Travel Times Conventional Thrusters

[rgrove0172]

Im trying to compute travel times for conventional thrusters in 2300AD. There are a few rules out there (Gurps, Orbital) that have forumulaes for figuring the time required to accelerate, cruise, then decelerate a given distance based on the space craft's performance but they require a DeltaV value - a combination of thrust and available fuel. Anyone know of a way to figure this with the info given for the Traveller ruled ships in 2300?

Would this work?

Time to Accelerate to desired speed (Kps) – Time(hours) = Speed (kps) x .0728/Acceleration in Gs

So a ship wanting to reach 50kps cruising speed at a comfortable .5G accleration would take 7.25 hours of accelerating. (and the appropriate full usage...

So if the ship was rated at max 3G acceleration, and had fuel for 3 hours of maximum thrust (page 194 MgT2300) it would consume .16 per hour of that .5G burn using 1.16 hours worth of its 3-hour fuel supply. (assuming it will have to decelerate again at the end of the trip, it would consume another 1.16 for a total of 2.32 hours and still have a bit of a reserve left for landing)

Am I getting this right? Some of you math wizzes please help me out.

 

[shaunhilburn]

Im trying to compute travel times for conventional thrusters in 2300AD. There are a few rules out there (Gurps, Orbital) that have forumulaes for figuring the time required to accelerate, cruise, then decelerate a given distance based on the space craft's performance but they require a DeltaV value - a combination of thrust and available fuel. Anyone know of a way to figure this with the info given for the Traveller ruled ships in 2300?

Would this work?

Time to Accelerate to desired speed (Kps) – Time(hours) = Speed (kps) x .0728/Acceleration in Gs

Yes, velocity divided by acceleration. Subtract starting velocity from inital velocity and divide by acceleration

So a ship wanting to reach 50kps cruising speed at a comfortable .5G accleration would take 7.25 hours of accelerating. (and the appropriate full usage...

So if the ship was rated at max 3G acceleration, and had fuel for 3 hours of maximum thrust (page 194 MgT2300) it would consume .16 per hour of that .5G burn using 1.16 hours worth of its 3-hour fuel supply.

0.5G for 7.25 hours? It works out to 128 km/s! That's quasi-magical tech from Traveller. Real spacecraft only burn, best case, a few gees for a few minutes - which consumes most or all of the available propellant. Consider the Shuttle's external tank. That entire massive load of propellants -about 760 tons - is consumed in 510 seconds, trajectory average of 1.8 gee for 8.5 minutes. Pretty much the same tech as in 2300.

(assuming it will have to decelerate again at the end of the trip, it would consume another 1.16 for a total of 2.32 hours and still have a bit of a reserve left for landing)

Am I getting this right? Some of you math wizzes please help me out.

You may want to consider working up a star system delta-v diagram in advance, like this one: http://i.imgur.com/duY2S.png or this one: http://upload.wikimedia.org/wikipedia/commons/c/c9/Deltavs.svg. With a tool like this, you can work up a delta-v budget to move between various bodies, then calculate your fuel consumption. Seriously, I wouldn't worry about it. Most of the thrusting in 2300 involves flight into orbit (one big long burn, then a little burn), coming down from orbit, or transfers between orbits (two little burns). You just stutterwarp around for everything else.



Just for general info:

Time t = (Vf - Vi)/a

Vf = 4350 m/s
Vi = 2000 m/s
a = 19.6 m/s² = 2G
Then t = (4350 - 2000) / 19.6 = 120 seconds (2 minutes)

Final velcoity Vf = Vi + at

Vi = 2000 m/s
a = 19.6 m/s² = 2G
t = 120 seconds
Then Vf = 2000 + 19.6 × 120 = 4352 m/s

One G-minute of burn adds 0.5884 km/s to your velocity. The examples above represents a burn of four G-minutes (2G for 2 minutes)

 

[atpollard]

Some other general trends and observations:

Shorter faster burns use less total fuel than longer slower burns (for the typical burn-coast-burn travel scenario).

A couple of minutes at 4G is the upper limit for 'normal people' space travel.

Very low thrust (0.001G) high efficiency engines [Hall Electric Thrusters] burning from orbit to orbit can make a faster trip (for cases where you should be using Shutterwarp, but are not for whatever reason).

The general problem is that using even super-duper real world equipment, dedicating 80% of your ship to fuel just to reach orbit, refueling and burning another full fuel tank to reach Mars in a month ... just ain't all that much fun.
Gravity and Inertia kind of suck in a RPG.

 

[rgrove0172]

Where I will be using the conventional thrusters is on "moon runs" and the like. Freighters delivering supplies, rotating work crews etc. I dont see a company spending money on stutterwarp ships for trip of a few hundred thousand km when a nuc with a thruster can provide the same service, if a bit longer and less comfortable for the passengers.

Is there a way to come up with a max delta-V for each ship? Given their max G acceleration and the number of hours they are able to sustain that thrust?
(That info per the MgT2300 ship construction rules)

If the end result was a given acceleration maximum, it would easy to figure in. This seems to be the way the Gurps Spaceship Book approaches it, a ship is given say 34kps-Max Dv and each time it uses a portion, it reduces. Seems a really easy way to work it out during play but Im not sure how they got the value.

 

[shaunhilburn]

Where I will be using the conventional thrusters is on "moon runs" and the like. Freighters delivering supplies, rotating work crews etc. I dont see a company spending money on stutterwarp ships for trip of a few hundred thousand km when a nuc with a thruster can provide the same service, if a bit longer and less comfortable for the passengers.

Is there a way to come up with a max delta-V for each ship?

Yes. The mission delta-v is: DV = xv × Ln(R)
R = propellant mass ratio mass before burn / mass after burn
vx = exhaust velocity, 4.5 km/s for H2/O2 rockets

Say I burn 25 tons, 100 tons pre-burn, 75 tons post-burn = 100/75 = 1.333
so, 4.5 × Ln(1.333) = 1.293 km/s

Given their max G acceleration and the number of hours they are able to sustain that thrust?

Not realistically. Sustained thrust at high G levels is a different RPG. Spaceships gulp down their propellants quickly, so the reality is short duration burns, usually at low acceleration.

(That info per the MgT2300 ship construction rules)

If the end result was a given acceleration maximum, it would easy to figure in. This seems to be the way the Gurps Spaceship Book approaches it, a ship is given say 34kps-Max Dv and each time it uses a portion, it reduces. Seems a really easy way to work it out during play but Im not sure how they got the value.

Acceleration is not constant. As spacecraft burn off fuel, they become lighter very quickly, so the acceleration (hence gee level) increases even though the thrust level is constant.
a = F/m, so since m is decreasing as you burn propellant, a is increasing, even though F remains constant.



It sounds like you're looking for "Thrust specific fuel consumption". Okay:

TSFC = 101972 / Isp rated in grams per kilonewton per sec, where Isp = specific impulse of your propulsion system. For H2/O2 rockets, Isp ~ 460 in vacuum.

101972 / 460 = 0.2217 kg propellant per kN thrust per second, and 1/0.2217 = 4.51 kN thrust delivered for each kg burned per second.

Thrust for a 100-ton ship accelerating at 2 gee = 100,000 × 9.8 × 2 = 1,960,000 N = 1,960 kN
1,960 kN thrust × 0.2217 = 434.5 kg/s

See? The 100-ton 2G ship is gulping down propellants, 0.43 tons per second.

 

[rgrove0172]

I have no doubt your right, but in the arena of managing space travel in a roleplaying game, Im not sure such detail and accuracy is necesary. Im starting the equation with the rules as they are, then trying to fit a system in that uses them. I totally realize there are some massive issues with the physics in there, but its a game afterall.

All Im looking for is consistency and at least a semblance of logic handling something like the following scenario:

The 3rd moon is currently 500,000km distant. The ship delivering supplies is rated (as per Trav/2300) maximum thrust 2G, with 3 hours worth of thruster fuel (at max G)

How long will it take to get there? How much fuel will it require?

Sure there are some variables in there like how much acceleration do they intend to use? What cruising speed do they intend to reach? But if the formula is simple enough, its not a problem to run a couple quick tests to see what makes sense.

This is going to be happening during game time so a lengthy calculation just isnt going to work. Ill be happy to trade accuracy and realism for a bit of playability, as long as Im consisent.

It does seem how however that thrust is calculated in terms of "hours at full thrust" for the ships in Traveller/2300 while, as you say, seconds is about all we can muster today. Granted, its 300 years in the future but physics is physics, you know?

The interplanetary transit time table on page 145 CT would be great but its geared toward the gravitic Traveller drives, jump points etc.

It amazes me that this seems to be such an oddity. The comparitivrely low tech feel of the 2300 setting would seem to indicate a heavy use of conventional drives for orbital and short interplanetary travel. Why didnt they provide rules for it?

 

[BytePro]

You could make up some tables based on constant acceleration - but you'd need a lot with those 'variables' or resort to extra math.

Seriously, just use a computer/tablet/smartphone.

Then you can easily account for initial orbital velocity and final orbital velocity (even orbits) as well as fuel use and times - heck, aside from fictional impulse and fuel consumption rates, you can be as real as you want - and still be playable.

(Traveller:2300s intent of 'playable realism' is really an oxymoron without a computer.)

 

[shaunhilburn]

All Im looking for is consistency and at least a semblance of logic handling something like the following scenario:

The 3rd moon is currently 500,000km distant. The ship delivering supplies is rated (as per Trav/2300) maximum thrust 2G, with 3 hours worth of thruster fuel (at max G)

How long will it take to get there?

Without stutterwarp, five or six days. The Earth-Moon flight time is four or five days.

How much fuel will it require?

It depends on the mission deltavee, vehicle and payload mass, and propulsion efficiency. I can work through some examples if you like.

Sure there are some variables in there like how much acceleration do they intend to use? What cruising speed do they intend to reach? But if the formula is simple enough, its not a problem to run a couple quick tests to see what makes sense.

This is going to be happening during game time so a lengthy calculation just isnt going to work. Ill be happy to trade accuracy and realism for a bit of playability, as long as Im consisent.

Astronautics really slows down game play. Its' easier to pull numbers out of the air, and they're just as plausible as the figures you get by calculating hours of sustained thrust. Just "wing it" and be consistent about it.

It does seem how however that thrust is calculated in terms of "hours at full thrust" for the ships in Traveller/2300 while, as you say, seconds is about all we can muster today. Granted, its 300 years in the future but physics is physics, you know?

The interplanetary transit time table on page 145 CT would be great but its geared toward the gravitic Traveller drives, jump points etc.

Figuring NASA-style interplanetary transit times isn't really that difficult. You might be able to do it with a simple dime-store calculator. P²=D³, so P=sqrt(D³/M)/2, something like that.

It amazes me that this seems to be such an oddity. The comparitivrely low tech feel of the 2300 setting would seem to indicate a heavy use of conventional drives for orbital and short interplanetary travel.

Yes, for short orbit transfers. Interplanetary travel takes months or years with rocket propulsion unless the star system is very compact.

Why didnt they provide rules for it?

Because it's (quite literally) rocket science. It would turn off a lot of potential players.

 

[shaunhilburn]

(Traveller:2300s intent of 'playable realism' is really an oxymoron without a computer.)

I've added two new spreadsheet files to the 2300NonCanon yahoo group.

They're named "Launch_Profiler" in Excel and OpenOffice formats. It calculates the delta-v for orbital ascents and hohmann transfer orbits, flight times, orbital periods, and the fuel requirements. Among other things.



You're welcome

 

[rgrove0172]

Ill give them a look, Thanks.

Its seems it would have been so easy for the rules to include a table of typical flight times based on distance and acceleration using accepted 'standard procedures' cruise velocities etc.

Sure it would be full of holes but really helpful.

 

[atpollard]

MegaTraveller Hard Times has rules for Pre-Gravitic Travel that might fill your need. If you already have it, then you might check it out.

 

[rgrove0172]

Thanks guys -

At the risk of offending anyone, and I truly hope I dont, the insightful but decidedly "real world scientific" answers to many of my questions here on the forum are impressive, interesting and appreciated - but honestly only marginally helpful.

When I have asked about problems with the sensors rules, I really didnt expect nor need a lesson in real world radar, physics or electromagnetism. I need a fix for the rules in the game.

Similarly here, Im looking for a rules system to allow conventionally driven ships (sense they are provided for in the spacecraft design rules) to actually fly around, work, fight or what have you. The real-world formulas and such are great, but only for inspiration. Ive still had to come up with rules on my own, when honestly I expected to find many of you gamers on this forum to have filled the gaps years ago.

I hope you guys understand and dont think Im complaining, just trying to make my position clear.