Engineering limits to speed

[Bomb#20]

We all know that relativity forbids us going to light speed or beyond. But from an engineering perspective, what is the practical speed limit? Is there any reason to think we will be able to achieve 0.99c? Or is that just a engineering impossibility?

SLD

Based on known science, yes, it's an engineering impossibility. There are two problems, the energy requirement and drag. From an energy perspective, the fastest we more or less know how to go based on current science is a couple percent of c, in a nuclear rocket. If we ever achieve controlled fusion, we could bump that up to maybe five percent. If you want a rocket to go seriously fast there's no substitute for using antimatter for fuel. If we can ever make that in quantity and store it safely then speeds on the order of half the speed of light become energetically possible.

Alternately, we could skip the rocket and use an external energy source. With enough materials science advances we could build a big enough laser and a thin enough light sail to push a ship up to several percent of light speed just with the momentum of all the photons we hit it with -- probably somewhere intermediate between fusion rockets and antimatter rockets.

But at speeds like that the interstellar medium stops looking like a vacuum -- even an atom every two cubic centimeters is a lot of material to push your way through per second when you're covering thousands of km per second. And as you get closer to light speed those atoms effectively get relativistic mass increases, which means the power it takes to overcome the drag will rise faster with speed than it does in air -- and in air it rises as the cube of speed. Even if half your ship is antimatter fuel you aren't going to get anywhere close to 99% of c.

 

[Malintent]

It was impossible to have every household in the US have a working telephone. completely impossible. If every single American WOMAN (cause, you know, operators can only be women back in 1900) was employed as a telephone operator, then we still would not be able to handle the demand for all of the connections that need to be made every minute of every hour (that's 1900 US population, not even today's).
this was a physical limitation on what is possible (using the technology of the day).

So, it is impossible for telephones to catch on as a piece of every day infrastructure... just a passing fad, said many.

They needed an automatic switching device to solve that problem. So, the automatic switching device was invented. It was called a "computer", and it was invented specifically to accommodate the demand for telephone connections.

These 'impossibilities' are the mothers of invention... never really an impossibility.. but an opportunity.

 

[repoman]

Speaking of going to the center of the earth why don't we build tunnels, evacuate them making them more or less like dirty space and then use MAGLEV to propel our speed needing folks along at about, SAY 30,000 KPH. Don't need to think about sonic booms unless someone's figured out the rate earth bleeds into vacuum that provides sufficient air like capacity to cause problems. Hell. We might seal the tunnel well enough to generate a launch capacity before the missile leaves the tunnel and it might be long enough so G forces tolerable to humans can be maintained.

IOW fuck the air and fuck limits to G forces. What was that mining species back in the day of Star Trek?

This guy is pretty cool, though he sometimes entertains questions that are totally impossible.

 

[Loren Pechtel]

If you've got plenty of time on your hands a light sail might be practicable, but accelerating mass with conventional rockets becomes cost prohibitive at high speeds

Would a light sail continue to generate much thrust once you're far away from the main star, say at a Neptune orbit? Seems to me that a light sail might be useful to get on around the inner solar system but not really much further.

SLD

Lightsails are good for no more than about 1% of lightspeed unless laser boosted. With a booster the limit becomes either how much booster you're willing to build (Read Robert L. Forward's Flight of the Dragonfly for some engineering numbers) or how much the sail can take before being worn away by the interstellar gas. (Something completely ignored in the book.) While I have never seen numbers I suspect the latter becomes a serious issue as your speed gets up there.

 

[Derec]

But at speeds like that the interstellar medium stops looking like a vacuum -- even an atom every two cubic centimeters is a lot of material to push your way through per second when you're covering thousands of km per second. And as you get closer to light speed those atoms effectively get relativistic mass increases, which means the power it takes to overcome the drag will rise faster with speed than it does in air -- and in air it rises as the cube of speed. Even if half your ship is antimatter fuel you aren't going to get anywhere close to 99% of c.

And hitting even a small meteoroid (there is such debris even in interstellar space) at such speeds could punch through the hull of your spacecraft.

 

[Bronzeage]

What do we do, once we achieve near SoL? The next solar system would still be at least 5 years away. If near SoL space travel is going to carry humans, we'll need some Star Trek style technology to keep our gravity dependant bodies functioning in deep space. It would cut the trip to Mars down to about 10 minutes or so, which makes the trip quite appealing.

The Star Trek solution was to ignore the reality of zero gravity, the same way almost every other sci-fi movie and TV show handled the problem.

Even Han Solo Understood the perils of light speed travel. He was willing to expose his spacecraft to enemy fire, just so he could program the navigation computer for the trip.

The Dune stories opted for an organic solution. Promising young students were chemically modified to increase brain capacity, and then permanently installed in a spacecraft as part of the hardware to be the pilot for the rest of their life. Dune used this kind of plot trick quite a bit. Most of their technology existed with no scientific basis, either in reality, or theory. Technology was treated as sorcery. Nobody asked how it worked, it just did.

 

[Jimmy Higgins]

We all know that relativity forbids us going to light speed or beyond. But from an engineering perspective, what is the practical speed limit? Is there any reason to think we will be able to achieve 0.99c? Or is that just a engineering impossibility?

SLD

Based on known science, yes, it's an engineering impossibility. There are two problems, the energy requirement and drag. From an energy perspective, the fastest we more or less know how to go based on current science is a couple percent of c, in a nuclear rocket. If we ever achieve controlled fusion, we could bump that up to maybe five percent. If you want a rocket to go seriously fast there's no substitute for using antimatter for fuel. If we can ever make that in quantity and store it safely then speeds on the order of half the speed of light become energetically possible.

Anti-matter fuel? Didn't that ship pass by a long long time ago (when most anti-matter was destroyed)? Isn't it physically impossible to develop enough anti-matter to fix a match box, or to do so would require much much much more power than you'd ever get out of the anti-matter?

But at speeds like that the interstellar medium stops looking like a vacuum -- even an atom every two cubic centimeters is a lot of material to push your way through per second when you're covering thousands of km per second. And as you get closer to light speed those atoms effectively get relativistic mass increases, which means the power it takes to overcome the drag will rise faster with speed than it does in air -- and in air it rises as the cube of speed. Even if half your ship is antimatter fuel you aren't going to get anywhere close to 99% of c.

The only way to approach c would seemingly be to manipulate space-time and hop and skip across undulations within it. Of course, that'd probably require way too much energy as well if it were even technically possible.

I do ponder the idea of a warp bubble. If you get in a bubble outside space-time, how do you get back in?

 

[Loren Pechtel]

What do we do, once we achieve near SoL? The next solar system would still be at least 5 years away. If near SoL space travel is going to carry humans, we'll need some Star Trek style technology to keep our gravity dependant bodies functioning in deep space. It would cut the trip to Mars down to about 10 minutes or so, which makes the trip quite appealing.

So long as your ship is big enough it's no problem--spin it.

 

[Bomb#20]

What do we do, once we achieve near SoL? The next solar system would still be at least 5 years away. If near SoL space travel is going to carry humans, we'll need some Star Trek style technology to keep our gravity dependant bodies functioning in deep space.

The crew go in one capsule; the tons of inanimate stuff they'll need at their destination goes in a second capsule; there's a long cable from one to the other; the two capsules rotate end over end around the common center of mass.

The Star Trek solution was to ignore the reality of zero gravity, the same way almost every other sci-fi movie and TV show handled the problem.

2001 did it pretty nicely.

Anti-matter fuel? Didn't that ship pass by a long long time ago (when most anti-matter was destroyed)?

Yes; I'm talking about making it, not mining it.

Isn't it physically impossible to develop enough anti-matter to fix a match box, or to do so would require much much much more power than you'd ever get out of the anti-matter?

The latter, yes. You'd need to build a giant solar-powered anti-matter factory next to your home star, use a lot more energy making it than it actually stores, and just stockpile the stuff until you have enough for your trip. The 2nd Law of Thermodynamics applies to every energy storage technology. But hey, you've got a star pumping energy in. True, our current technology for making anti-matter is horrendously inefficient; but I'm not aware of any fundamental theoretical limit to increasing the efficiency a lot. Get the efficiency up to, say, 1%, and we'd probably be in the starship business.

 

[barbos]

Just wanted to point out that drag from interstellar gas is not a problem.
If you travel a light year through 1 atom/cc medium you will get 16micrograms per m^2.

 

[Malintent]

What do we do, once we achieve near SoL? The next solar system would still be at least 5 years away. If near SoL space travel is going to carry humans, we'll need some Star Trek style technology to keep our gravity dependant bodies functioning in deep space. It would cut the trip to Mars down to about 10 minutes or so, which makes the trip quite appealing.

So long as your ship is big enough it's no problem--spin it.

.. and walk on the walls.. but in Star Trek, they did not 'ignore' the issue of gravity.. they just had 'gravity plating' as a very basic piece of life support that was built into either the exterior or interior surfaces of starships... and utilization of the gravity plating as a plot device was integrated into several episodes of Enterprise and Deep Space 9.

 

[Bronzeage]

So long as your ship is big enough it's no problem--spin it.

.. and walk on the walls.. but in Star Trek, they did not 'ignore' the issue of gravity.. they just had 'gravity plating' as a very basic piece of life support that was built into either the exterior or interior surfaces of starships... and utilization of the gravity plating as a plot device was integrated into several episodes of Enterprise and Deep Space 9.

The problem of gravity was never used as a plot device on the original series. Roddenberry knew it would be too expensive to produce an episode with actors hanging from wires.

 

[fromderinside]

Speaking of going to the center of the earth why don't we build tunnels, evacuate them making them more or less like dirty space and then use MAGLEV to propel our speed needing folks along at about, SAY 30,000 KPH. Don't need to think about sonic booms unless someone's figured out the rate earth bleeds into vacuum that provides sufficient air like capacity to cause problems. Hell. We might seal the tunnel well enough to generate a launch capacity before the missile leaves the tunnel and it might be long enough so G forces tolerable to humans can be maintained.

IOW fuck the air and fuck limits to G forces. What was that mining species back in the day of Star Trek?

Seriously. What are calculations and costs for creating large space, say equivalent to a 30 foot diameter tunnel that is 1000 mile long with sustained molecular density as low as that 100 miles above the earth? Stupid, aging, neuro-scientist wants to know.

 

[barbos]

Speaking of going to the center of the earth why don't we build tunnels, evacuate them making them more or less like dirty space and then use MAGLEV to propel our speed needing folks along at about, SAY 30,000 KPH. Don't need to think about sonic booms unless someone's figured out the rate earth bleeds into vacuum that provides sufficient air like capacity to cause problems. Hell. We might seal the tunnel well enough to generate a launch capacity before the missile leaves the tunnel and it might be long enough so G forces tolerable to humans can be maintained.

IOW fuck the air and fuck limits to G forces. What was that mining species back in the day of Star Trek?

Seriously. What are calculations and costs for creating large space, say equivalent to a 30 foot diameter tunnel that is 1000 mile long with sustained molecular density as low as that 100 miles above the earth? Stupid, aging, neuro-scientist wants to know.

Elon, is that you?

 

[fromderinside]

Seriously. What are calculations and costs for creating large space, say equivalent to a 30 foot diameter tunnel that is 1000 mile long with sustained molecular density as low as that 100 miles above the earth? Stupid, aging, neuro-scientist wants to know.

Elon, is that you?

Да, это я Барбос Элон

 

[barbos]

Elon, is that you?

Да, это я Барбос Элон

google translate is working great.

 

[bilby]

Да, это я Барбос Элон

google translate is working great.

"Yes, I am mongrel Elon"

I suspect FDI was trying for "Yes, it is I, Barbos - Elon!" or something similar, which is what I get using the "If only it were English" method of translation.

It will be a while before the machines take over as masters of language.

 

[fromderinside]

"Yes, I am mongrel Elon"

Works for me.

 

[fromderinside]

Shit. FDI just called us out on greatest accomplishment. So here is a non physicist beginning to churn. Elements are requirements for creating large high vacuum environment permitting human injection of man into space. Constraints are injection of large mass into atmosphere at very high speed space.

Overcoming constraints. Launch from very high altitude.

Permitting man to use system requires system creating less than 5 or 6 G forces.

Creating high vacuum:  Ultra-high vacuum


Limiting restriction, forces of injection. Example volcano eruption.