Space Time Travel

http://en.wikipedia.org/wiki/Gravity_assist#Why_gravitational_slingshots_are_used

<...what rate of acceleration, is needed to feel a constant never ending force of 1 G...>

About 32 feet per second per second, or 9.81 metres per second per second, would do the job nicely. Bear in mind that:

• Mars' surface gravitational acceleration is less than the above.
• The moon's surface gravitational acceleration is less than the above.
• There is a gravity "col" or "saddle" that a vehicle must overcome to approach another planet or moon, otherwise it cannot be captured by its gravitational field. The col is closer to the body of lower mass, therefore "halfway" is not applicable.
• The reason constant acceleration/deceleration is not used is that it is heavy on propellant, and getting all that propellant out of the gravity well that is the Earth is not practicable. It is easier/cheaper/more practicable to use a Hohmann Transfer Orbit.
  
  

Calculating the speed at the half-way point to the moon is a relatively simple exercise in calculus. Perhaps someone will throw in their 3 cents worth (inflation!).

Probably the most significant reason for not having acceleration (1g or some smaller value) during spaceflight is due to the consumption of fuel. Generating acceleration WILL take fuel to produce, and getting fuel into orbit takes more fuel during launch, and taking more fuel during launch takes a larger launch vehicle, and ....

You can quickly see that adding a requirement for acceleration to a space vehicle greatly increases the complexity of every aspect of a space flight, and this adds to its risk as well. One day, perhaps, we will be more able to add acceleration but this is not too likely in the near-term future.

Yes I know all about the slingshot stuff. As well as the problem with propellants and their weight. I did mention these problems in the question.

Well not the slingshot, but of course with the slingshot you still have the bad effects of zero G on the passengers. but that no such magic propellant exists yet.

But the question is.

If there indeed was a new magic propellant that would allow such a flight to happen, what speeds and timelines would have happened?

Thanks.

That is a relatively simple question to answer. What answer have you come up with?

Well we could make propellant on our Moon Base, if we had a moon base...At it's closest point Mars is about 55 million km it would take about 42 hrs.....Your top speed would be a little more than 1.6 million mph...

Wow Really? That Short?

Wow!

Actually, not quite. It would be a little longer due to Mars and Earth moving in their orbits, but not far off.

That is also the optimum shortest distance between Earth and Mars and those conditions do not happen all that often. The current trip to Mars for Curiosity was about 250,000,000 kilometers.

The calculations are very simple.

a = Δv/Δt = (Vfinal - Vinitial) / (Tfinal - Tinitial)

If the initial velocity and time are taken to be zero, the equation simplifies to:

a = v / t

You can rearrange the equation to solve for any one to the terms you want. If you want to solve for distance, then the following is true:

d = 1/2 v × t

The issue with what you are proposing is simply a matter of energy. For a chemical rocket most of the mass of the rocket is fuel. To generate enough thrust for 247 million kilometers of flight is simply prohibitive.

Chemical rockets have a limit to velocity based on the exhaust speed of the propellant. The Tsiolkovsky rocket equation will tell you why. Obviously some other propulsion is required and the only propulsion system that might have a shot at generating that kind of acceleration would be nuclear. No such systems even exist yet.

For a trip to Mars (250 million kilometers) you would need enough thrust for almost 3.75 days (both acceleration and deceleration times) and your maximum velocity would be just over 1,500 km/s.

Your exhaust velocity would need to be even higher to provide the 1 G of thrust. So, you need an extremely powerful engine.

??

Chemical rockets have a limit to velocity based on the exhaust speed of the propellant. The Tsiolkovsky rocket equation will tell you why. Obviously some other propulsion is required and the only propulsion system that might have a shot at generating that kind of acceleration would be nuclear. No such systems even exist yet.

??

Umm, almost every rocket we use now makes many many times 1 G force now,

So this statement confused.

I need to make a correction to what I said. There is no theoretical limit to the velocity of a chemical rocket, but there is a practical limit. That limit is the amount of propellant you need to achieve a constant acceleration for a duration long enough to get through a Mars or even a lunar burn.

It is a "Catch 22" situation. To achieve a very long burn time you need lots of propellant. However, the more propellant you carry, the more energy you need to move that huge mass, so you end up exhausting all of your fuel just to get that huge mass going.

Look at the Space Shuttle. The amount of propellant carried by the external tank is 730,000 kg (about 10 times the mass of the Shuttle Orbiter). There is also 504,000 kg of solid propellant. That is all gone in about 8 minutes.

To sustain a one-way trip to Mars at that burn rate would require 630 times that amount of fuel if the additional fuel had zero mass. However, since the mass is also 630 times, you need to burn a whole lot more fuel just to get to 1 G acceleration, but it becomes an impossible feat because we simply do not have enough engine power to ever lift the huge mass of fuel needed and sustain the burn for days. It is a vicious cycle; you need more and more fuel to move the fuel you need to keep the rocket going.

Now, here is where exhaust velocity comes in. The energy derived from a chemical rocket (or any other rocket that has thrust) is KE = 1/2mv^2, where KE = energy, m = mass of the propellant, v = the velocity of that propellant out of the exhaust nozzle.

The cue here is the velocity is squared in the equation, so doubling the velocity of the exhaust yields a geometric expansion of the energy produced.

A solid rocket booster produces 2.7 km/s exhaust velocity. If we arbitrarily assign a fuel mass of 1000 kg, we get about 3.6E9 kg•m/s^2.

Double the exhaust velocity and we get 14.6E9 kg•m/s^2. That is a lot more bang for the buck. The solution is getting ultra high exhaust velocities so we can keep the overall fuel mass low. Chemical rockets just can't do that because the maximum exhaust velocity is so low (relatively speaking).

You need exotic propulsion systems that require low masses of fuel so that we do not need Herculean forces to move the space craft, but generate very, very high exhaust velocities so as to take advantage of the square of the velocity of the exhaust.

The bottom line is there is no current technology or even technology on the horizon this century that will provide the kind of thrust you propose (9.8 m/s^2) for the durations needed to get to the Moon, let alone Mars.

The only one that can do that is Hollywood.

"The only one that can do that is Hollywood."

Up till now.

The transistor was only invented in the 1920's and not practical till '47.

Look where we've gone from there.

We did just land a 1 ton SUV on Mars.

Who knows???

I would like to think so, but the snail's pace we are making into space says otherwise.

It's easy (relatively) to generate massive acceleration while traveling at low speeds. Acceleration is a function defined by the inverse relationship of vehicle velocity to exhaust gas velocity. While the vehicle is slow, and exhaust gas velocity is at max, the acceleration is also at max, but as vehicle velocity increases toward max, it also increases toward exhaust gas velocity, with the result that as the delta between the two decreases, so does the acceleration that is available.

Hence, we can build aircraft (the F-16, in the US) which are capable of such massive acceleration under the thrust of their engines alone (no catapult launch, since they aren't shipboard capable) that they can literally accelerate, straight up. But, realizing that the limit of available altitude is also affected by thinning of air (their engines are air-breathing jet engines, as opposed to oxidizing rocket engines), that capability is also limited by the fact that as they accelerate at multiple g-s, they also approach their exhaust gas velocity limits.

Thrust producing chemical engines, regardless of design, fuel, or any other factor, will always have an upper limit driven by exhaust gas velocity. Eliminate that, and other factors will weigh heavier, and possibly up the vehicle velocity limit. But without eliminating that consideration, we're still stuck at the relatively slow velocities currently available.

Very good points about exhaust velocity versus acceleration.

The F-16 really doesn't have much acceleration. At takeoff weight the thrust to weight ratio is about 1, so a true vertical climb is pretty slow (almost nil) without the advantage of having significant forward airspeed before initiating the climb.

There is a good video that Top Gear did that demonstrated the takeoff acceleration of a Eurofighter fighter jet (thrust to weight ratio of 1.25:1) compared to a Bugatti Veyron.

The Veyron covered 188 mph in 18 seconds, which is an average acceleration of 4.67 m/s^2, which is less than 1/2 G. Even the zero to 100 time is less than 1 G (7.8 m/s^2) and kicked the Eurofighter's butt, at first. However, the jet has the advantage of more power and in the end builds up a lot more speed.

K. Thanks. I never looked at specific T-W ratio for an F-16, and confess that I was basing example on word of mouth from friends who fly (or flew) it. And it's probable that, like most macho studly people who fly, they think it (or at least are willing to have us think it) to be the fastest, most powerful, tail-kickingest airplane in the world.

I am shamed. As an engineer, I should have looked it up, instead of spouting about what I haven't seen/verified/checked myself.

Anyway, thanks for the gentle flogging. I needed that. And the accurate data. I trust. I still didn't check.

Not a big deal. Aircraft jet engines just do not have a lot G kick, but they can develop a lot of velocity given time.

Perhaps the exception might be the SR-71, which has a very high thrust to weight ratio. Amazing technology for something out of the 1950s.

However, your point about the delta between exhaust velocity and vehicle velocity when vehicle velocity becomes close to the exhaust velocity was an excellent point. There simply isn't enough oomph to generate meaningful acceleration at that point.

Very much similar, and for very similar reasons, to the limitations on a gas turbine engine used for direct torque output in a land vehicle. At zero turbine wheel speed, but maximum internal pressure/gas jet velocity, you get maximum torque out. But as the turbine wheel approaches it's max rotational velocity, the torque drops off in a linear fashion, to near zero with the result that turbine horsepower out is a linear, fairly flat curve over the entire RPM range (since HP is directly related to Torque X RPM).

Too bad the Granatelli's lost that turbine bearing at Indie.

Maybe a Higgs Bosun's chair would work?

The God Stool.

If you hollowed out a big rock and put in some form of high heat chamber, say a nuclear fusion plant and threw rock into the chamber where it vaporized and squirted out of the back, propelling the rocket. The ship would gradually lose mass material as it was consumed as propellant, material because Relativity says it gains mass with velocity.

So what velocity would the propellant need to accelerate the ship to say, C/2 while only expending 50% of it's material?

That's how a dilithium-powered anti-matter warp drive works.

During the cold war, the US developed a nuclear powered missile and it was functional, but never used.At the time it was designed to be an ICBM and deliver a nuclear warhead as well.I remember the nuclear reactor was the heat source, but don't remember the gas that was expelled as a propellent.

Perhaps if this info is still available, it may still be viable as a power source for a rocket.

Anybody want to dig that up?

Me brain hurts.

Bazzer

Some have scratched the surface of this point, but almost half of the fuel would be needed to decellerate the vehicle (presumably also at 1G) for orbit or entry to Mars atmospere. So where does that put the timeframe? Is it also intended to return to Earth?

No. The first flight (B Ark) is only for politicians, telephone sanitizers, and attorneys.

The rest of us will follow at a later date.

If we can send all of them there, why would we need to leave? And why would we want to go wherever they are, anyway?

I'd prefer to get away from it them all, if I had a choice.

Ah, you need to read more. For instance, The Hitchhiker's Guide to the Galaxy would be a good start. :)

I did that once. Started it, I mean. Just couldn't get into it. Not enough gore, I guess.

Or something.

Do a search for "The B Ark".

Ah. Done, and mightily enlightened. So, WHY was Ford Prefect there? And how did HE get back?

OK, never mind. My wife always tells me to read it myself (or watch it, or listen to it, or ...) before asking questions.

I mean, I read an average of 150 books a year, and hate watching TV or movies because their plot development is too slow. But I just HATE coming into the middle of it and not being able to get answers to the questions that follow.

But I'll be good, and read it myself.

Almost every engineer has read that book. You will love it.

Yeah. But as an engineer who learned by doing, as opposed to learning by college and THEN doing, I "escaped" (quotes, because it might not have been an escape) a lot of the fun my engineering brethren took part in. Including reading that one.

But you may be right. If it isn't too painfully silly (I AM much older now, I suspect, than most engineers are when they read that) I probably will have to revisit the old, and underexperienced, joys.

It is full of irony. Just don't wear a magnet.

Now THAT was low! I mean, my punny-bone IS rusty, but ... well, that was just the bottom of the well.

Or, as my daughter would say, "Well played".

:)

Plus Jet Engine Thrust and Rocket Motor thrust are different.

A Jet engine even if it had a source of air to run on, would provide zero or very little thrust in space. Whereas a Rcket motor will. because of the nature of how the thrust of expanding gasses is used. a Rockets thrust is mainly produced by pushing on the specially shaped rocket nozzle. NOT by pushing on the air behind the motor.

Hmm. Interesting point. And I WOULD (though I can see the error, now that you name it, just considering the relative shapes of both the jet and the rocket nozzles) have made that mistake. Thanks for pointing it out.

That is not quite right.

You are not pushing on anything with a jet, you expelling fuel and air.

The formula for net thrust for a jet engine is:

F = (Mair + Mfuel)•Vexhaust - Mair•V

Where:
Mair = mass rate of the air flow through the engine
Mfuel = mass rate of fuel entering the engine
Vexhaust = Velocity of exhaust plume
V = Velocity of air intake (aircraft true airspeed)

Nowhere in that equation is there an expression for pushing against anything (air or otherwise). The thrust generated is a function of the quantity of mass ejected rearward and the square of the velocity of that mass (Ke = 1/2m•v^2).

For example, if you are in a still lake on a boat full of bricks and you start hurling those bricks behind into the lake the boat will move forward as you toss those bricks back.

The boat moves forward not because the bricks push on the air or the water, but by obeying Newton's Laws of motion.

Furthermore, the jet engine and the rocket nozzle both eject mass in the same direction. The function of each is to eject mass as much as possible directly opposite the direction of travel to maximize thrust. If you think about it, you get much better thrust throwing those bricks directly opposite of your desired travel than you would throwing them at 45° angles from due aft. Simple vector math demonstrates that.

Sooo, the rocket's thrust is NOT due to pushing against it's nozzle, either?

I mean, that makes sense, because the rocket, too, has to obey the third law. But the nozzles are certainly different in design and shape, and the relative shape differences make his point look pretty good, also.

All nozzles shape the exhaust plume so that it is as much as a straight line pointing aft as possible.

I was given to understand that once you reach space and are free of Earths gravity, you can shut down the propulsion engines and your speed would remain the same, so why couldn't you start the engine again in bursts to keep accelerating something like firing a big gun?

Bazzer

Yes, you can coast, but it is no different when driving a car on a flat freeway. When holding a steady speed you feel no acceleration.

To get acceleration you must apply throttle. The moment you stop applying throttle you return to zero acceleration and zero G inside the cabin.

Needless to say, if you repeated that goosing of the throttle once every second you would have a very bumpy ride. You would not want to eat dinner like that.

Are you saying that when Capt Kirk says "ahead warp factor 8" every one on board ends up flat against the wall?

Bazzer

Ha!

Well, it's just a TV show, but like anything on Star Trek they tend to try to support it with a theory.

For warp drive there is no specific impulse to the ship or its contents. It is the folding of space (warping) that takes place. Essentially, nothing can travel faster than light. This does not apply to space (and the hyper inflation of space during the Big Bang supports this), so the theory is that you move space around you. Such is the power of dilithium. :)

For the ship's impulse engines they use the artificial gravity generators to counter the effects of the acceleration or something like that.

When you fall out of a plane you are accelerating due to the force of gravity. In free fall you feel weightless, but you are actually accelerating. The reason you feel weightless is because every atom in your body is being accelerated uniformly at the same magnitude of force.

Rather than using the thrust to provide the G force, what if we spun the rocket around it's longitudinal axis like a rifle bullet. Could the centrifugal force generated by spinning substitute for a gravitational force?

It would have the problem not being constant. Zero Gs at the axis and maximum Force at the outer circumference. It may still be useful for the health of any human cargo.

Once the spin is established it should remain reasonably constant and not require the constant effort of the thrust form of artificial gravity.

BAB

Looks like a donut, or a tube?

How fast will you need to spin? Spinning Wheels

Yes.

However, when you turn your head to talk to the person next to you, you will throw up.

When I took my first centrifuge ride at NASA I was specifically warned about doing just that. Turning your head while spinning disturbs the vestibular region of the ear and spooks the brain into thinking you are moving in a direction that you are not as compared to what your other senses and eyes tell you.

The result is a bad case of motion sickness!

The fix is to make the centrifuge very large so that it only has to spin at a low angular rate. I don't know what that minimum size is, but I think it is huge and it becomes a mechanical and engineering nightmare to build a spacecraft that incorporates that.

The other problem with spinning is that any redistribution of weight upsets the center of balance of the system, so some form of an active system needs to be used to compensate for people and objects moving within the spinning wheel.