Starships & torpedos, a physics question

Chmee · Post by **Chmee** » 2005-03-01 10:11pm

Ok, been a while since physics ..... help me out here.

A starship is traveling at .75 lightspeed and fires a torpedo which cannot, by itself, achieve that velocity, but has engines which accelerate it at a constant 3G .... will the torpedo slow to less than .75 lightspeed or accelerate to above .75 lightspeed?

Jadeite · Post by **Jadeite** » 2005-03-01 10:16pm

With nothing to slow it down, the torpedo will keep it's velocity, and in this case, accelerate.

Wicked Pilot · Post by **Wicked Pilot** » 2005-03-01 10:30pm

The torpedo will accelerate. The velocity it acheives will depend on how long the burn is. And just to add, with constant thrust it is not possible to maintain a constant 3g acceleration. This holds true with both Relativity and Newtonian mechanics.

Chmee · Post by **Chmee** » 2005-03-01 10:37pm

Wicked Pilot wrote:The torpedo will accelerate. The velocity it acheives will depend on how long the burn is. And just to add, with constant thrust it is not possible to maintain a constant 3g acceleration. This holds true with both Relativity and Newtonian mechanics.

Ok, I can see that but I wanted a simple hypothetical.

J · Post by J » 2005-03-01 10:41pm

Depends on which way it's pointed...

kheegster · Post by **kheegster** » 2005-03-02 05:36am

Chmee wrote:Ok, been a while since physics ..... help me out here.

A starship is traveling at .75 lightspeed and fires a torpedo which cannot, by itself, achieve that velocity, but has engines which accelerate it at a constant 3G .... will the torpedo slow to less than .75 lightspeed or accelerate to above .75 lightspeed?

Without throwing out any equations, in free space it will accelerate. However, 3G is an almost insignificant acceleration when talking about relativistic speeds (0.75c = 2.25E8 m/s, while 3G = 29 m/s^2 ).

Sarevok · Post by **Sarevok** » 2005-03-02 06:45am

Assuming it is fired forward the torpedo will start out at 0.75c and will keep accelerating till it has run out of fuel. If it has enough fuel it will reach relativistic velocities and then it's acceleration will start falling as mass increases.

Prozac the Robert · Post by **Prozac the Robert** » 2005-03-02 10:02am

From the spaceship's perspective it pretty much has to accelerate normally. Otherwise you would be able to use the acceleration of missiles to define an absolute zero speed.

From an outside perspective where the spaceship is moving at .75c compared to your definition of rest, I expect that the missile would move away from the spaceship more slowly than you would expect without relativistic effects. If the spaceship is not accelerating then I can't see any mechanism for the torpedo to fall behind it.

Batman · Post by **Batman** » 2005-03-02 04:50pm

Prozac the Robert wrote:If the spaceship is not accelerating then I can't see any mechanism for the torpedo to fall behind it.

aft-firing torpedo launcher

But the general consensus is correct. A torpedo fired forward will cuntinue to accellerate at 3g until
a) it runs out of fuel or
b) hits a velocity where mass increase becomes a considerable factor, from which its acceleration will gradually decrease

Post by **Darth Wong** » 2005-03-02 04:55pm

Of course, this is assuming that the ship is not using some sci-fi technobabble method for attaining its velocity, such as the spacewarp technobabble that is used for just about everything in Star Trek.

Batman · Post by **Batman** » 2005-03-02 05:10pm

Darth Wong wrote:Of course, this is assuming that the ship is not using some sci-fi technobabble method for attaining its velocity, such as the spacewarp technobabble that is used for just about everything in Star Trek.

Well, assuming this was asked in SLAM by a poster who is at least to me at least of yet not known for his scientific ignorance, I think we can safely assume that is the default position.

Chmee · Post by **Chmee** » 2005-03-02 05:15pm

Darth Wong wrote:Of course, this is assuming that the ship is not using some sci-fi technobabble method for attaining its velocity, such as the spacewarp technobabble that is used for just about everything in Star Trek.

Yes, definitely not presuming any kind of warping technology here, it's one reason I made sure to specify a sublight speed.

Kuroneko · Post by **Kuroneko** » 2005-03-02 11:10pm

Batman wrote:A torpedo fired forward will cuntinue to accellerate at 3g until
a) it runs out of fuel or
b) hits a velocity where mass increase becomes a considerable factor, from which its acceleration will gradually decrease

Under the straightforward interpretation of 3g being the acceleration in the torpedo's frame of reference, (b) is not a factor. While there is still fuel remaining, maintaining a constant acceleration in a torpedo would not be too difficult, since this would involve throttling down to compensate for decreasing total mass, although why one would want that in a space weapon is anyone's guess.

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Starships & torpedos, a physics question

Starships & torpedos, a physics question

Re: Starships & torpedos, a physics question