1g to 4g Acceleration (I should know this)

4G is not that bad when you have pressure suits on and inside of a can that is pressurized...

Rocket engines do NOT produce acceleration (cf RichardGarfinkle above). Rocket engines provide force. The result of a force applied continually on a mass over time is acceleration.

This is, of course, different than a force applied instantaneously to a mass, such as happens with a rifle bullet. The rifle bullet is accelerated (nearly instantly) to a certain velocity by the explosive force of the gunpowder. From that moment on, it slows down, due to friction, but usually strikes something before all the energy imparted by the initial application of force is gone. In a vacuum, the bullet would continue forever until it hit something.

But you're asking about rockets. A rocket engine provides a nearly constant amount of force until its fuel is used up. During that time, the mass upon which that force acts (the rocket ship) goes faster and faster (the definition of acceleration). If you add another engine of the same force rating, but add no additional mass, the rocket will accelerate more rapidly.

Remember, though, that as a rocket uses up fuel, its mass decreases. Assuming the engine is metered so that it produces the same force always until its fuel is used up (the case in liquid fueled rockets, generally, and in some solid rockets), then rate of acceleration of the rocket ship will go up. Once the fuel is gone, however, the rocket ship is now nothing more than a glorified bullet, reaching a maximum velocity, which decreases only due to friction. In space, there is little to cause friction, so the ship will continue to travel at that velocity until some newly applied force serves to stop it.

You might think of this as pushing a car down the road. All by your self (one engine) you can get the car moving only so fast--say 1 MPH. But have a friend help you (two engines) and it is either 1) easier to push the car at 1 MPH or 2) if you both expend the same effort, the car goes faster--maybe 2 MPH--because the force applied to it is now double. Assuming you and your friend have had your Wheaties that morning, you can theoretically accelerate the car to highway speed. Unfortunately, it's unlikely, because despite all the Wheaties in the world, you guys will run out of fuel before that happens.

This analogy is super simplistic; there are many other factors ignored here that affect the result. Fortunately, a rocket ship in vacuum is perhaps one of the most simplistic examples of Newton's 2nd Law of Motion, which directly applies here.

*walks into thread*

*feels all kinds of stupid*




Carry on!

The acceleration resulting from the force of four identical engines will be four times that of the acceleration of one of those engines.

But you call these "backup" engines - does each engine have its own fuel and oxygen pumps and all that? Are they made to all run at the same time? What happens when all engines are started with the fuel tanks low (whereas full tanks account for maybe half the mass of the rocket)? If they give 4gees acceleration on a full tank, they will give 8gees on an near-empty tank. How long before the pilot and passengers pass out? Is the structure designed to withstand 8 gees?

Yeah, lots of niggling questions like that come to mind.

One guy pushing the car = acceleration 1g (for guy).
Two guys pushing the car = 2g
18 guys pushing the car = 18g.

Answer to initial question: yes.

I don't know, Greg, but I think it'd hurt.

FOTSGreg said:

Oh, my... A sudden thought...

What's the instantaneous force (?) of a 1-second transition to a 1g (9.8m/sec^2) acceleration?

Or vice versa...?

That could be, um, quite a lot I'm thinking...

Ie, a rest mass on a 1g planetary surface experiences a 1g instantaneous acceleration in 1 second.

Umm, gimme' the formula for determining the answer myself, please - not the answer itself.



(yeah, I'm sick - I like maths)

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Hold on, now (pun intended). I'm not sure what you're describing.

If a rocket fires and gives 1g of acceleration straight up on the surface of the Earth, it's just going to hover there. It's just barely counteracting the 1g acceleration of Earth.

You may be asking what if a space vehicle in orbit (in zero g as far as an occupant is concerned) fired its rocket and gave a 1g acceleration. The (rough) equivalent on Earth would be if a rocket were launched with a 2g acceleration. There would be a sudden change of 1g.

This change of acceleration is called jerk.

No, that's not a joke, that's what it's called. A lot of this mechanics part of physics is described by calculus.

Speed is the rate of change of position in relation to time.

Acceleration is the rate of change of speed in relation to time.

Jerk is the rate of change of acceleration in relation to time.

http://en.wikipedia.org/wiki/Jerk_(physics)

If I started reading techie stuff like this in the middle of an SF story, my eyes would glaze over and I'd probably go looking for another book.

caw

blacbird said:

If I started reading techie stuff like this in the middle of an SF story, my eyes would glaze over and I'd probably go looking for another book.

caw

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Which is why we do this kind of discussion here, in the Science Fact sub-forum, so that we technogeeks can get it all out of our systems before we attempt to write something compelling.

Acceleration is the rate at which the velocity of a body changes with time. The idea of "instant" acceleration is useful only in calculation, as it ignores time.

Say a body is in motion, moving left to right. Say that body has a velocity of 1 meter per second. Now apply a force to the left side of that body to increase its velocity. Because we're discussing rockets, let's assume the force is the reaction of a rocket motor mounted on the body. As long as the force continues to be applied, the rate of change of the velocity of the body is acceleration.

As an example, let's imagine an Apollo mission to the moon. The rocket launches the Apollo capsule, its service module and the LEM (the payload) into orbit. Once the rocket motors have shut down, the velocity of the payload is constant. Until there needs to be a course correction (we want the payload to arrive at the moon, after all).

The rocket motors on the payload fire for some period of time, altering the trajectory and affecting to some degree, the velocity of the payload. During the time the rockets are firing, there is acceleration. But once those rockets cease firing, acceleration stops and the payload now moves at a new velocity.

The difficulty with the original question is terminology. The engines do not "have" acceleration to impart to the ship. They provide force. If the force of each engine is 2 megajoules, using one of them alone will change the velocity of the ship a certain amount over a certain period of time. If you use two of these engines, the force is 4 megajoules and if you do not change the time these engines are used, the acceleration is twice as much as before. Use three engines, but change nothing else, the acceleration is three times as much. Etc.

So attempting to parse the original question, let's assume the ship is accelerating at 1g (or a change in velocity of 10 meters per second per second). Now you add the force of three other engines. During the time those engines are firing, your ship should indeed experience an acceleration of 4g (a change in velocity of 40 meters per second per second).

But once you shut down the auxiliary engines, the ship's acceleration is restored to 1g, and if you shut them all down, the occupants of the ship experience what is known as 'zero g' (weightlessness).

These changes are as "instantaneous" as the time it takes for the rocket motors to come up to full thrust or for them to shut down (pretty darned quick, usually). In other words, not at all like the changes in velocity you experience by slowly pressing on your car's gas pedal.

Hehee, a thread for me! (I’m a rocket scientist. No, really)

Assuming all your engines point in the same direction, the force generated by all of them is the sum of the force generated by each. So if they all produce the same thrust, x, four engines give you a force of 4x. You will, of course, use up your propellant four times as fast.

3G is about what the Shuttle pulled getting to orbit, and roughly what you feel on a rollercoaster. Mercury/Gemini/Apollo rockets pulled closer to 9G, which is the level fighter jets are rated to (because there’s no point making an aircraft that can pull more G than the pilot). It’s tolerable in certain directions, but requires training/speed jeans/strapping into a seat. Remember also, there’s nothing to stop your spacecraft once it’s moving; you need to deliver the same impulse at the other end or you’ll carry on forever (or do something clever with aerobreaking). And, while the spacecraft may be in zero-G, your occupants will still get thrown around inside when you start and stop thrusting. Jolly good explanation here.

Spacecraft stuff tends to talk about delta-V (change in velocity, which is acceleration) more than force, because your velocity dictates whether and what you’re in orbit of. You can achieve the same change in spacecraft velocity quickly (high G, biprop engine), or slowly (low G, ion engine), which is better depends on what orbital windows you have to meet. Ion propulsion gets very very fast, eventually (is pretty efficient in terms of mass of fuel required), conventional rockets have pretty impractical fuel requirements for the same acceleration, but can get you going slowly much faster (wow, that doesn’t sound confusing at all ) People on your spacecraft are going to be the limiting factor.

Now for some maths…: to accelerate a payload mass of 31500kg (35tons) by 10ms[SUP]-2[/SUP], a biprop with an ISP of 400s[SUP]-1[/SUP] needs ~80kg propellant. If you want to stop again at some point, double it, 160kg. For an ion thruster with an ISP of 3000s[SUP]-1[/SUP], it’s ~11kg and 22kg.

That gets you going at 10ms[SUP]-2[/SUP] faster than you were going… which isn’t very much. What is it you’re trying to do? To leave orbit you’re looking at a delta-V of kms[SUP]-2[/SUP]: from a low earth orbit, delta-V is 3.4kms[SUP]-2[/SUP], propellant required is ~86,800kg (biprop) or ~7,700kg (ion) which is starting to be a sizeable amount of your spacecraft; from the ground delta-V 11.2kms[SUP]-2[/SUP], propellant will be at least 1,000,000kg (biprop, ignoring the atmospheric drag which will make an ion engine useless). It's actually worse than this, because we've been ignoring the inert mass of the propellant tank which increases with the propellant, and isn't included in the payload mass...

If I tell you the escape velocity for solar orbit (what you’re doing when you’re not orbiting a planet…) is 620kms[SUP]-1[/SUP] you can guess the propellant required gets pretty silly. Even for an ion thruster we’re talking 6x10[SUP]13[/SUP]kg…so if you’re planning interstellar travel you need to get very lucky with planetary alignments, and it still takes decades, or ditch Real Science and make something up On the subject of how much fuel…

FOTSGreg said:

I'm actually trying to figure out if the travel equations in Classic Traveller (an old rpg game) make any sense outside of a reactionless universe.

For example, 1g of instant thrust applied to an entire 15 thousand ton mass vessel. What kind of experience is that for the passengers. The key word is "instant" as in a sudden change from no acceleration to 10ms-2 in a second or less.

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That would be like floating in the air and then suddenly falling to the floor. You better already know which way the "floor" is going to be, and already BE on the floor when the acceleration hits, else you'll fall and hit it, just like on Earth. The one second to full acceleration might help it not be quite as bad.

My original question dealt with a 1g thrust engine versus 4 1g thrust engines and that's pretty much been dealt with (light all four off, it's 4g's, light just one off it's 1g).

I'm also cross-checking for calculations based on space opera style ships that use a "field effect" thrust system capable of up to 20%c. I had somebody tell me once that this is incredibly high instantly, but I want to know what passengers might experience if the total was applied over a 3-hour period - and trying to do it without mixing up terms, terminology, etc. (and so I understand the math because I'm kind of a (simple) math junkie).

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I recall doing some rough calculation of acceleration in recent years. Continuous acceleration at 1g will take about a YEAR to become relativistic (a significant percentage of lightspeed, maybe 5 to 20 percent). Making it several gees will reduce the time proportionately, but that means several months at several gees.

Tele is right, a fighter pilot can go to ten gees for short periods of times (probably enough time to make a U turn, maybe ten seconds), but only because of training in centrifuges and knowing what internal muscles to tense to keep him from passing out. The Space Shuttle takes about ten minutes to get into orbit, and does so with an acceleration of 3 to 5 gees. The limitation is how much a person can reasonably take for that long. Rockets and spaceships can be designed for higher acceleration, but not human bodies.

Let's see how fast you get accelerating at 1 gee for three hours - one gee is 32 feet per second squared, so at the end of every second your speed has increased by 32 feet per second. That times three hours is 3 * 60 * 60 * 32 is 345,600 feet per second, or 65 miles per second. That's a "good" speed in space, but it's still many orders of magnitude below lightspeed's 186,000 miles per second.

FOTSGreg said:

I'm also cross-checking for calculations based on space opera style ships that use a "field effect" thrust system capable of up to 20%c. I had somebody tell me once that this is incredibly high instantly, but I want to know what passengers might experience if the total was applied over a 3-hour period - and trying to do it without mixing up terms, terminology, etc. (and so I understand the math because I'm kind of a (simple) math junkie).

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Electric propulsion (field effect/ion/etc) can reach enormous speed (0.2c) eventually. What it's doing is accelerating electrons to insane speeds, and the action/reaction is what gives you thrust. The problem is the mass of the spacecraft is orders of magnitude higher than the mass of the electrons, so the reaction force is tiny.

The reason it's so good is that for a tiny amount of propellant you can keep on accelerating for years, slowlyslowlyslowly building up speed. The acceleration is very very small, but applied over a long time results in velocities vastly in excess of anything you can achieve with conventional rockets. The downside is that it takes epic amounts of time to get up to that speed...and you have to do the same process in reverse to stop at the other end. I can't remember which mission it was (in the last 10 years or so), instead of the standard Hohman transfer from Earth orbit to Mars (? possibly Moon/Venus/Mercury, can't recall), the electric propulsion mission was a crazy two-year spiral because an electric propulsion system can't do that hard-burn-to-leave-orbit-then-coast maneuver that gets you there in months (days if it was the moon...).

If you're accelerating to 0.2c in three hours, your passengers are going to be a stain on the back wall (unless you fabricate something to eliminate this minor SNAFU ). Thats a mental amount of G.

In terms of what a person on board would feel...a lot like this, for a realistic electric propulsion kind of acceleration (~an order of magnitude less than 1g). This video (or anything else from the Vomit Comet) is pretty good for the transition between ~zero-G and Earth/Moon/Mars-ish gravities, but the nature of the flight path means it's not an instant change. You won't achieve accelerations like this without conventional rockets or a fictitious brand of electric propulsion. Maintaining higher-than-Earth G for extended periods (longer than a rollercoster ride) is going to be a challenge for your passengers: the lady says something a little over 1g (when she's talking about having to lie down for the first parabola) pins them all immobile to the floor because moving makes them feel sick. The problem only gets worse if your passengers have been exposed to extended periods of zero-G before hand.

FOTSGreg said:

For example, 1g of instant thrust applied to an entire 15 thousand ton mass vessel. What kind of experience is that for the passengers. The key word is "instant" as in a sudden change from no acceleration to 10ms-2 in a second or less.

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Greg, I think you're mixing apples and neutrons.

"g" is g-force, which is not actually a force; it's a measure of acceleration felt as weight. It is the acceleration of a system necessary for items within the system to weigh the same as as they would on Earth. (Or multiples thereof - 2 gs, 6 gs, etc.)

But "Thrust" is a description of force. You can describe the force generated by each engine - but the subsequent acceleration - and the g forces felt by passengers in the ship - depends on both the FORCE and the MASS of the ship. Thrust can be measured in newtons or dynes or other units, but not g.

(Also, expecting physics consistency of an RPG like Traveller is just asking for pain and sorrow.)

Best of luck.