Max speed a spaceship could go?

[Meatball]

Hey folks,

Got a question that I've been mulling over for a while and thought you guys might have some good answers or insight into.

I think Helios 2 is the fastest manmade object in space so far at 150,000 miles per hour. I know theoretically a ship can travel up to the speed of light in space since there's no resistance in space to slow you down (assuming you don't run into something...)

My question is how fast do you think you could actually make a ship go under some type of propulsion power before the engine itself tears the ship apart?

For example, if you were to take a jet engine and strap it a balsa wood airplane, the engine itself would be too powerful for the airplane and would destroy it.

So, assuming materials that are available today, would there be some sort of max speed a ship could accelerate/decelerate from without using any other type of forces like gravity, etc to effect it's velocity?

Of course, now that I think about it, if you just impart small amounts of thrust continuously, that would build up a lot of speed, so that might be the answer to my question.

What's everyone's thoughts?

[Shadow_Ferret]

Well, I think if there was such a powerful engine the engineers would try to make the ship structurally sound enough to handle it.

[rmgil04]

There was a show on Discovery Channel called The Universe. They had an episode where scientists discussed/presented some theories about traveling near the speed of light. The closer you get, the more power you need (increaasing exponentially). IIRC, the only type of thrust they said might reach the speed of light would be some type of ion propulsion. However, the acceleration would be very slow.

In the book I'm writing, I use the theory of controlled particle collisions to create an artificial black hole to get from one place to another faster than light.

[FOTSGreg]

Theoretically, the absolute maximum speed of a spacecraft is 99.99 ad infinitum 9's percent of the speed of light. You cannot actually attain 100% of the speed of light without using more energy than the combined output of the entire universe.

Properly designed spacecraft will not experience most of the effects of what we think about here on Earth when the engines torch off (or whatever it is they do). There is nothing to resist the "push" of the engines or warp the actual structure of the spacecraft itself. If the engines "push" they will provide thrust to the entire craft in some manner. It could be made of balsa wood and still have a "jet engine" (problems with the concept begin with the fact that jet engines operate by pushing air through a portion of the engine at high speed where it is combined with fuel explosively - something that cannot happen in space) attached if the attachment is strong enough to absorb the push.

It's been therorized by some science fiction writer and scientist friends of mine that a "field effect" drive might have a top speed limited by the effect of a sort of "drag" effect (wrong word, I know) or resistance of the local ions, dust, gas, etc. in the vacuum against the thrust of the drive. Essentially, the field itself might encounter enough local material or draw in such amounts of material that the infall against the field ahead of the spacecraft effectively becomes so large it negates the thrust of the drive itself. This is, of course, only theoretical (although some research on Bussard ramjets and local hydrogen densities suggests that Bussard ramjet driven ships might have a theoretical top speed).

I think I got most of that correct but Lhun will correct me if I missed the boat anywhere (thanks ahead of time, Lhun).

[Lhun]

Originally Posted by Meatball

For example, if you were to take a jet engine and strap it a balsa wood airplane, the engine itself would be too powerful for the airplane and would destroy it.

So, assuming materials that are available today, would there be some sort of max speed a ship could accelerate/decelerate from without using any other type of forces like gravity, etc to effect it's velocity?

Speed has no effect at all on a vehicle, neither in space nor on earth. Acceleration does, but sustained acceleration can theoretically get a vehicle close to the speed of light.

Originally Posted by FOTSGreg

You cannot actually attain 100% of the speed of light without using more energy than the combined output of the entire universe.

More actually. To accelerate an object with rest mass to or beyond lightspeed would theoretically take an infinite amount of energy. It's more of a theoretical problem than one of how much energy is available in the universe.

I think I got most of that correct but Lhun will correct me if I missed the boat anywhere (thanks ahead of time, Lhun).

Damn, i'm predictable.

[Pthom]

Originally Posted by Lhun

Speed has no effect at all on a vehicle, neither in space nor on earth. Acceleration does, but sustained acceleration can theoretically get a vehicle close to the speed of light.

What matters, practically, is how fast you want to get to this state of "almost light speed." A solar sail might do the trick but would take longer than the lifespans of the occupants.

Damn, i'm predictable.

Yes, you are. So where's the corrections?

[Lhun]

Originally Posted by Pthom

Yes, you are. So where's the corrections?

Surprisingly () there's really nothing to correct here. Though i just have to add that when writing about solar sail powered spaceflight, despite the temptation, please do not start using nautical terminology. A solar sail does not work like a nautical sail, and never, ever will a solar sail ship be able to beat to windward with respect to the light pressure.

[Meatball]

Awesome info/answers so far, thanks everyone!

To clarify a bit, I'm really looking to figure out how fast someone could actually get to another planet in our own system, say Jupiter or Saturn as opposed to a long range trip that is slowly building up speed.

That's why I was thinking more of some type of direct thrust system as opposed to a sail or something like that.

[Lhun]

There's no way to answer that without the specifics of the drive. Although in general, if you have magical anti-inertia technology, it can be arbitrarily fast, if not, no matter what the drive, 1g sustained acceleration is pretty much the maximum.

[Meatball]

What would 1G acceleration actually equate to in velocity?

On a secondary note. I'm debating some type of 'jump' technology using natural phenomena and debating whether I should use black holes or wormholes. I'm not sure how I could make a black hole able to transport someone without mashing them to bits, while wormholes are hypothetical and feel like a cop-out Any thoughts?

[Pthom]

Acceleration is defined as an increase in speed: the rate at which something increases in velocity. The longer you accelerate (at whatever rate) the greater your velocity becomes. Up to the speed of light.

Regarding transport through black holes or wormholes, from a scientifically factual standpoint, it's impossible. You might consider posing that question in the main forum.

[rmgil04]

Originally Posted by Meatball

What would 1G acceleration actually equate to in velocity?

Could you be more specific? On Earth, I'd say 1G sounds like 1 unit of Earth gravity or 9.8 m/s^2.

Originally Posted by Meatball

On a secondary note. I'm debating some type of 'jump' technology using natural phenomena and debating whether I should use black holes or wormholes. I'm not sure how I could make a black hole able to transport someone without mashing them to bits, while wormholes are hypothetical and feel like a cop-out Any thoughts?

In the Lost Fleet, ships travelled to jump points near stars. They couldn't jump to their version of FTL in open space. In the novel I'm writing, I use artificial black holes as a conduit to FTL space. The transition is very stressful on crew. As the ship passes through the event horizon, shields protect the ship and crew from being torn apart by the gravity, but the crew still feel the effect of gravity trying to pull them apart.

[Meatball]

I'm moving the wormhole/black hole question to another thread.

As for being more specific about the 1G acceleration, I'm basically trying to determine how quickly a ship could get to other objects in the solar system, say Jupiter or Saturn for example, if they were able to apply a constant 1G acceleration. Though I'm still not positive why a ship couldn't go faster than 1G other than some crew discomfort.

Assuming they don't go past 1G, I assume they'd only be able to accelerate to the half way point and then they'd need to reverse the 1G acceleration to stop at the correct location.

[small axe]

You might search their website or past issues of Discover magazine for a few years back, I recall they had an excellent overview article on possible propulsion systems.

[Lhun]

Originally Posted by rmgil04

Could you be more specific? On Earth, I'd say 1G sounds like 1 unit of Earth gravity or 9.8 m/s^2.

Anywhere. 1g is defined as the gravitational acceleration of earth. The earth not an earth.

Originally Posted by Meatball

As for being more specific about the 1G acceleration, I'm basically trying to determine how quickly a ship could get to other objects in the solar system, say Jupiter or Saturn for example, if they were able to apply a constant 1G acceleration. Though I'm still not positive why a ship couldn't go faster than 1G other than some crew discomfort.

The human body evolved to live on earth which has a 1g gravity, no more, no less. Big deviations from that will cause serious health problems, not just discomfort. With a superbly trained astronaut crew, long periods of different gravity are possible, if it doesn't deviate too much, but for normal passengers it's not a fun ride. Free fall is a problem even then, but a good fraction of normal gravity can do much to prolong the maximum safe time.
On the other side however, there are many additional consideration. For one, while 2g doesn't sound like too much (it's less than you can experience in a sports car after all) it's a very different thing to be under 2g 24/7 for a while than experiencing it for a few seconds. Even for a reasonably fit man or woman, weighing twice as much is a huge strain, the average modern couch potato would be pretty much immobilized.
And that's just the immediate biological concerns.
If you drop something in 2g, it accelerates twice as fast. You're not going to catch it, since your reflexes are as slow as before. So you better never drop anything. And don't trip and fall, you'll break a bone much more easily. Having a hammer fall on your foot will likely result in broken toes. Everything weighs twice as much, so something that could normally be carried by one person now needs two.

Assuming they don't go past 1G, I assume they'd only be able to accelerate to the half way point and then they'd need to reverse the 1G acceleration to stop at the correct location.

That's true no matter what acceleration you use. Accelerating hard and coasting is much less efficient than constant acceleration. So, you need to take half the distance, calculate the time it takes to travel under 10m/s² acceleration (rounding up for convenience) and double it. Wiki has all the relevant distances on the pages for the respective planets. As s=0,5at², the time it takes is t=2(s/a)^0,5, units are metre for s, metre/second for a and second for t.

[benbradley]

Originally Posted by Meatball

Awesome info/answers so far, thanks everyone!

To clarify a bit, I'm really looking to figure out how fast someone could actually get to another planet in our own system, say Jupiter or Saturn as opposed to a long range trip that is slowly building up speed.

That's why I was thinking more of some type of direct thrust system as opposed to a sail or something like that.

Originally Posted by Meatball

What would 1G acceleration actually equate to in velocity?

How long can your ship supply 1g acceleration? The formula for velocity in this context is acceleration times time. 1g is 32 feet per second squared. If you accelerate (from a dead stop) at 1g for 1 second, you'll be going at 32 feet per second.

The Space Shuttle goes into orbit in about ten minutes, and accelerates at something like 3 to 4 g's. At the end of ten minutes it's going at about seven miles per second. I think the limitation on acceleration is more because it has astronauts inside that can only take so much acceleration than other design limitations. I've heard of missiles (for delivering nuclear warheads) having acceleration of ten g's, designed to deliver their payload to the target as quickly as possible, as in crossing oceans in a few minutes.

But yes, if there's a human on board, acceleration is limited to what the human can take. Fighter pilots trained to take g forces can take more than an average person, but it's still limited to the 2 to 4 g range, depending on the duration (lower g's for longer duration - I doubt anyone could handle the Shuttle's acceleration for hours, even if it had that much fuel) and training, or about 1 g if passenger comfort is a must.

The main limitation in moving between planets within the solar system is fuel. Fuel-efficient travel with chemical engines involves relatively short burns with long times of coasting in space, The Apollo astronauts took about 2 or 3 days to go to the Moon, and planned trips to Mars involving current chemical fuel rockets likewise takes rocket burns that last for minutes or hours, and coasting times of months. The rocket burns just change the ship's orbit around the Sun enough that it becomes an ellipse that crosses the target planet's orbit (and are timed so the ship and the planet are both at the crossing at the same time).

If you have enough fuel (think of the Space Shuttle, but with many of those big external tanks instead of just one), you can accelerate all the way, and go "straight" there (okay, maybe not perfectly straight, but the optimum path would be a lot closer to straight). Actually, you'd accelerate halfway there, then decelerate the other half, so you're at a stop when you get there instead of whizzing by or hitting the planet at some high speed. The amount of fuel needed for this goes up by some huge amount (at some power or exponentially, I forget the exact equation, that's why I have books) as your acceleration goes up and your travel time goes down. In short, it becomes hideously expensive (as if space travel weren't already expensive enough). But doing that, you can reduce your travel time between planets from months to days, or in the best cases hours.

[Lhun]

Originally Posted by benbradley

The amount of fuel needed for this goes up by some huge amount (at some power or exponentially, I forget the exact equation, that's why I have books) as your acceleration goes up and your travel time goes down. In short, it becomes hideously expensive (as if space travel weren't already expensive enough). But doing that, you can reduce your travel time between planets from months to days, or in the best cases hours.

The reason for that is inertia. Any additional fuel and reaction mass also need to be accelerated. I.e. let's say one unit of fuel accelerates you to a speed of one. If you take two instead, your first unit of fuel will only accelerate you to a speed of 0,5 since your ship was twice as heavy. The second unit of fuel will then accelerate you to a total of 1,5 which means you doubled the fuel and got only 50% more total speed out of it. Take three units of fuel and it adds up to 0,3+0,5+1 and so on. And that's taking straight acceleration in a line. If you have to take twice that fuel to brake down at the end again, it just gets worse. This is the calculation for chemical drives, since there the fuel(+ reaction mass) is the single most massive part of the ship. And that's why chemical drives are far, far from the theoretical maximum speed/acceleration.
With something like a fusion powered plasma drive or even arcjet, one can get reaction mass efficiencies that allow for constant acceleration, and fuel is even essentially infite.

[Meatball]

Alright, so, dropping back to our 'theoretical' situation of being able to accelerate at a constant 1G. Tell me if I'm figuring out the travel time from Earth to Jupiter right.

Looking at the Brachistochrone equation over on Project Rho, the calculation is:

T = 2 * sqrt[ D/A ]

Where:
T = transit time in seconds
D = distance in meters
A = acceleration in m/s2 - (1G is 9.8 m/s2)
sqrt[x] = square root of x

The distance from Earth to Jupiter varies from 628311058.2 km at it's closest to 927506800.2 km at it's furthest (I'm going off the 4.2 AU - 6.2 AU listed over on WikiAnswers, don't know how accurate that is)

So, at it's closest...

T = 2 * sqrt [628311058200 / 9.8] comes out to 506412.3745949117 seconds or 5.8 Days.

At it's furthest...

T = 2 * sqrt [927506800200 / 9.8] comes out to 615283.8403235252 seconds or 7.1 Days.

Not sure if I'm doing that right, but, 5-7 days? Wouldn't that be nice?

[Lhun]

Almost correct. If earth and Jupiter are both at Perihelion the distance even shrinks to 3,9AU, but that's rare of course.

For the calculation, keep in mind you only accelerate half the way, then brake again, so the complete calculation isn't 2(s/a)^0,5 but 4(s/2a)^0,5 i.e. the time needed for half the distance, multiplied by two.

[Pthom]

I believe you can accelerate for more than half the way. At the beginning of the trip, your ship is full of fuel. At the end of the trip, it's nearly empty, so because the mass to decelerate is less, it can be done in a shorter distance.

[Lhun]

1g is 1g. If you have constant acceleration you need to switch direction halfway. You need less energy to accelerate less mass at the same rate of course.
But it's not going to be too relevant, because fuel mass is mostly a factor for chemical and other similarly inefficient engines, and those couldn't keep up a constant acceleration anyway. Those would just accelerate at the start, and then coast along.

[Maraxus]

If the ship was moving like this with no Star Trek Inertial damper and artificial gravity ... they would obviously fly not like in a plane with the direction of travel forward but with the direction of travel "up". Would the contact pressure due to acceleration be distinguishable from gravity by anything, if they accelerated with 1g?

That sounds cool and I can imagine a mean captain on the way to Alpha Centauri giving his crew some hours of 1.1g if they have been unproductive and maybe a bit of 0.9g on Sundays (a journey which would take about 6 years and I have no idea how much fuel but definitely too much)

Oh and one thing I remember: The turn point has to be very exact in this case. Shouldn't be much problem for a computer but it's something that happens, when the atomic clock and the ship sensors tell the navigation computer, that they have reached exactly half point, not when the captain awakes and thinks: Hey, today it's roughly 3 years anniversary of the mission, we should turn and deaccelerate.

[benbradley]

Originally Posted by Pthom

I believe you can accelerate for more than half the way. At the beginning of the trip, your ship is full of fuel. At the end of the trip, it's nearly empty, so because the mass to decelerate is less, it can be done in a shorter distance.

Originally Posted by Lhun

1g is 1g. If you have constant acceleration you need to switch direction halfway. You need less energy to accelerate less mass at the same rate of course.
But it's not going to be too relevant, because fuel mass is mostly a factor for chemical and other similarly inefficient engines, and those couldn't keep up a constant acceleration anyway. Those would just accelerate at the start, and then coast along.

Actually we're talking about chemical engines "with enough fuel to accelerate the whole way." Yes, it would be a big f'ing humongous fuel (and oxygen) tank(s) and not practical, but if you Absolutely Positively Had To make the trip in days rather than months...*

I can see what Pthom is saying, if the engines create constant force (presuming chemical rocket engines are on-and-off devices rather than throttleable), the acceleration will increase as fuel is burned, because the mass to be accelerated will be less.

Originally Posted by Maraxus

If the ship was moving like this with no Star Trek Inertial damper and artificial gravity ... they would obviously fly not like in a plane with the direction of travel forward but with the direction of travel "up". Would the contact pressure due to acceleration be distinguishable from gravity by anything, if they accelerated with 1g?

Other than practical limitations such as engine noises, it would be indistinguishable. Einstein showed this with a thought experiment:
http://www.astronomynotes.com/relativity/s3.htm

* Such a rocket would likely be cheaper with nuclear power and the reactant being steam or ions shot out the engine at high velocity, but for this argument we're just blindly scaling up current chemical rockets, and especially their fuel tanks.

[Lhun]

Originally Posted by benbradley

I can see what Pthom is saying, if the engines create constant force (presuming chemical rocket engines are on-and-off devices rather than throttleable), the acceleration will increase as fuel is burned, because the mass to be accelerated will be less.

Chemical rockets are some of the easiest to create with adjustable thrust so on/off is a non-issue. And the reasons to not have accelerations much beyond 1g have been mentioned above.

* Such a rocket would likely be cheaper with nuclear power and the reactant being steam or ions shot out the engine at high velocity, but for this argument we're just blindly scaling up current chemical rockets, and especially their fuel tanks.

Scaling up is what star drives are all about. Nuclear salt water rockets or an orion drive are pretty cool concepts (well, you don't want to use a NSWR for planetary take-offs) and with those a trip under a constant 1g is possible, even with current technology. The most important part about them is the significantly increased fuel efficiency and high thrust, as well as increased reaction mass efficiency. Without going nuclear, you have to pick either of the two. Anyway, even when not going nuclear though, any engine with high reaction mass efficiency will likely not lose a significant part of it's mass so. The theoretical limit would be a photon drive.

[Pthom]

Given that this is the "Science Fact" subforum, can someone describe a propulsion system for space ships that does NOT consume mass?