Author Topic: Constant Acceleration at 1G and Beyond  (Read 43713 times)

Offline tea monster

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Re: Constant Acceleration at 1G and Beyond
« Reply #100 on: 05/03/2015 12:32 PM »
There were various proposals for using the original nuclear pulse Orion ships for interstellar journeys. Before you condemn it for using nuclear bombs, and thus being dangerous, I think that once the general public find out how dangerous antimatter really is, they will embrace pulse ships with open arms.

Offline KelvinZero

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Re: Constant Acceleration at 1G and Beyond
« Reply #101 on: 05/03/2015 01:11 PM »
That ridiculous scheme is not ridiculous enough for this particular goal ;)

You might also want to look up Robert Zubrin's Nuclear Salt Rocket as a fun thing to try some day.

Online Stormbringer

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Re: Constant Acceleration at 1G and Beyond
« Reply #102 on: 05/03/2015 01:57 PM »
There were various proposals for using the original nuclear pulse Orion ships for interstellar journeys. Before you condemn it for using nuclear bombs, and thus being dangerous, I think that once the general public find out how dangerous antimatter really is, they will embrace pulse ships with open arms.
The latest proposed Orion derivatives don't use bombs. they use deuterium pellets fused by lasers. so no proliferation hazard and little in the way of environmental risk if a carrier rocket malfunctions or otherwise crashes and burns.

Also the type of antimatter ship likely to be available to us near to medium term would be hybrid systems that require either a nanogram or a microgram depending on which scheme is selected. E.g; AIMSTAR. Either quantity is essentially without risk.
When antigravity is outlawed only outlaws will have antigravity.

Offline Esteban

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Re: Constant Acceleration at 1G and Beyond
« Reply #103 on: 12/14/2017 07:44 AM »
I didn't expect this from a NASA forum. I figure you mostly aren't scientists, but neither readers of science fiction?

The key is "constant" acceleration. When in your car you stop going faster and maintain your speed, _irrespective_ of it, you'll feel about 0g from acceleration. Keep in mind that maintaining speed or acceleration aren't synonymous. To keep feeling 1g, you need to never stop accelerating *at 1g" and spend ever more energy, because  as Einstein said, you'll have more mass the faster you go. You'll reach c very fast, if only were impossible of course.

And that is really the difficulty, 1g is not a speed, as 80km/h, but a rate of acceleration. Your speed will forever increase, but your acceleration will remain constant.

So that means all your calculations about energy needed, are moot.

Offline Nilof

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Re: Constant Acceleration at 1G and Beyond
« Reply #104 on: 12/14/2017 07:51 PM »
http://www.projectrho.com/rocket/supplement/dvNomogram01.pdf

Fusion can do interplanetary 1g brachistochrones with somewhat reasonable mass ratios. It definitely can't do interstellar brachistochrone trajectories. You need antimatter or laser sails for that.

It gets better at ultrarelativistic speeds because proper acceleration isn't the same thing as coordinate acceleration, and proper velocity increases exponentially with rapidity in that range. But getting to that speed range is a pure fantasy to begin with.

Edit:

Ah, this got lifted by necroposting.

I didn't expect this from a NASA forum. I figure you mostly aren't scientists, but neither readers of science fiction?

This isn't a nasa forum. It's a spaceflight forum that has nasa in its name because most of it is devoted to US spaceflight.
« Last Edit: 12/14/2017 07:54 PM by Nilof »
For a variable Isp spacecraft running at constant power and constant acceleration, the mass ratio is linear in delta-v.   Δv = ve0(MR-1). Or equivalently: Δv = vef PMF. Also, this is energy-optimal for a fixed delta-v and mass ratio.

Offline Esteban

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Re: Constant Acceleration at 1G and Beyond
« Reply #105 on: 12/16/2017 03:56 AM »
http://www.projectrho.com/rocket/supplement/dvNomogram01.pdf

Fusion can do interplanetary 1g brachistochrones with somewhat reasonable mass ratios. It definitely can't do interstellar brachistochrone trajectories. You need antimatter or laser sails for that.

It gets better at ultrarelativistic speeds because proper acceleration isn't the same thing as coordinate acceleration, and proper velocity increases exponentially with rapidity in that range. But getting to that speed range is a pure fantasy to begin with.

Edit:

Ah, this got lifted by necroposting.

I didn't expect this from a NASA forum. I figure you mostly aren't scientists, but neither readers of science fiction?

This isn't a nasa forum. It's a spaceflight forum that has nasa in its name because most of it is devoted to US spaceflight.

Humm, I've always assumed orbits to be brachistochrone enough to be worth mentioning, but of course you're right and they're different.
And I'd truly be amazed the day we have the capacity to "set brachistochrone curve to yonder star, Mr. Sulu!" in the actual definition of the word.

Sorry, didn't know necroposting was bad form.

Ah, I see, the name fooled me, thanks!


Offline D_Dom

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Re: Constant Acceleration at 1G and Beyond
« Reply #106 on: 12/16/2017 03:47 PM »

Sorry, didn't know necroposting was bad form.


Not bad form, merely an observation. Often threads like this are inactive because the interest level has dropped off. No reason to not bring it back into discussion if you have a question or something else to add. I learned a new term today, thanks for that!

Looking into this I found Wolfram Alpha solving the problem "Find the shape of the curve down which a bead sliding from rest and accelerated by gravity will slip (without friction) from one point to another in the least time...
In the solution, the bead may actually travel uphill along the cycloid for a distance, but the path is nonetheless faster than a straight line (or any other line)."
http://mathworld.wolfram.com/BrachistochroneProblem.html

Sun is our closest star, wonder what the solution is to Alpha Centauri?
« Last Edit: 12/16/2017 04:05 PM by D_Dom »
Space is not merely a matter of life or death, it is considerably more important than that!

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