Speed and breaking a gravity well.

[kiahoga]

I need help! A friend of mine does nothing but go on andon about why ship of Terren Empire makew no sense because they dont (In his opinion) have enough speed to break a gravity well. Ive tried telling him that The ship are not designed to land on planets but he always ignores it please. give me something so i dont shoot him.

 

 

 

 

 

A Frustrated Gamer

[Captain Obvious]

Re: Speed and breaking a gravity well.

 

Agree with him. Then tell him that's why they DON'T FREAKIN' LAND ON THE PLANET!

[Jaxom]

Re: Speed and breaking a gravity well.

 

Actually, speed and breaking the gravity well have absolutely *zero* to do with one another.

 

Energy, sure. Speed, no.

 

Why?

 

If you fly straight away from the earth at 1 foot per *hour* and your speed never changes you *will* leave the Earth's gravity well. It's that last part that is hard, the speed never changing. Escape velocity is literally the speed you have to be going if you never ever turn on the engines again. It only applies if you are a rock and your launch mechanism is a giant sling-shot. If you have the luxury of an engine that you can burn continuously then you don't even have to exceed the speed of sound. (E.G. Chuck Yeager has flown a large number of test planes into space. The clear indication that you have done this is that your jet turbine flames out because it requires air. The "space plane" is just a normal plane with an engine that doesn't require air from outside the plane to function.)

 

What *does* matter is the ratio of power per gram of fuel you use. Note that this kind of thing never comes up in Spaceship design docs for games.

 

Arguing about whether or not a ship in an RPG can reach escape velocity is like arguing whether the red laser beam does more damage than the blue one. You have no relevant stats, no way to get the relevant stats and the system doesn't use the relevant stats in ship building anyway.

[Old Man]

Re: Speed and breaking a gravity well.

 

The blue beam does more damage, BTW.

 

 

[Erkenfresh]

Re: Speed and breaking a gravity well.

 

Play the rubber science card. If you want ships to land on planets, they have enough power to do so. If not, they don't. They can land in my campaign, it makes their adventures more flexible, and therefore the ships have enough power to do so.

 

If the player still can't get over it, then don't invite them anymore. They are just being obnoxious for attention.

[keithcurtis]

Re: Speed and breaking a gravity well.

 

Actually, speed and breaking the gravity well have absolutely *zero* to do with one another.

 

Energy, sure. Speed, no.

 

Why?

 

If you fly straight away from the earth at 1 foot per *hour* and your speed never changes you *will* leave the Earth's gravity well. It's that last part that is hard, the speed never changing. Escape velocity is literally the speed you have to be going if you never ever turn on the engines again. It only applies if you are a rock and your launch mechanism is a giant sling-shot. If you have the luxury of an engine that you can burn continuously then you don't even have to exceed the speed of sound. (E.G. Chuck Yeager has flown a large number of test planes into space. The clear indication that you have done this is that your jet turbine flames out because it requires air. The "space plane" is just a normal plane with an engine that doesn't require air from outside the plane to function.)

 

What *does* matter is the ratio of power per gram of fuel you use. Note that this kind of thing never comes up in Spaceship design docs for games.

 

Arguing about whether or not a ship in an RPG can reach escape velocity is like arguing whether the red laser beam does more damage than the blue one. You have no relevant stats, no way to get the relevant stats and the system doesn't use the relevant stats in ship building anyway.

While most of the above is technically correct, I's like to point out that in the case of Chuck Yeager, flying into space is not the same thing as escaping a gravity well. But yes, with sufficient fuel and the appropriate drive, you can escape the gravity well of earth at just about any speed. Seven miles per second is escape velocity from the surface for a ballistic object.

 

 

Keith "likes ships that travel at the speed of plot" Curtis

[Jaxom]

Re: Speed and breaking a gravity well.

 

While most of the above is technically correct' date=' I's like to point out that in the case of Chuck Yeager, flying into space is not the same thing as escaping a gravity well. But yes, with sufficient fuel and the appropriate drive, you can escape the gravity well of earth at just about any speed. Seven miles per second is escape velocity from the surface for a ballistic object.[/quote']

 

A-yup-yup. The point there was supposed to be that your space plane can fly into space and continue boosting so if you build it with something other than a normal jet turbine it happily flies away at speeds like 500 mi/hr which is substantially lower than "escape velocity" (about 1/7 miles per second, a factor of 50 below escape velocity).

[Manic Typist]

Re: Speed and breaking a gravity well.

 

Wait, is he saying that it can't take off and leave atmosphere...

 

Or just leave the gravity well?

 

Because, after all, the gravity well extends beyond the atmosphere IIRC, so that would explain why your statements to him about the ships not landing wouldn't appease him: it wasn't actually answering his question. It was answering a different question.

 

However, Jaxom's answer should resolve escaping the gravity well, even if you start in space.

[austenandrews]

Re: Speed and breaking a gravity well.

 

If you fly straight away from the earth at 1 foot per *hour* and your speed never changes you *will* leave the Earth's gravity well. It's that last part that is hard' date=' the speed never changing.[/quote']

Just to be pedantic, you do need enough acceleration to overcome the pull of gravity. That's trivial in most cases, but for a gigantic spaceship with a huge mass, maybe not so much. Especially if its primary drive is FTL and its STL engines are strictly slow & attitudinal.

[Manic Typist]

Re: Speed and breaking a gravity well.

 

Also, you could explain to him, that as GM, you decide the laws of reality. Suddenly, a hummingbird can escape the gravity well if it wants! Deal with it.

[Nyrath]

Re: Speed and breaking a gravity well.

 

Just to be pedantic' date=' you do need enough acceleration to overcome the pull of gravity. That's trivial in most cases, but for a gigantic spaceship with a huge mass, maybe not so much. Especially if its primary drive is FTL and its STL engines are strictly slow & attitudinal.[/quote']

The way I like to look at it is that being close to, say, the Earth imposes an "acceleration tax" of one gee. That is, every second your spacecraft is close to the Earth it has to "spend" one gee worth of acceleration just to keep its velocity from decreasing, then more acceleration on top of that to increase the ship's speed.

 

This is why NASA optimizes its rockets with huge accelerations, in order to spend as little time as possible in the "tax region".

 

So in order to achieve orbit around Earth, the rocket has to increase its speed by about five miles per second. Plus 32 feet per second for every second spent in the tax zone.

[Rapier]

Re: Speed and breaking a gravity well.

 

The way I like to look at it is that being close to, say, the Earth imposes an "acceleration tax" of one gee. That is, every second your spacecraft is close to the Earth it has to "spend" one gee worth of acceleration just to keep its velocity from decreasing, then more acceleration on top of that to increase the ship's speed.

 

This is why NASA optimizes its rockets with huge accelerations, in order to spend as little time as possible in the "tax region".

 

So in order to achieve orbit around Earth, the rocket has to increase its speed by about five miles per second. Plus 32 feet per second for every second spent in the tax zone.

 

This is exactly the point we struggle with in SciFi games.

 

The cheapest way for limited fuel vehicles (eg solid fuel rockets) is to reach orbit in the smallest amount of time possible. However, if you have a vehicle that doesn't use a limited fuel (eg some kind of anti-grav or space-warp) there is no reason to take the least arduous path. You only need 32.5 ft/s/s (or 9.9 m/s/s) of acceleration to overcome gravity. So take it nice and slow!

[Jaxom]

Re: Speed and breaking a gravity well.

 

This is exactly the point we struggle with in SciFi games.

 

Ok, see... I have to disagree. Some people geek out and argue about SFX but the truth of the matter is simple and in black and white in every ship write-up. Appealing to "real physics" is a Pandora's Box that you don't want to touch with a 10' pole (and THAC0 -20). To make a system playable we make some simple assumptions that on further inspection are *way* rubbery. That was the point in the silly quote in my first post (which OldMan obviously got, by the way). Physics has an answer. The game does not.

 

You are using the HERO system. Your vehicle has SPD and some kind of movement power. The power lists a movement in inches. This is the number of inches it can move per phase (distance per time, a speed). Maybe it has limitations on it.

 

Unless you have taken such spiffy limitations as "not usable in atmosphere" or built it in such a way as to use more END than post-phase-12 REC, then it *can* leave the gravity well. Now, this may mean that you didn't correctly build what you wanted for your SFX, but that is a different statement...

 

The point I am trying to make (and have been since my first post) is that with 1" Flight which you can use every phase without ever having to turn it off you eventually leave the gravity well. Flight doesn't have any parameters dealing with acceleration under normal rules. Anything dealing with structural integrity and such is SFX or Limitations. You can make some really sneaky calculations, see below, but there is a good reason not to.

 

The *wrong* argument is that d = vt + (at^2)/2. We're going to start with someone standing (at rest) on the ground and who starts flying straight up. We know that (without special Limitations) the distanced travelled is just the number of inches of Flight. We know that the time spent flying is either 1 second (so segments look like fly, hover, hover, hover, fly, hover, hover, hover) or 12/SPD to yield more "continuous" flight. Using the latter and solving for acceleration that reduces to a = 2d/t^2 = Flight (in inches) * SPD^2 /72.

 

Cool! So a 1G engine needs a of about 11 yds/s/s or 5.5 inches/s/s (remember, the units on SPD are (/s)), right? Wrong on many counts. From rest on the planet's surface, you use up 1G of acceleration just to take the weight off the landing struts. If you actually accelerate that fast from the surface pointing straight up then you have an acceleration of 2Gs (but worse things are going to make us abandon this anyway). But then what happens the next phase? Answer? You quit accelerating. You don't get *faster*, you just maintain the same speed. Technically, unless your acceleration was instantaneous (we're going to come back to this, by the way), you actually decelerate a bit and then speed back up the next phase... You're distance covered is constant (by rule) but you'll find that since you're looking at a quadratic equation, holding time and distance constant (X" of flight per 12/SPD seconds) your velocity starts oscillating (probably with a bit of decay but I have nout gone looking for the critical points of the solution) because you accelerate too much the first phase and then decelerate too much the next, etc.

 

Since the above makes no sense, the only thing you can do is pretend that acceleration is instantaneous. It's rubber physics, but at least it fits the power description. Now, the second half of most arguments in this kind of discussion lead to, "Yeah, but it'd tear itself to bits," we might should look at that. The fact is that you *can* answer that if you really feel the need to do so (but you won't like the answer).

 

The situation is that we have a nice big ship at total rest in the launch pit. It instantaneously accelerates to some fixed velocity. Specifically, the *engines* accelerate and we're going to hope that the ship does too (since that's the crux of the "tears itself apart" argument).

 

We can assume no inertia (in which case the argument is moot) or we have inertia. Barring really wonky physics or disintegration of the ship, the physics is entirely reversible so takeoff and landing can be viewed as time-reversed events. (For this to be physically true I need a ship that doesn't have landing struts and comes to rest directly on its engines, but that's close enough.) Take a ship moving at whatever speed you want and bring it to rest in the landing pit instantaneously. And since we must have inertia for this argument to matter, *THAT* should ring bells....

 

Knockback, anyone?

 

Yep, the mechanics of instantaneous acceleration is what happens when you get knocked back into the bulkhead (since deceleration is just acceleration in the other direction, this shouild make a lot of sense). So, if your ship is moving along at Mach 1 (chosen randomly, much less than escape velocity which is where we started) then you're moving 360 yds per second... 180". That's right... If you can hit Mach 1 instantaneously, that is the same as hitting a brick wall with 180" of knockback left to take. Your ship still intact? I thought not.

 

Why don't we do something like this all the time? Imagine taking the power Leaping and arguing whether or not your hero's legs should break every time he lands. Heck, I know some GMs add flavor-text about he concrete spidering every time the hero lands but they don't add the same flavor text when a hero goes flying into a wall and it's the same physics.

 

So, now that I have pegged out the geek-o-meters, can we stop worrying about whether or not ships can land? Your ship sheet is only going to help you if you start trying to apply real physics and if you go that route then you're going to have far more serious problems than "can I land this thing"...

 

And if we persist in continuing with this, then I demand to know how you feed the ship's crew on a prolonged voyage and whether or not they are going to suffer rickets and why they are not a fine red mist on the inside of the hull every time you accelerate.

[austenandrews]

Re: Speed and breaking a gravity well.

 

And if we persist in continuing with this' date=' then I demand to know how you feed the ship's crew on a prolonged voyage and whether or not they are going to suffer rickets and why they are not a fine red mist on the inside of the hull every time you accelerate.[/quote']

Dehydrated gruel, no and it depends on the definition of "fine mist."

[Jaxom]

Re: Speed and breaking a gravity well.

 

Dehydrated gruel' date=' no and it depends on the definition of "fine mist."[/quote']

 

See, if it is just gruel they're not going to get some thing that humans (or transhumans or whatever the heck they are) need and rickets is what happens if you do that for a few weeks. If you're not going to be in space a few weeks then you're going to be accelerating like mad at some point and that means best case is very *flat* crew or more likely crew that splatter nicely when they hit the bulkhead. Or you've solved the inertia problem and again the argument is rendered moot.

 

/runs in circles looking for the tigers tail so he can grab on!

[Talon]

Re: Speed and breaking a gravity well.

 

Ships in Terran Empire don't make sense for many reasons, but that's not one of them.

[keithcurtis]

Re: Speed and breaking a gravity well.

 

Flight doesn't have any parameters dealing with acceleration under normal rules.

Actually it does. 5ER pp122-5

 

Keith "unless I misunderstood your point" Curtis

[input.jack]

Re: Speed and breaking a gravity well.

 

Jaxom, the problem youre having is one of trying to assume that phased movement is continuous movement.

 

Its not.

 

If you have a Speed of 4, you move on segments 3,6,9, and 12. For the sake of playability and convenience, yout figure is moved to the limit of your Flight (in the case of a flying character or vehicle) on your Phase, but in the "reality" of the character, you dont really reach the end of your movement until the beginning of your next Phase.

 

What I mean (if that got unclear) is not that you character goes "hover, hover, hover, fly 10", hover, hover, hover, fly 10"", and so on. Instead, between Post 12 and the end of your move on 3 you moved 10". Between your move on 3 and your move on 6 you moved another 10". But in the "reality" of the campaign, the way the characters who live in it see the world, your movement was smooth and continuous.

 

Phases are a playability convenience.

 

Therefore, if a character or a ship can fly 1" up each Phase, even with a Speed of 1, they will eventually break free of Earth's gravity. There are no complicated physics here; the ability to fly that 1" upwards each Phase is what the character can do -after- all the physics ahve been taken into account.

 

So, without Limitations to the contrary, any Character with Flight can escape the Earth's gravitational pull. Eventually.

 

However, most Characters have some kind of Limitation that prevents them from doing so, or lack the requisite Life Supports to survive, or most commonly, lack a motivation for doing going there

 

I agree with the poster who said your Player was being a pain in the keister.

[Zeropoint]

Re: Speed and breaking a gravity well.

 

See, if it is just gruel they're not going to get some thing that humans (or transhumans or whatever the heck they are) need and rickets is what happens if you do that for a few weeks.

 

IIRC, rickets is caused by a vitamin D deficiency, and takes longer than a few weeks to set in. Scurvy, caused by a vitamin C deficiency, is a more immediate hazard.

 

And it's a moot point anyway, because Dehydrated Space Gruel includes vitamin supplements ensuring that it provides 100% of a human's dietary needs.

 

That's why it tastes so bad.

[Jaxom]

Re: Speed and breaking a gravity well.

 

Jaxom, the problem youre having is one of trying to assume that phased movement is continuous movement.

 

Its not.

 

Ah, not my problem, just the particular place in the rubber that I chose to pull at for my example.

 

If you have a Speed of 4, you move on segments 3,6,9, and 12. For the sake of playability and convenience, yout figure is moved to the limit of your Flight (in the case of a flying character or vehicle) on your Phase, but in the "reality" of the character, you dont really reach the end of your movement until the beginning of your next Phase.

 

What I mean (if that got unclear) is not that you character goes "hover, hover, hover, fly 10", hover, hover, hover, fly 10"", and so on. Instead, between Post 12 and the end of your move on 3 you moved 10". Between your move on 3 and your move on 6 you moved another 10". But in the "reality" of the campaign, the way the characters who live in it see the world, your movement was smooth and continuous.

 

Phases are a playability convenience.

 

Therefore, if a character or a ship can fly 1" up each Phase, even with a Speed of 1, they will eventually break free of Earth's gravity. There are no complicated physics here; the ability to fly that 1" upwards each Phase is what the character can do -after- all the physics ahve been taken into account.

 

So, without Limitations to the contrary, any Character with Flight can escape the Earth's gravitational pull. Eventually.

 

However, most Characters have some kind of Limitation that prevents them from doing so, or lack the requisite Life Supports to survive, or most commonly, lack a motivation for doing going there

 

I agree with the poster who said your Player was being a pain in the keister.

 

This is *exactly* my point. In the absence of other factors, anything with flight can leave the gravity well. Arguing about whether or not a ship can reach "escape velocity" is a meaningless argument.

[Jaxom]

Re: Speed and breaking a gravity well.

 

Actually it does. 5ER pp122-5

 

Keith "unless I misunderstood your point" Curtis

 

Ah yes, the special rubber room where time-travel saves us from speedsters. (If you move 5" per turn in the first hex that is clearly 1/5th of a turn. 10" in the second is 1/10th of a turn. Expand the series and sum and let's see how many turns one turn really is!)

 

Actually, this misses my point. I took out the big rubber mallet and beat the rubber horse to death (and a bit beyond). I rubbered him out, so to speak.

 

The whole point was to apply real physics to the game system and demonstrate that no matter how true to "reality" you try to get, somewhere you wave your hands and say, "pay no attention to that man behind the curtain!" Those of us who don't wave our hands are called physicists or engineers and playing one turn takes a *really* long time.

[Jaxom]

Re: Speed and breaking a gravity well.

 

IIRC, rickets is caused by a vitamin D deficiency, and takes longer than a few weeks to set in. Scurvy, caused by a vitamin C deficiency, is a more immediate hazard.

 

And it's a moot point anyway, because Dehydrated Space Gruel includes vitamin supplements ensuring that it provides 100% of a human's dietary needs.

 

That's why it tastes so bad.

 

Thank you, yes. Scurvy was what I had in mind. And thank you for picking up the next rubber mallet.

 

By the by, can Dehydrated Space Gruel be used to patch stress cracks in the main hull caused by acceleration as well?

[Captain Obvious]

Re: Speed and breaking a gravity well.

 

With a name like that, why not?

[austenandrews]

Re: Speed and breaking a gravity well.

 

Sometimes they call it Pistachio Spackle.

[Dale A. Ward]

Re: Speed and breaking a gravity well.

 

[Homer Simpson Voice]

 

Mmmmmmm... spackle...

 

~drool~

 

[/Homer Simpson Voice]