# Questions on Reaction Drives for a Techie

#### Sturn

##### Mongoose
I'm trying to develop some reaction drive ships (per MGT High Guard page 42) and have some questions about travel times.

Space Combat

Makes complete sense for combat, no questions. 4G at 2 hours gives X number of G/Turns.

Interplanetary Travel

The example shows a ship giving up 20% of space for 4G for 2 hours. Looking at the travel charts (Core page 145) means this ship would run out of fuel before reaching anything beyond surface-to-moon travel. I have no problem with this. This assumes using 1 hour of acceleration at 4G, turn around at midpoint, deaccelerate for 1 hour until arrival.

But, what about attempting much longer trips to planets further out?

Doubling the fuel in the HG example I could dedicate 40% of the tonnage for 4G and 4 hours, and still never be close to the 18.6 hours needed to reach a, "Close neighbor world" using constant acceleration.

You could still use the 4G ship with 2 hours of fuel to reach a planet, but you would need a different chart or equation (hopefully not a complex one). As in accelerate at 4G for 1 hour, drift at whatever speed you reached for the majority of the trip, then deaccelerate for 1 hour to stop at the target planet. How could I determine trip times easily? I'm hoping for a simple equation that gives me a ballpark figure, doesn't have to be exact. What I'm hoping to avoid is something like XG squared divided by distance in AU multiply by pie squared times 22.36 for time in months. I guess complex is ok if I could do all of the math before play and have a chart like on Core page 145 standing by.

Interstellar Travel

The same concept could be used for "Generation" ships to travel to a nearby star system. How to compute the time needed (that could be decades or centuries?) to travel a light year or parsec?

#### rust

##### Mongoose
At the moment I do not have the material at hand, but I seem to remem-
ber that Traveller New Era had the required formula and / or a series of
tables to determine interplanetary travel times with different G-hours of
acceleration.

Again, if I remember it right, TNE calculated the distances in "light units",
for example light seconds, so it should not be too difficult to convert the
interplanetary travel times to interstellar ones for generation ships.

#### rust

##### Mongoose
A search on the net led to a formula, but I think you will not like that
one very much:

t = ( 3/2 ) { m S^2 / [ 2P ( ln [ ( M + m ) / M ] )^2 ] }^( 1/3 )

It comes from here, one of the last paragraphs:
http://www.dangermouse.net/gurps/science/reaction.html

There is also a tiny "spreadsheet" to calculate trip times.

I thought about using reaction drives for my Pandora setting, after a little
research on the mathematics of the use of reaction drives I decided to
keep the "magic wand" of a gravitic maneuver drive for starships, the use
of reaction drives was simply too much of a challenge for my very limited
mathematical skill.

#### BP

##### Mongoose
Well, ignoring most of reality† - the concept can be really simple:

1) Constantly Accelerate using fuel
- 2) Coast at a constant 'speed'
-- 3) Constantly Decelerate using fuel.

Basically, you have the acceleration and time - like your 4G at 2 hours - you just need to know the distance traveled. Then subtract that from the distance you need to travel. Using this remaining distance - figure the time you need to coast based on the coasting speed which is simply acceleration multiplied by the time. The units can make this seem tricky (distance is in km, acceleration in meters...).

So, say you need to go 45,000,000 km to a 'Close neighbor world'...
• 3G for 2 hours takes one 400,000 km (according to table). This covers stages 1 and 3 above.
• The distance remaining, 44,600,000 km, is gonna be done coasting based on the 'speed' attained the first half (1 hour = 60 min = 3600 s) of acceleration at 3G (3 x 10 m/s²).
In this case that is 30 x 3.6 km/s or 108 km/s. (30 m/s^2 x 1 km / 1000 m x 3600 s).
• Take the distance remaining and divide it by the speed to get the time. In this case, 44,600,000 / 108 s, is approx. 413,000 s. Doing the seconds to hours and minutes we have about 114 hours and 45 minutes.
(Note - mentally estimating so you might want to check all that with a calculator).
• So, add that time and the original 2 hours and you have about 4 days, 20 hours and 45 minutes.
Simple. :roll:

Not guaranteeing any of this - long day and the mind is mush!

The formula that could be used to derive the MGT table would be D = (A x T²) / 4, where D is distance in meters, A is acceleration (10 m / s²), and T is time in seconds. One can then convert m to km and seconds to minutes and hours. Rearrange the formula for other things like T = 2 x sqr(D / A), or A = (4 x D) / T²

Reaction drives don't have advantage of 'ignoring' gravity wells, so first ignore gravitation pulls (sun(s), solar bodies, etc. etc.). Then, forget about orbital positions (and thus Hohmann transfer orbit trajectories and other optimal fuel consumption vs. time options). And then, ignore gains from reaction mass losses and variable reaction rates, relativity, etc., etc., etc...

#### rust

##### Mongoose
BP said:
Well, ignoring most of reality†
...
Reaction drives don't have advantage of 'ignoring' gravity wells, so first ignore gravitation pulls (sun(s), solar bodies, etc. etc.). Then, forget about orbital positions (and thus Hohmann transfer orbit trajectories and other optimal fuel consumption vs. time options). And then, ignore gains from reaction mass losses and variable reaction rates, relativity, etc., etc., etc...
That was the point that finally prevented my use of reaction drives for the
Pandora setting - if I have to ignore most of reality anyway to keep the
mathematics comparatively simple, I can just as well go one step more
and use a standard Traveller maneuver drive instead.

#### BP

##### Mongoose
rust said:
...t = ( 3/2 ) { m S^2 / [ 2P ( ln [ ( M + m ) / M ] )^2 ] }^( 1/3 )
That one looks like it takes into account the masses of ship and used up reactants (i.e. a traditional rocket simplification). A bit more realistic to be sure - which doesn't fit with MGT as there is no ship 'mass' to work from.

Yep - realism is a lot easier with gravitics! :lol:

#### BP

##### Mongoose
Sturn said:
...
Interstellar Travel

The same concept could be used for "Generation" ships to travel to a nearby star system. How to compute the time needed (that could be decades or centuries?) to travel a light year or parsec?
Yes - that is the same basic approach (again we are ignoring most of reality here...)

The 3G for 1 hour example above resulting in 388,800 km/hr - if any of that math is correct - means a trip of 9,460,000,000,000 km (~1 ly) would take about 24.3 million hours - i.e. about 1 million days! That is over 2 and 3/4 millennium!

#### BP

##### Mongoose
FYI - New Horizons (Pluto probe launched beginning of 2006) was the fastest launch 'speed' of any man made device yet - reaching over 58,000 km/hr.

Of course, 'speeds' are relative - in this case relative to the Earth (which is orbiting the sun, which is moving in relation to other parts of the galaxy, etc. etc.).

#### locarno24

##### Cosmic Mongoose
The 3G for 1 hour example above resulting in 388,800 km/hr - if any of that math is correct - means a trip of 9,460,000,000,000 km (~1 ly) would take about 24.3 million hours - i.e. about 1 million days! That is over 2 and 3/4 millennium!

And this is why we don't do slower-than-light....

#### Sturn

##### Mongoose
So interstellar times will be so large they are undoable even in a generation ship.

So I'm left with figuring and making charts for interplanetary travel using reaction drives (based upon G-rating and fuel available instead of constant acceleration as depicted in Core). I would probably need 10 charts (?) ranging from 1G to around 10G (MGT allows ships over 6G). Then for each chart, a fuel available column (in hours) versus distance column for time needed.

When I get a chance will try my hand at figuring (using BP's notes, CT equations).

#### BP

##### Mongoose
A generation ship is still 'doable' - the example I gave was based on a 'short' one-hour burn (and no gravitational assists and no sling shot 'speed' advantage by aiming to a system that had relative high delta v in relation to the launching system).

Expendable burn stages (as done with RL rockets) could greatly increase the burn time - the higher the 'speed' while coasting, the shorter the journey.

As for tables - you need to know the distance traveled (accel and decel) and max change in speed for a given burn time and Gs. Finding the time is then just a matter of picking from that table, subtracting the uncovered distance from the desired distance and dividing by the speed.

#### Rikki Tikki Traveller

##### Cosmic Mongoose
Interstellar times can be done in a more reasonable time (hundreds or thousands of years rather than millions), but not using the type of propulsion that MGT uses.

You need to use a propulsion system that gives you much better fuel efficiency (usually Ion or some such) but to get that, you have to give up thrust; so you end up with a ship that accelerates at 0.001 G but can do it for years. In the end, you get higher velocities.

Another possibility is to use a Bussard Ramjet which does not carry fuel. It uses a huge magnetic field to draw in interstellar hydrogen for use as fuel. The device theoretically would be some big fraction of the ships total mass (say 20% - 40% for a rough guess), but would replace the fuel tanks and would never go dry. HOWEVER, you have to get the ship to about 1% of the speed of light (30,000 kilometers per second) before it will work effectively.

Hope that helps.

#### hdan

##### Mongoose
Rikki Tikki Traveller said:
Interstellar times can be done in a more reasonable time (hundreds or thousands of years rather than millions), but not using the type of propulsion that MGT uses.

I don't have my rules handy, but what about a solar sail to get you out of the system first? "Run" that until you clear the useful range from the star, THEN kick in your fuel-driven engines to the half-way speed. What would that do to the travel time?

Just a random thought.

#### DFW

##### Mongoose
hdan said:
What would that do to the travel time?

Just a random thought.

Nothing worth measuring. Also, the sail "thrust" drops per the inverse square law..

#### Sturn

##### Mongoose
Another randomn thought. Heplar considered a reaction drive or grav drive in MGT? Or would Heplar be a new drive, possibly more efficient then reaction, less so then grav?

#### Sturn

##### Mongoose
Over on that other forum, there was a recent similar discussion by pure coincidence. This equation was posted which I think is exactly what I needed:

T = ((D - (A * t^2)) / (A * t)) + (2*t)

T = transit time (seconds)
D = distance (meters)
A = acceleration (m/s2)
t = duration of acceleration phase (seconds), just the acceleration phase only, NOT the acceleration+deceleration phase.

Note that the coast duration time is of course = T - (2*t)

(stolen from Brasel5048 at CotI forums)

Does the above seem correct for computing interplanetary times with a reaction drive with coasting in the middle (a.i. not enough time for a constant burn).

#### rust

##### Mongoose
Sturn said:
Another randomn thought. Heplar considered a reaction drive or grav drive in MGT? Or would Heplar be a new drive, possibly more efficient then reaction, less so then grav?
If I remember it right, Colin has recently been working on a Heplar drive
for the Mongoose Traveller system, perhaps you should write him a PM ?

#### DFW

##### Mongoose
Sturn said:
Another randomn thought. Heplar considered a reaction drive or grav drive in MGT? Or would Heplar be a new drive, possibly more efficient then reaction, less so then grav?

Reaction.

"The High efficiency plasma recombustion (HEPlaR) is a very high impulse and high thrust reaction drive. First developed at TL-10. ..."

http://misc.thefullwiki.org/HEPlaR

#### captainjack23

##### Cosmic Mongoose
Sturn said:
Another randomn thought. Heplar considered a reaction drive or grav drive in MGT? Or would Heplar be a new drive, possibly more efficient then reaction, less so then grav?

heplar is a reaction drive. Unfortunately, from what I recall, its implementation in TNE* had some serious reality problems, so it doesn't score much more than Grav if you're looking for hard.

*IIRC, part of the reason it was a very effective reaction drive was that at the upper ends, the tables required that the exhaust move faster than the speed of light......oopsie.

It may be somthing more technical, too, and not the above. But there were problems. I'm not a rocket scientist.

Without starting a sh*tstorm, I'd suggest a search on COTIs archives.

EDIT: Oh, hey...look here
http://www.pocketempires.com/pe/drax000.htm

#### Sturn

##### Mongoose
Sturn said:
T = ((D - (A * t^2)) / (A * t)) + (2*t)

T = transit time (seconds)
D = distance (meters)
A = acceleration (m/s2)
t = duration of acceleration phase (seconds), just the acceleration phase only, NOT the acceleration+deceleration phase.

If the equation above is correct, I made a simple spreadsheet that allows entering distance in kilometers, G-rating of craft, and hours of fuel burnt to give trip times. This is not for a constant burn (a.i. the tables in Core page 145), but for Reaction Drives that will be forced to coast for a large part of their trip.

Reaction Drive Trip Times

I may work on it later to look a little fancier and output tables for various drive ratings and burn times.

I made it with OpenOffice, but converted to Excel, tell me if something doesn't work right. It's non-glamorous, just type in the values (calculations hidden on sheet 2).

The calculations may fail if you enter a burn time that is less then the total trip time. A.i. you decide to burn 2 hours of fuel, but your trip would only take 1 hour anyway - you can constantly accelerate no need to coast.

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