'Hypersonic'

[pendell]

Didn't know where to put this, so I'll try it here and just ask.  

I assume torpedoes are FTL weapons which can strike enemy ships at interstellar distances. 

So why exactly is this attack called 'hypersonic'?  

hypersonic means speed in excess of Mach 5 -- 5 times the speed of sound. While this is technically correct, Pretty much everything in space moves that fast, given escape velocity from Earth is 8000 m/s, which equates to Mach 23.5.  

So 'hypersonic' isn't a particularly meaningful term; EVERYTHING in space is hypersonic.  

A better word might be ' Hyperluminal ' or perhaps 'superluminal'. This would distinguish FTL torpedoes from a mass driver, whose projectiles are presumably subluminal, as accelerating an object to FTL speeds would pretty much ruin any kinetic energy impact , since things travelling above C have 0 effective mass, as I understand it. 

So may I ask why this particular word was chosen?  Or is a different definition being used? 

Respectfully, 

Brian P. 

[Cory Trese]

A different definition is being used. Consider it a replacement for the world "Hyperspace"

This is a in-game term that is used throughout Star Traders RPG and 4X.

[squee]

I vaguely remember in Star Traders firing torpedoes basically means warping or teleporting bombs into enemy ships. Or am I just very confused?

I completely understand what you're saying pendell, but a few physics technicalities

Spaceships are not projectiles, and can use thrusters on to keep raising altitude regardless of speed. They might also use ground-based structures to basically fling ships into space, so ships themselves don't necessarily have to be that fast. Also, that escape velocity is the speed an object needs only at ground level, and then it can cruise away from the planet. Ergo, they don't necessarily need to be going 8000 m/s when they enter space. Just technicalities, because to travel across solar systems in a timely manner these ships obviously need to be very fast, so you're right, Mach 5 doesn't make sense. Plus there's no sound in space. Velocity is also relative (during a space battle you'd be more concerned about velocities between ships, which is completely unrelated to your ship and a planet you left way back). Because there is no air resistance or gravity in space, the concept of velocity there kind of gets thrown out of whack. You can basically keep accelerating to any velocity as long as you have the time and fuel (due to relativity yada yada you can't surpass lightspeed). Acceleration is critical in outer space.

[pendell]

Very well, I will add it to my internal dictionary of words.

Good thoughts , Squee! I'll have to think on your answer.

Respectfully,

Brian P.

[pendell]

Okay, I've been thinking about what you said, Squee. And before I start, I must acknowledge that we are agreed on the important thing: That in order to get anywhere in space in any reasonable time frame, spacecraft need the ability to generate significant acceleration.

But I'm still niggling at the little things, purely because I like science .

"Spaceships are not projectiles, and can use thrusters on to keep raising altitude regardless of speed. They might also use ground-based structures to basically fling ships into space, so ships themselves don't necessarily have to be that fast."

Actually, I believe they do. While it is true that escape velocity can be provided by something other than an onboard source, such as a ground-based mass driver or a space elevator , you're still going to have to be able to go really fast. The 'fast' may not be provided by an onboard source. But the important thing is that you're going to need an 8000 m/sec vector if you're going to leave earth orbit, and while you don't necessarily need the ability to have an *onboard* source generate that acceleration, you need to achieve that acceleration *somehow*.

So you're going to be fast, whether you run yourself or someone else throws you, if that makes sense.

Also, that escape velocity is the speed an object needs only at ground level, and then it can cruise away from the planet.

From a strictly technical perspective, you're right. Once a vehicle escapes from orbit, it doesn't need to retain that 8000 m/sec vector. It can travel as slowly as it like and remain in space forever.

However, in the first place there's not a lot of reason to simply cancel an 8000 m/sec vector -- for this you would have to apply an equal and opposite burn in the other direction, and where would you go? Not back to earth -- simply canceling the vector, once you've left orbit, won't bring you back. It just means you're traveling into empty space at a much slower vector -- meanwhile earth is NOT standing still in space. It's ripping along around the sun at -- according to Wikipedia here -- 3000 m/sec, or 108,000 km/h .

The planet Mars, by contrast, is orbiting the sun at 86,900 km/h as shown here .

So if you're going to get from Earth to Mars, You're going to need some way to shed the excess velocity you picked up *just from leaving earth in the first place*.

And that's just for a close by planet. It gets more complicated when traveling to another star, because our solar system is moving too -- towards the constellation Leo -- at a speed of 390 km/sec, which is faster than 500,000 miles an hour.

The TARGET solar system is moving as well.

The long and the short of it is, while it is technically true an object in space doesn't need much in the way of acceleration just to be in space, getting anywhere interesting in ANY time frame is going to require a LOT of acceleration. At this point, we haven't succeeded in launching a single object clear of the solar system. Given the sun is strong enough to keep Jupiter in orbit, it wouldn't surprise me to learn that the voyager space probes are in fact on cometary orbits, and they'll come back around in a few thousand years.

You can basically keep accelerating to any velocity as long as you have the time and fuel (due to relativity yada yada you can't surpass lightspeed).

I don't believe that's *entirely* true.

It is true that at the accelerations we can currently apply to spacecraft, you can accelerate forever and achieve as much velocity as you want.

Where that breaks down is when you actually start getting up to lightspeed -- C -- 3x10^8 m/sec. -- inertia radically increases . That's why you can't go above the speed of light -- because the inertia of an object approaches infinity as its velocity approaches light speed.

Thus, the faster you are going the harder it is to continue accelerating. This effect is unnoticeable at the speeds *we* typically think of, but it's much hard to push from .90c to .95c, and still harder to push from .95c to .97c. And it gets worse from there until it becomes impossible at C proper.

That's why I mentioned elsewhere that the guns must be firing sub-luminal rounds -- because known particles that travel at lightspeed are massless , thus evading the inertia problem massless particles . Tachyons -- faster than light particles -- have not been proven to exist, but if they existed they would have imaginary mass , and therefore would probably not have any measurable impact on a physical object. Thus unsuitable for throwing at Xenos.

Thanks for the discussion!

Respectfully,

Brian P.

[squee]

Hehe, physics discussion! First of all, we're going to have to neglect atmospheric air resistance, since it doesn't affect our points anyway.

Jul 22, 2014 20:40:55 GMT -5 pendell said:

"Spaceships are not projectiles, and can use thrusters on to keep raising altitude regardless of speed. They might also use ground-based structures to basically fling ships into space, so ships themselves don't necessarily have to be that fast."

Actually, I believe they do. While it is true that escape velocity can be provided by something other than an onboard source, such as a ground-based mass driver or a space elevator , you're still going to have to be able to go really fast. The 'fast' may not be provided by an onboard source. But the important thing is that you're going to need an 8000 m/sec vector if you're going to leave earth orbit, and while you don't necessarily need the ability to have an *onboard* source generate that acceleration, you need to achieve that acceleration *somehow*.

So you're going to be fast, whether you run yourself or someone else throws you, if that makes sense.

You only need to achieve the 8000 m/s (actually I looked it up and apparently it's 11000 m/s) ground level escape velocity if you don't plan on thrusting while leaving the planet. In this case, you actually need kinetic energy (1/2 m * v^2) to overcome the gravitational potential energy (not m*g*h, that is only for uniform gravity, but rather -GmM/r using law of gravity F=GmM/r^2 with Calculus) at ground level, so yes, 11000 m/s velocity vector aimed in any direction that is not at the ground.

However, it is physically possible to leave a planet going at only 100 m/s, all you need to do is thrust against gravity while going up. In fact, the higher you go, the weaker gravity gets and the easier it becomes to leave the planet. Although it would take a LOT of fuel and energy, that is why the ground level escape velocity does not really pertain to rockets and ships that can thrust while leaving the atmosphere. Real life rocket ships start from rest on the ground and start accelerating slowly, not at 11000 m/s.

From a strictly technical perspective, you're right. Once a vehicle escapes from orbit, it doesn't need to retain that 8000 m/sec vector. It can travel as slowly as it like and remain in space forever.

However, in the first place there's not a lot of reason to simply cancel an 8000 m/sec vector -- for this you would have to apply an equal and opposite burn in the other direction, and where would you go? Not back to earth -- simply canceling the vector, once you've left orbit, won't bring you back. It just means you're traveling into empty space at a much slower vector -- meanwhile earth is NOT standing still in space. It's ripping along around the sun at -- according to Wikipedia here -- 3000 m/sec, or 108,000 km/h .

The planet Mars, by contrast, is orbiting the sun at 86,900 km/h as shown here .

So if you're going to get from Earth to Mars, You're going to need some way to shed the excess velocity you picked up *just from leaving earth in the first place*.

Yep, I did mention that in order to travel through space effectively, you definitely need to be able to accelerate quickly. I was just saying ships don't necessary have to be fast to get to space. If you fly upwards starting with just a little over 11000 m/s, you will not end up at that speed after you get further away from the planet, but rather your velocity will slow down considerably. There is no need to cancel 11000 m/s in space, gravity will slow you down as you rise. Whatever kinetic energy you had that surpassed the gravitational potential energy will be the amount kinetic energy you will have remaining after you get far away. Of course, if you are going 1 m/s over an escape velocity of 11000 m/s, that is still a decent amount of excess kinetic energy (148.3 m/s): 1/2 m * (11001m/s ^2 - 11000m/s ^2) = 1/2 m * 148.3m/s ^2.

The long and the short of it is, while it is technically true an object in space doesn't need much in the way of acceleration just to be in space, getting anywhere interesting in ANY time frame is going to require a LOT of acceleration. At this point, we haven't succeeded in launching a single object clear of the solar system. Given the sun is strong enough to keep Jupiter in orbit, it wouldn't surprise me to learn that the voyager space probes are in fact on cometary orbits, and they'll come back around in a few thousand years.

Yep, agree. However, it's important to also remember that efficient interstellar travel is really going to require traveling much faster than the speed of light, so our conventional forms of acceleration are not going to cut it.

I don't believe that's *entirely* true.

It is true that at the accelerations we can currently apply to spacecraft, you can accelerate forever and achieve as much velocity as you want.

Where that breaks down is when you actually start getting up to lightspeed -- C -- 3x10^8 m/sec. -- inertia radically increases . That's why you can't go above the speed of light -- because the inertia of an object approaches infinity as its velocity approaches light speed.

Thus, the faster you are going the harder it is to continue accelerating. This effect is unnoticeable at the speeds *we* typically think of, but it's much hard to push from .90c to .95c, and still harder to push from .95c to .97c. And it gets worse from there until it becomes impossible at C proper. 

Well, technically you do keep accelerating, it's just that if you approach the speed of light and inertia increases, actual increase in velocity slows to a crawl. However, you will still keep adding energy. Let's say it takes one whole day to accelerate from .95c to .97c. It will then take one whole day to decelerate ("acceleration in the opposite direction") from .97c to .95c. Every tiny fraction you get closer to the speed of light still matters a lot, including effects like time dilation.

And imaginary mass, I'm not sure what that means in a physical sense lol.

[Deleted]

Just goes to show, dump enough raw velocity into an object, there's not much it won't kill.