Interstellar travel

Factual Questions
21

John W Kennedy has inspired a new question in me. It’s a very simple question that I have trouble phrasing, so bear with me. Could you achieve a constant acceleration without upgrading your engine (or chucking out larger nuclear bombs or whatever)? Three (big) caveats: ignore the change in mass due to fuel consumption, assume a frictionless environment, and neglect relativistic effects.

What I’m asking is, does an engine of a given size provide a constant speed increase at non-relativistic speeds? Or does it provide a constant kinetic energy gain per second? If the latter is correct, your engine will give you 1 g acceleration in the first block of time, and 0.5 gs acceleration in the second block of time, etc.

22

A globular – or other – cluster might be a compromise between galactic core radiation levels and spiral arm interstellar distances.

Also, distant binaries or other multiples might not be impossible places for life to exist. More than half of all stars have at least one companion.

23

Scylla,
(

50 years is not the same amount of time as 10 years. It is the 10 year trip that I was disagreeing with, not the 50 year, and I think it was quite clear in my earlier message.

This just demonstrates that you’re not conversant with the way rockets work. The ratio of fuel to payload goes up exponentially with the peak velocity you want to obtain, so turning a 50 year trip into a 10 year trip does not mean you need to carry just 5 times as much fuel.

The amount of energy required to accelerate an appreciable mass to .43c is also a Very Big number. Fortunatley, with sophisticated modern mathematics we don’t have to just say ‘ohh, that’s big,’ but we can instead calculate using those Very Big numbers.

This is trivially true, and also doesn’t say that you can achieve a velocity of .43c. 1,290,000m/s is an enourmous velocity.

No, the problem here is DEFINATELY carrying enough fuel. As I demonstrate below, taking a 10 ton spaceship on a .43c journey with a perfectly efficient fission drive requires nuclear fuel massing close to 1% of the EARTH’s mass. We do not belive the Earth is made up of as much as 1% uranium, and we do not have the technology to mine/manufacture or store that quantity of fissionables, so it is clear that we do not have the technology to get a spacecraft up to .43c.

This is an appeal to authority and not a logical argument. You’re simply invoking Sagan’s name like a mystic spell to support what you say; you’re certainly not citing a source, as you’ve already admitted that Sagan’s figures do not match up with yours. It is simply a cop-out to wave around a famous person’s name in an attempt to disuade someone else from arguing with you.

The numbers: http://www.treasure-troves.com/physics/RocketEquation.html has the basic rocket equation. Since we’re looking for the mass ratio (the mass of the payload plus fuel (Mo) divided by the mass of the payload (M)), we solve for Mr and get Mr=Mo/M=exp(u/v) where u is the desired velocity and v is the exhaust velocity.

I would note that I’m using the Newtonian rocket equation here, as it’s simpler and I’ve already lost my first go at posting this. The relativistic rocket equation would worsen the fuel requirements. Below I also assume that the Orion drive is 100% efficient, while an actual Orion drive is much less efficient.

Nuclear fission releases about 8 x 10^13 J/kg, so your ideal exhaust speed is roughly 1 x 10^7 m/s, or ~0.04 c. Your desired velocity is .43c, so we find that Mr=exp(.43/.04), or ~5e4. This means you’d need roughly 50,000 kilograms of fuel for every kilogram of payload, or 5,000,000 kilos of fuel for a 100kg person.

But wait! We have to both accelerate and deccelerate to reach a destination, so we have to square that mass ratio, giving us a mass ratio of 2e9 for a round trip (you square because you’re carting around the fuel for the second leg while accelerating in the first leg).

While these numbers are high, you probably won’t have time to acquire these astronomical amounts of uranium in AC, so you’ll need to carry enough fuel for a return trip home, bringing your fuel ratio to a staggering 4e18.

This means that you need 4e20 kg of uranium/plutonium to get a single 100kg person up to .43c, or 4e22 kg of fissionables to get a 10 metric ton spaceship up to .43c. The mass of the Earth is around 6e24, so you’re going to require almost 1% of the mass of the earth in fissionables to make your journey once.

And it gets even worse! In reality, unless you want your passengers splattered across the ‘living’ quarters, you have to accelerate at a reasonable rate and not instantaneously. For a trip to AC, this roughly doubles the required peak velocity to .86c (because you’ll be below it for most of the trip).

I’m not going to use the relativistic rocket equation because I think I’ve written enough, but the answer comes out to a mass ratio of 1e56, which means that moving a 100kg payload requires more nuclear fuel than the estimated mass of the observable universe.

Kevin Allegood

24

Yes, it provides a constant speed increase. If it accelerates your spaceship at 1 g initially it will continue to accelerate at 1 g. In fact it the acceleration will increase, not decrease.

More precisely, a typical rocket engine (including the nuclear engine proposed above) provides a constant thrust, thrust being a fancy word, in this case, for force. Newton’s Law tells us that force * mass = acceleration. So constant thrust of a constant mass gives a constant acceleration.
But the acceleration actually increases, since the mass is decreasing as the fuel is consumed.

At relativistic speeds, of course, the mass increases and the acceleration therefore decreases.

“To do her justice, I can’t see that she could have found anything nastier to say if she’d thought it out with both hands for a fortnight.”
Dorothy L. Sayers
Busman’s Honeymoon

25

One more thing - even if we only assume a velocity of .1c is needed (thus making the trip in 86 years), the mass ratio for a perfectly efficient fission drive is only around 11 to get up to the speed (meaning your fuel only needs to be ~10 times your payload), but to accellerate, decellerate, and make the trip home your mass ratio is still ~20,000, meaning you’d need to get your hands on some 200,000,000 kg of fissionables to make the trip with a 10 ton ship.

Realistically, a 50% efficiency is optimistic for an Orion drive since a lot of the explosion goes off into empty space instead of impacting the pusher plate, but we’ll use that here just so you can see how quickly these things add up. You’d end up with a ratio of around 400 million for the round trip, so our 10 ton ship would require 4 (American) trillion kg of fissionables to make the trip.

Sagan always was a little over-enthusiastic about the ease of space travel; getting to .1c (what I’d call a significant fraction of c) in space travel requires absolutely incredible amounts of fuel.

Kevin Allegood

26

Riboflavin:

I never said it would be easy, just possible.

I didn’t say anything about returning from AC , just getting there.

You don’t need all that much uranium or plutonium, (or any for that matter) “Chucking nuclear bombs out the back” was gross simplification on my part, but I didn’t want to write ten pages.

You don’t need as much fuel to decellerate, because you weigh less.

Sagan’s spacecraft starts off fairly light, picks up momentum around the sun, does the same thing as it passes jupiter (picking up more fusion mass from the gas giant by scoop.)

Along the way it picks up more mass from interstellar dust and a magnetic scoop.

If their is a gas giant or sun in AC (which seems reasonable, they repeat the process to slow down.

Interstellar travel is not a simple accelerate/decellerate function of mass and velocity. You can pick up or lose significant mass and/or momentum by slingshotting around suns, and gas giants (the latter also providing fuel)

I am not about to attempt the calculus here to show you what I mean, but would you agree that the situation was not as bad as it first seemed?

Sheesh!

I admit 10 years is a WAG, and probablynot feasible, ok.

But, 50 ain’t so tough. We already did the slingshot thing with voyager, so there is proof that it can be done.

As for invoking Sagan. Yes, I do so as as he is a careful scientist who is well respected. He simply describes the Orion project, and the subsequent theoretical work, as it was performed by the scientific community. Works for me.

Often wrong… NEVER in doubt

27

Neither Uranium nor Plutonium are neccessary for the construction of a nuclear weapon.

Use your search engines, children. The subject is “laser fired cold squeeze fusion bombs”.

These are theoretically possible; & Heaven help us, somebody at the Pentagon may have been stupid enough to build one.

“Show me a sane man, and I will cure him for you.”----Jung

28

You’ve all forgotten yet another hazard of flying at high speeds: cosmic rays.

Cosmic rays are actually atomic particles flying at near-light speeds. And it makes no difference whether it’s you who’s going at .43c or if it’s the proton or neutron that’s travelling that fast or a combination of your velocity and the particle’s velocity: The damage would still be done. You’d need a combination of a positively-charged, VERY strong electromagnetic field on the bow of the ship to repel the protons (Is that a “force field?”) and some kind of lead shielding to absorb the neutrons. (And I don’t know how you’d protect yourself from the free-floating atoms of anything heavier than helium.) (Yeah, a ramscoop would collect free-floating hydrogen, but how powerful would it have to be to collect hydrogen moving at .43c?)

>< DARWIN >
__L___L

29

My recollection of 8.01 lectures may be a little shaky, pluto, but I think the equation goes f=ma, not fm=a as you posted. As far as the rest of the discussion, it seems to uphold my initial statement that reasonable people agree that earth-dwellers will not visit distant stars anytime soon. To reframe the OP: should I be envious of other civilizations with earth-level technology that happen to have the great good fortune of being located in a densely populated galactic neighborhood, with stars, say, 0.1 or 0.2 light-years apart? Or would such proximity not make any difference as far as ease of travel with our level of technology?

30

No, it’s a gross equivocation on your part. Project Orion was about developing a spacecraft that works by detonating small fission bombs roughly 100-200 feet behind the ship, and having the explosion his a pusher plate and so propel the ship. You seem to have had a rather clear idea of what Orion is, as demonstrated by the quotes below.

An Orion-style spacecraft COULD be built today, but COULD NOT make the 10-year trip you propose, as I’ve said many times. The craft you described in your last message has nothing to do with an Orion-type craft and could not be built today.

If I say that X is not possible, I’m not wrong simply because you begin talking about Y. I firmly stand behind saying that your claims that an Orion-type craft could make a trip to AC in 10 years, or even 50 years, are simply untrue.

You need much more fuel overall, because you have to accelerate the fuel reserved for decceleration during the initial acceleration. This is very basic physics, but is often overlooked by the over-enthusiastic.

Slingshotting will save you some marginal fuel costs, but is not going to get you anywhere near a significant fraction of c.

{QUOTE] (picking up more fusion mass from the gas giant by scoop.)
[/QUOTE]

If the ship Sagan is describing uses fusion, it is not an Orion type ship as an Orion drive, by definition, uses small fission explosions; it would be a Daedellus(sp) drive if it used small fusion explosions, or some other sort of fusion drive if you use another mechanism.

Aside from your changing the type of ship being discussed, we do not have a working fusion drive today, which means this ship could not be made with today’s technology as you had earlier claimed.

First of all, how do you use a magnetic scoop to get your hands on a significant quantity of unionized hydrogen? No one has come up with anything approaching a design for an actual working ramscoop, so although it’s theoretically possible to create such a device, it is AGAIN certainly not today’s technology.

Even if you can scoop up some hydrogen, what good does it do you? We’re only capable of getting one or two percent more energy out of hydrogen than we put in to fuse it, and that’s not using a ‘standard’ mix of hydrogen, but one using deuterium and tritium (which are relatively rare in interstellar hydrogen). Again, this is a technology that we do not have, although we’ve been 30 years away from having it for about 40 years now.

It is if you’re using a rocket, which (broadly speaking) is what an Orion drive is. If you’re talking about a variety of drive type, then you shouldn’t call them by the name of one specific drive.

Slingshotting doesn’t give you anywhere enough of a momentum gain to change your velocity by a large fraction of C unless you’re using a black hole or neutron star to slingshot around. Gas giants, again, don’t provide fuel for an Orion-type drive, and only provide you with a place for refueling a fusion drive; you’ve still got to carry enough fuel with you for the trip.

No, because the ship you described uses technologies that have not been developed yet, including one (ramscoops) that may not be practically possible. You can’t use a bunch of not-developed technology to say that a 10-year flight is possible now.

Voyager is going to take a few tens of thousands of years to pass the nearest stars, so slingshotting for significant fractions of c has not been proven, and won’t be possible unless the laws of gravity undergo a dramatic revision.

Well, Newton Einstein Hawking to you! Simply saying the name of a scientist who didn’t even support what you said doesn’t lend any credibility to your arguments.

If he describes the Orion project as using something other than nuclear fission, he’s simply wrong, much like he was about nuclear winter.

Kevin Allegood

31

Uranium and plutonium are the most common fissionable elements, and so are normally used when one is talking about a fission drive. As project Orion only dealt with fission explosions, fusion explosions are irrelevant to the discussion. Also, as laser-fired fusion bombs are currently only theoretical, they are not a technology available today that could be used for making an interstellar voyage.

Perhaps the ‘children’ should use their reading skills to read what is being written before telling people that they are incorrect.

Kevin Allegood

32

If the stars are that close together, you’d have problems getting Earthly life, since near passes by stars would be common enough to make planetary systems unlikely.

Assuming you did live where the stars were much closer together, shorter distances are much easier to cross, but whether there’s anything worth finding at your nearby stars is an open question; currently it looks like an area with very densely packed stars would not have many stars with planetary systems.

33

How do you guys do that nifty “quote” thing?

34

Here’s a link that shows a lot of the commands:
http://www.straightdope.com/ubb/Forum2/HTML/000194.html

To answere your immediate question, format it like this:

{quote}Words that you want to quote{/quote}

Format it exactly (includeing the words “quote.” However, instead of using braces {}, use brackets.

35

Riboflavin:

We seem to be hijacking Sputnik’s thread here.

This might be an interesting topic for Great Debates.

I merely use Orion for a starting point towards what may be possible using today’s technology, and fully admit to making not unreasonable extrapolations from this.

I respect the effort you made in your calculations, and have already said that 10 years was a “WAG.” (Wild-Assed Guess,)and not realistic.

Perhaps one should closely examine the posts of “the children,” before replying. Let’s not get unpleasant though.

Some of your objections are valid, some are not.

I see the major drawback to an interstellar nuclear-pulsed driven craft as construction.

It would probably take longer to build than the trip.

This could be an advantage because you can send fuel ahead.

Don’t be so quick to dismiss the concept of picking up momentum from the sun. A lot of your objection is the question of accelerating the mass of all that fuel. In this context it probably is significant.

Finally, an interstellar trip is likely to be a one way trip. There is no need to decelerate the entire craft, just the habitat, and such. You use the mass of the bulk of the spacecraft to help slow you down, then let it keep going.

If you want to get back, build another ship.

Obviously a fusion type drive is not possible “today,” but how far off is it? Perhaps I was wrong to include one based on the stipulation of “today’s” technology.

A light sail could also help you slow down.

The trip is not a simple start/stop equation as you seem to indicate. There is no need to accelerate all the fuel at the outset, and no need to stop the entire craft. Without attempting the math, (which you haven’t either.) it seems obvious that significant gain could be achieved using the gravity wells of both our sun and ACs to help accelerate/decelerate (a not quite Wag.)

Based on this, and your own obvious knowledge of this subject, does 50 years still seem unreasonable to you?

Often wrong… NEVER in doubt

36

Riboflavin–

Since you seem extreamly good at these types of questions, could the fission/fusion type of ship give us interplanetary travel?..More importantly, would that system be better than the current rocket construction?..I know I’m not being very clear here, I can just hope you understand my question

37

I’ll chime in and agree that it’s just not feasable right now.

I think the key to intersteller travel is to eliminate the whole concept of fuel. I am familiar with the previously mentioned Light Sail concept, but am unaware if it has ever been tested. Beaming the necessary energy to propel an expeditionary craft from Earth, the Moon or an orbiting station seems to be the way to go.

Such a craft would be ultra-light and would accelerate to enormous speeds quickly. There’s still the problem of slowing down - how would this be possible without carrying any fuel? So I suppose you would need 1/2 the fuel needed for a traditional journey. This helps the timeframe a bit.

We could send an unmanned relay power station ahead of the craft to arrive in the intended “landing zone” and direct energy at the craft to slow to orbital speed. That sounds like some complex physics, though - to predict the exact location of termination and develop a reliable power station (and get THAT there with conventional power) and have the whole thing work: yikes. If it doesn’t work, the manned craft will just zoom right by the landing zone until it strikes something.

Any journey which takes more than 10 or 20 years is futile because of the unfeasability of men living comfortably in a small craft for such a period. Plus they know that the return trip (if possible) will take an equally long time. It is also pointless because of the likelyhood that in that timeframe we will develop better, more efficient technology, which will allow a craft to pass right by the previously launched one.

Hell is Other People.

38

Ah. 10-250 or 26-100?

jrf

39

posted 01-19-2000 11:27 AM

                Sake:

                Buzz Aldrin talks about such a ship in _Encounter with Tiber_.

                Aldrin's ship accelerates by a "laser" from it's home system, makes a slight course change
                and launches a "mirror" ahead of itself.

                It then decelerates by the deflection of light from the "mirror." The mirror just keeps
                going.

                Good point:

                I've stretched present technology in my posts (and been called on it.)

                We're not close to anything like what Aldrin or you describe yet, though I have read
                about somebody launching a "pie plate," to significant altitude with a laser.

                ------------------
                Often wrong.... NEVER in doubt
40

I went and purchased a book by Dr. Robert Zubrin Astronautical Engineer. Entering Space.

He wrote a paper for The Journal of The British Interplanetary Society, describing just some of these problems, including the math. It concerns “How much acceleration can be applied at what altitude over a dense object of a given mass”

If I’m reading this correctly (which is a big if, but the charts around pg 214,) you can multiply your velocity by somewhere between ten and a hundredfold depending on how much thrust you apply (he pegs Orion at a thrust to mass of 100 to 1.) He doesn’t tell you how long this trip would take to AC. I would guess that the primary drawback here is how many G’s you could reasonably apply to a manned vehicle and expect it to survive.

On page 204 he pins down quite definitively a case for a manned craft leaving the solar system at .05c, and accelerating to .15c within two months that could make the trip in 29 years! (this is beyond an Orion’s capability though. I would still guess 50 years.

A 400 meter orbital laser projector powered saleship could make the trip in about 15 years, (Consuming about 1/2 the earth’s current yearly average powere usage within a month or two.)

This does not count stopping, but on the next page he describes a magnetic sail that could be used for braking upon approach to another solar system, with a peak deceleration of about 18gs as you approached the equivalent of Mercury’s orbit.

Both of these technologies (laser and magnetic sail,) are potentially within the next decade. We could have had them 10 years ago, if we were trying according to Zubrin

Sake Samarai:
Thanks for pointing this stuff out.

Often wrong… NEVER in doubt