of reaction drive ships, I have a simple spreadsheet for trip times that could be useful:

Reaction Drive Trip Times

Say Sturn. Would you care to add that to the files area of the MGT-Aids elist?

 

[Captain Jonah]

Found it pg. 58 HG - "The power plant rating must be at least equal to the manoeuvre drive rating for gravitic M–Drives. For reaction M–Drives, the power plant rating must be at least 1."

Aha good find.

Guess the Cstars guys missed that one since all of the ships have power plants equalt to to R-drives.

That will save a few Dtons on the ships.

 

[zero]

One point... seeing as all ships have to make a journey of 5,000,000 metres to get to the Transit Point and then from its same sized exit to destination, that simplifies the total needed somewhat, right?

At some point I'll have up here or in the Ctech forum the amount of time in seconds needed for each Thrust to travel those 5 Million metres.

Of course I'm sure I'll be beaten to the punch with the math, but I think I'm getting a grip on how these ships work now. Might even put my hand to making a small craft for a change

 

[Sturn]

If you are planning on a campaign with lots of reaction drive ships, I have a simple spreadsheet for trip times that could be useful:

Reaction Drive Trip Times

Say Sturn. Would you care to add that to the files area of the MGT-Aids elist?

I don't have a Yahoo ID, but feel free to add it if you wish.

 

[zero]

Going off the equation that time in seconds from constant accel or constant deccel equals;

Transit time (seconds) = sqrt (5,000,000/metres in accel/deccel)

I stumbled into a problem. I have come up with Thrust 1 = sqrt of (5,000,000 / 9.81) = 713.9 seconds. That would be half of an accel then deccel of 5,000,000 metres. So the whole accel/deccel should be more than that.

Does anyone know the equation for a transit time based on constant acceleration?
How would Transit time = sqrt (2 x 5,000,000/9.81) work?

I get 1009.6 seconds, which is more than the original calculation, is that right?
If so, Thrust 1 works at nearly 17 minutes to get to the Transit point. Thats pretty fast, which means you would only need a single hour at 1G Thrust for an Orbital-only spaceship (shuttles would be different).

That doesnt sound right to me at all, why dont all the Cstars ships just go with that? A 100dton ship would need 3 tons of xeon and another ton of hydrogen for the P-plant.

Some help please???

 

[barnest2]

Suvat equations are youre friend:
a=9,81
u=0
s=5,000,000
v=?
t=?

 

[apoc527]

And for your convenience, 1 AU is 149,598,000 km.

Sturm, that is TREMENDOUSLY awesomely valuable. Thank you for doing the work.

 

[Captain Jonah]

A few other things to consider for your ships Zero.

Deckplans for Traveller ships are all based on the ships gravity point being its belly, all those artificial gravity fields and inertial dampers allow for long thin decks and belly lander designs.

Reaction drive ships will be tail landers inthat thrust based gravity comes from the tail. This makes small decks stacked on top of each other more practical. Those long corridors down the length of the ship may be fine for grav drives but they turn into very deep drops under reaction drives. Think mercenary cruiser.

Also I'm not sure why so many ships have such high G ratings. Humans handle 1G very well and a ship can cruise at 1G for several hours then turn and burn later for only a little bit longer trip than getting crushed into your chair under 4G for a hour or two. NASA types are very fit and train hard for those takeoffs which last a lot less that 2 hours. The sprint passenger shuttle being a case in point, do you put your passenegers into the pods for an hour each time you burn or is it normal for VIPs to arrive with 4G bruises :lol:

Ring ships could bumble along at 0.1G with little problem for the people in the ring, much like a trrain trip. Rotating pods would need to be hinged to allow them to swing down and out for 1G trips. Lots of good images in 2300 of this.