Spaceships for Dummies

[korbeau]

Help me with Spaceships or i'll blow my mind!

I try to incoporate the Space Travel and for helping me, I use the Example but for some weird reasons, my math is not right! I have a LOT of questions but i'll start with theses 2:

1) Princess of Helium travel Mars to Earth. It said the distance is 1.5AU. Traveling Mars to Eart should be .5AU and not 1.5 for AU is the unit distance Sun-Earth. I look to wikipedia, Nasa and the like and they place Mars at 0.5AU from Earth.

2) The distance Mars to Earth have a cruise time of 64.46 days but when I calculate it - (1.5 - 0.0022) * 1.076/25 - i have a result of 0.064!! Look like a * 1000 is miss somewhere.

Someone can explain me thoses numbers and revise my math? I would'nt like to GM my last Space game because of this :P

[Ulzgoroth]

Originally Posted by korbeau

1) Princess of Helium travel Mars to Earth. It said the distance is 1.5AU. Traveling Mars to Eart should be .5AU and not 1.5 for AU is the unit distance Sun-Earth. I look to wikipedia, Nasa and the like and they place Mars at 0.5AU from Earth.

The planets aren't arranged in a line pointing out from the sun. They orbit at different rates. Mars orbit with a radius of 1.5 AU and Earth orbit with a radius of 1 AU means that the two may be anywhere from 0.5 AU to 2.5 AU apart depending on where in their orbits they are at the time.

Originally Posted by korbeau

2) The distance Mars to Earth have a cruise time of 64.46 days but when I calculate it - (1.5 - 0.0022) * 1.076/25 - i have a result of 0.064!! Look like a * 1000 is miss somewhere.

Your problem is that the book doesn't say 1.076, it says 1,076. While in some countries I think that a , is used as a decimal point, that is not the case in GURPS books. 1,076 = 1076.

[vierasmarius]

Originally Posted by korbeau

Help me with Spaceships or i'll blow my mind!

I try to incoporate the Space Travel and for helping me, I use the Example but for some weird reasons, my math is not right! I have a LOT of questions but i'll start with theses 2:

1) Princess of Helium travel Mars to Earth. It said the distance is 1.5AU. Traveling Mars to Eart should be .5AU and not 1.5 for AU is the unit distance Sun-Earth. I look to wikipedia, Nasa and the like and they place Mars at 0.5AU from Earth.

The orbits of Mars and Earth are indeed 0.5 AU apart from each other. However, that is the minimum distance between the two planets; at most points in their orbits they will be much further apart - to a maximum of roughly 2.5 AU if they are on opposite sides of the Sun.

2) The distance Mars to Earth have a cruise time of 64.46 days but when I calculate it - (1.5 - 0.0022) * 1.076/25 - i have a result of 0.064!! Look like a * 1000 is miss somewhere.

Someone can explain me thoses numbers and revise my math? I would'nt like to GM my last Space game because of this :P

What you're missing there is that it's 1,076 (one-thousand and seventy-six) not 1.076 (one and 76 thousandths). Some countries use the comma to indicate a decimal, but in the US and Canada at least, we use the comma to separate every thousand, and a period for the decimal place.

EDIT: Ninja'd, almost word-for-word. =P

[Fred Brackin]

Originally Posted by korbeau

1) Princess of Helium travel Mars to Earth. It said the distance is 1.5AU. Traveling Mars to Eart should be .5AU and not 1.5 for AU is the unit distance Sun-Earth. I look to wikipedia, Nasa and the like and they place Mars at 0.5AU from Earth.

2) The distance Mars to Earth have a cruise time of 64.46 days but when I calculate it - (1.5 - 0.0022) * 1.076/25 - i have a result of 0.064!! Look like a * 1000 is miss somewhere.
:P

0.5 AU is the _minimum_ distance. 2.5 AU is the maximum. At any given time the current distance can be anywhere in between. I.5 AU is the radius of Mars' orbit the way 1 AU is for Earth. The minimum is orbit of Mars minus orbit of Earth when Earth and Mars are on the same side of the Sun and lined up just right.

People with limited space travel resources (like us in the real world) tend to wait for the minimum time to come around (a little longer than a year's time apart).

With about 8x as much delta-V as any current rocket the Princess of Helium can afford to go when ever the owners want. 1.5 AU just happens to be how far Earth and Mars are apart when this is.

You can't really ignore accel and decel time but it's only hours of many days so the major determinant of trip time is a simple Distance divided by speed.

1.5 AU is approx. 93 million miles x 1.5. or 139,000,000 miles. Divide that by 25 (speed in miles per second) and I get c.5,800,000 seconds or 64.5 days.

[korbeau]

... 1,076... damn i feel stupid XD loll

I'll read again without that mistake. Thanks again!!

[vierasmarius]

You may find Langy's Spaceship Travel Calculator helpful. Just plug in the acceleration, available delta-V and distance, and it does the rest. The second page of it is useful for Relativistic (ie, near-light-speed) travel using a reactionless drive or Bussard ramjet.

[Flyndaran]

Originally Posted by vierasmarius

You may find Langy's Spaceship Travel Calculator helpful. Just plug in the acceleration, available delta-V and distance, and it does the rest. The second page of it is useful for Relativistic (ie, near-light-speed) travel using a reactionless drive or Bussard ramjet.

I tried that calculator but found it to not allow accelerations below 0.01 gravities, or a delta V above 100 mps accel/decel total.

[jeff_wilson]

Originally Posted by Fred Brackin

People with limited space travel resources (like us in the real world) tend to wait for the minimum time to come around (a little longer than a year's time apart).

Negative, minimum time trips are avoided because they are also maximum delta-v trips. Longest-time trips are sought after because they are also least delta-v trips.

http://en.wikipedia.org/wiki/Hohmann_transfer_orbit

[vierasmarius]

Originally Posted by Flyndaran

I tried that calculator but found it to not allow accelerations below 0.01 gravities, or a delta V above 100 mps accel/decel total.

I haven't had these problems. While it only displays acceleration down to 0.01G, that's just the cell formatting (which isn't hard to change). It still registers lower totals.

For Delta-V, it seems to only pay attention to what you enter into the Initial Burn cell. Final Burn is equal to that, and Total Trip Burn is the sum of the two. Total Delta-V seems to only be used for "accounting" purposes (ie, to let you know if your selected Burns exceed the available fuel).

[Fred Brackin]

Originally Posted by jeff_wilson

Negative, minimum time trips are avoided because they are also maximum delta-v trips. Longest-time trips are sought after because they are also least delta-v trips.

http://en.wikipedia.org/wiki/Hohmann_transfer_orbit

It'd be more like minimum delta-v rather than maximum time. You could make the trip last arbitrarily long with gimmick orbits but those wouldn't be minimal delta-v.

Also, in the context of _manned_ spaceflight you do indeed want the minimum time possible for your available delta-v. Indeed, current quantities of delta-v are too low and result in trip times too long for many human factors.