The dumb questions thread

[Bomb#20]

What's the thrust impulse distinction?

To expand on what Loren wrote, a rocket, unlike an aircraft, has to do everything itself. So it has to be hyper-efficient. The trouble is, there are two different measures of efficiency -- energy efficiency and reaction-mass efficiency -- and they conflict with each other. To get a given amount of thrust, i.e. a given push, you have to throw some reaction mass out the back, at some speed. The strength of the push is just the amount of mass you jettison multiplied by how fast you shove it away. So to be reaction-mass efficient you want to throw it out as fast as possible: make it fly away from you twice as fast and you only need to throw away half as much stuff to get the same push. But kinetic energy is E=1/2mv², and that square term means that to be energy efficient you want to throw it out as slow as possible. Throwing away twice as much stuff at half the speed takes only half as much energy to give you the same push. Since the rocket is carrying with it both the energy and the reaction mass, there's always this balancing act going on, where the rocket scientists had to figure out how fast to throw the reaction mass in order to not run out of either mass or energy too soon. "Specific impulse" is basically the same thing as the speed of the rocket exhaust, just expressed in funny units. The idea is, if you're throwing your reaction mass out so fast that you accelerate in the opposite direction at 1 g, "specific impulse" is however long can you keep doing that before you run out of reaction mass. (And a bigger fuel tank doesn't help, because that just means you'll have to throw out more fuel per second, just to accelerate your bigger fuel tank at 1 g.)

There are basically two kinds of space rockets we've built: high-thrust low-specific-impulse rockets like the Space Shuttle, and low-thrust high-specific-impulse rockets like the Dawn space probe. We don't make the high-thrust high-specific-impulse rockets you need for your proposal, because the energy requirements are severe -- you shoot out a lot of reaction mass very fast, and pay the v² penalty. The only things we know how to build that deliver that much energy per kilogram are nuclear reactors and atomic bombs.

 

[Bomb#20]

The point of my dumb question was to weed out my possible misinterpretation of escape velocity. Given a sufficiently advanced technology to overcome the volume and weight of fuel, there seems to be no reason why we couldn't eventually stray far from earth without ever reaching such speeds.

For instance, with the right propulsion system capable of enormous light weight regenerating power, we should be able to travel towards the sky and take a break and just hover after every mile if we chose. We could travel at 20MPH and eventuallly escape the gravitational pull of earth. It might take a long while, but the point is that escape velocity has a ring to it that seems to suggest 25,000 mph or so is necessary to go to the moon. Maybe to get into a very slow decaying orbit perhaps.

So, escape velocity is the orbital speed necessary to escape the pull of gravity so that we can stay in space and not be pulled back to earth. With unlimited thrust capability and no concerns for fuel, however, reaching an orbital trajectory with sufficient speed to maintain a safe stay is unnecessary.

We just need to constantly produce thrust to maintain a position. Reaching orbit is just the practical route to take given our technological limitations.

Remember, escape velocity declines with altitude. So there's no need for even a conventional rocket to go to 25,000 MPH to get to the moon, or even to leave earth orbit altogether. 17,000 MPH is enough. First you launch into low earth orbit; then you gradually raise your orbit until you reach the moon or beyond. You're slowing down the whole way, so your speed tops out at 17,000 MPH. To escape altogether you'd still need to reach escape velocity, from wherever you are when you leave earth orbit, but at, say, the distance of the moon, escape velocity is only 3000 MPH. In general, escape velocity from a given radius is always circular orbital velocity at that radius times 1.414 (the square root of 2).

The Apollo missions went at 25,000 MPH because the spiral-outward method takes too long -- we couldn't have kept the astronauts alive that long with 1969 technology.

 

[humbleman]

So, you only canreach just over the clouds with chemical rockets, but going to the moon will require a rocket which will cause such an impulse to reach the moon: the antimatter rocket!

Lets see the SLS

To expand on what Loren said, I know it's counterintuitive, but getting to deep space (or even just to geostationary orbit) quickly is an awful lot easier than getting there slowly. Everybody who wants to do more than an Alan Shepard-style suborbital hop starts out by accelerating at high g-force up to 17000 MPH or thereabouts; after that you can speed up or slow down at your convenience, depending on where you want to go and what route you want to take. Fast's question was about skipping that first step -- doing the whole trip with a speed limit of 12,500 MPH, or even 1000 MPH. The lower the speed limit, the harder the challenge and the crazier the technology it would take.

Even when numbers can be manipulated and this way fit in our wishes of traveling to the moon in very easy steps, my dumb question is, where the hell comes that antimatter rocket?

It comes from fast's question: "Is it physically possible to". Everything not ruled out by the laws of physics is physically possible, and we don't know of any law of physics that proves antimatter rockets are impossible. Presto: one antimatter rocket!

Wait a minute.. wait one more minute...

I think this thread about skyrockets is asking dumb questions, not giving dumb answers...

 

[rousseau]

My dumb question is whether complicated questions about physics that need extended discussion can be properly classified as dumb questions?

 

[Angry Floof]

My dumb question is whether complicated questions about physics that need extended discussion can be properly classified as dumb questions?

If a question becomes a long discussion, we can move it to its own thread if needed.

 

[bilby]

My dumb question is whether complicated questions about physics that need extended discussion can be properly classified as dumb questions?

If a question becomes a long discussion, or in the (highly unlikely) event that it isn't dumb enough, we can move it to its own thread if needed.

FTFY

 

[rousseau]

My dumb question is whether complicated questions about physics that need extended discussion can be properly classified as dumb questions?

If a question becomes a long discussion, we can move it to its own thread if needed.

I was just joking but that's sort of the thing. If you're asking an interesting question that needs extended debate, why not it's own thread? That way people know you're talking about something interesting rather than the convo getting buried in this thread.

 

[Loren Pechtel]

To expand on what Loren said, I know it's counterintuitive, but getting to deep space (or even just to geostationary orbit) quickly is an awful lot easier than getting there slowly. Everybody who wants to do more than an Alan Shepard-style suborbital hop starts out by accelerating at high g-force up to 17000 MPH or thereabouts; after that you can speed up or slow down at your convenience, depending on where you want to go and what route you want to take. Fast's question was about skipping that first step -- doing the whole trip with a speed limit of 12,500 MPH, or even 1000 MPH. The lower the speed limit, the harder the challenge and the crazier the technology it would take.

Nitpick: 18,0000 mph.

Bigger issue: That speed must be horizontal. You want to build as much horizontal speed as soon as you can. It's very counterintuitive but you are better off burning horizontal in the upper fringes of the atmosphere than climbing above it before doing the burn.

What's going on is that until you reach orbital speed you are spending some of your fuel just to keep from falling back. The more horizontal velocity you have the less this loss. At orbital velocity there is no longer any loss.

Anyone who wants to understand how this stuff works should go play the game Kerbal Space Program. The physics simulation isn't anything like perfect but it's good enough that all basic aspects of powered rocket flight are realistic. (The thermal model is quite another matter--I've brought home rockets with a core temperature far hotter than an oven--no problem. If the capsule survives so does the crew.)

 

[Loren Pechtel]

There are basically two kinds of space rockets we've built: high-thrust low-specific-impulse rockets like the Space Shuttle, and low-thrust high-specific-impulse rockets like the Dawn space probe. We don't make the high-thrust high-specific-impulse rockets you need for your proposal, because the energy requirements are severe -- you shoot out a lot of reaction mass very fast, and pay the v² penalty. The only things we know how to build that deliver that much energy per kilogram are nuclear reactors and atomic bombs.

The problem with reactors is that you can't push the energy all that fast. I'm not aware of any way of lifting your rocket off Earth with one. The closest I'm aware of is NERVA (basically, run hydrogen through a reactor running very hot) and it has an ISP a bit over 800 and it's nowhere near powerful enough to lift itself off Earth, let alone any fuel or payload.

There are designs that run a lot hotter--but at the expense of not containing the reaction. The exhaust is dirty.

The only thing that can actually lift off Earth is Orion and it's dirty, it will turn it's launch site into a radioactive crater and it can't land--once it's launched it stays in space forever. Planetary landings--including bringing the crew back to Earth--have to be by auxiliary craft.

 

[bilby]

There are basically two kinds of space rockets we've built: high-thrust low-specific-impulse rockets like the Space Shuttle, and low-thrust high-specific-impulse rockets like the Dawn space probe. We don't make the high-thrust high-specific-impulse rockets you need for your proposal, because the energy requirements are severe -- you shoot out a lot of reaction mass very fast, and pay the v² penalty. The only things we know how to build that deliver that much energy per kilogram are nuclear reactors and atomic bombs.

The problem with reactors is that you can't push the energy all that fast. I'm not aware of any way of lifting your rocket off Earth with one. The closest I'm aware of is NERVA (basically, run hydrogen through a reactor running very hot) and it has an ISP a bit over 800 and it's nowhere near powerful enough to lift itself off Earth, let alone any fuel or payload.

There are designs that run a lot hotter--but at the expense of not containing the reaction. The exhaust is dirty.

The only thing that can actually lift off Earth is Orion and it's dirty, it will turn it's launch site into a radioactive crater and it can't land--once it's launched it stays in space forever. Planetary landings--including bringing the crew back to Earth--have to be by auxiliary craft.

I am unconvinced that Orion need be dirty enough to worry much about. Just launch it from a former atmospheric bomb test site - they are cool enough to be as safe to work in as LEO anyway. If you are getting less than 1mSv/day from the launch site, then the exposure you get once in orbit is worse.

The Nevada test range would be a suitable location. And a well designed fusion device need not be particularly dirty - certainly not on the scale of the stuff they did there in the '50s.

 

[Loren Pechtel]

There are basically two kinds of space rockets we've built: high-thrust low-specific-impulse rockets like the Space Shuttle, and low-thrust high-specific-impulse rockets like the Dawn space probe. We don't make the high-thrust high-specific-impulse rockets you need for your proposal, because the energy requirements are severe -- you shoot out a lot of reaction mass very fast, and pay the v² penalty. The only things we know how to build that deliver that much energy per kilogram are nuclear reactors and atomic bombs.

The problem with reactors is that you can't push the energy all that fast. I'm not aware of any way of lifting your rocket off Earth with one. The closest I'm aware of is NERVA (basically, run hydrogen through a reactor running very hot) and it has an ISP a bit over 800 and it's nowhere near powerful enough to lift itself off Earth, let alone any fuel or payload.

There are designs that run a lot hotter--but at the expense of not containing the reaction. The exhaust is dirty.

The only thing that can actually lift off Earth is Orion and it's dirty, it will turn it's launch site into a radioactive crater and it can't land--once it's launched it stays in space forever. Planetary landings--including bringing the crew back to Earth--have to be by auxiliary craft.

I am unconvinced that Orion need be dirty enough to worry much about. Just launch it from a former atmospheric bomb test site - they are cool enough to be as safe to work in as LEO anyway. If you are getting less than 1mSv/day from the launch site, then the exposure you get once in orbit is worse.

The Nevada test range would be a suitable location. And a well designed fusion device need not be particularly dirty - certainly not on the scale of the stuff they did there in the '50s.

1) Ask the downwinders about the safety of the test site. (Although the claims are overstated, there is an actual issue.)

2) What scrapped Orion is they estimated 10 radiation deaths per launch. That's not at the launch site, that's from the fallout.

3) Orion uses fission devices. Fusion is simply too big.

4) Your dose for LEO is 6x what you actually get on the ISS.

 

[humbleman]

Dumb question.

Before "going to Mars" (which is opium dreams until health of astronauts is maintained stable for long periods of time in outer space) the suggested step is going to the Moon "and return back from" the moon as well.

No sh*t is accepted that "we did it before, we don't need to do the same again".

Fact is that first, man must go to the moon in this new adventure, and return back, after testing this way the new spacecrafts, then, probably the next step must be Mars.

However, going to the moon first: what is the reason why the "old technology" is not used for traveling to the moon?

Please understand that "economy, green world protesters", and so forth are not a credible answer. One more travel using old technology won't double the current national debt. Using kerosene as fuel will make us compete for a few minutes with the black carbon produced in Africa, but hey! it's just a few minutes...

Why not traveling to the moon with the old technology when is "obvious" it does work perfectly for this purpose?

 

[Keith&Co.]

However, going to the moon first: what is the reason why the "old technology" is not used for traveling to the moon?

No one builds it anymore... That's the main reason submarine fire control can't use modules that were state of the art ten years ago, and have to redesign subsystems on a regular basis.

One more travel with old technology will require much more financial investment than merely buying the parts. We would have to stand up the old production methods, make sure the old materials are available in sufficient quantities, train the designers in the old tech's characteristics, limitations, and functionality, basically ride our Segways to the 'reinvent the wheel' factory and learn to handle the slide rulers.

Why not traveling to the moon with the old technology when is "obvious" it does work perfectly for this purpose?

But if the purpose of the trip to the moon is to prepare for the trip to mars, it is not 'obvious' that old moon-landing tech works perfectly for THAT purpose. It would be a pointless exercise in expense. We'd still have to develop new tech for the Mars trip. And test it, possibly by a trip to the Moon... Again...

Redundant PR stunt is redundant.

 

[Loren Pechtel]

Dumb question.

Before "going to Mars" (which is opium dreams until health of astronauts is maintained stable for long periods of time in outer space) the suggested step is going to the Moon "and return back from" the moon as well.

No sh*t is accepted that "we did it before, we don't need to do the same again".

Fact is that first, man must go to the moon in this new adventure, and return back, after testing this way the new spacecrafts, then, probably the next step must be Mars.

However, going to the moon first: what is the reason why the "old technology" is not used for traveling to the moon?

Please understand that "economy, green world protesters", and so forth are not a credible answer. One more travel using old technology won't double the current national debt. Using kerosene as fuel will make us compete for a few minutes with the black carbon produced in Africa, but hey! it's just a few minutes...

Why not traveling to the moon with the old technology when is "obvious" it does work perfectly for this purpose?

We went to the moon with rockets that used canned supplies for a short stay.

The reason to go back to the moon first is to do it with the sort of technology intended for Mars. Find the problems when they're three days from home, not up to 18 months from home.

Also, if there's enough water at the poles it might be cheaper to launch your Mars rocket with only enough fuel to reach orbit and then refuel it from lunar resources.

 

[Bomb#20]

To expand on what Loren said, I know it's counterintuitive, but getting to deep space (or even just to geostationary orbit) quickly is an awful lot easier than getting there slowly. Everybody who wants to do more than an Alan Shepard-style suborbital hop starts out by accelerating at high g-force up to 17000 MPH or thereabouts; after that you can speed up or slow down at your convenience, depending on where you want to go and what route you want to take. Fast's question was about skipping that first step -- doing the whole trip with a speed limit of 12,500 MPH, or even 1000 MPH. The lower the speed limit, the harder the challenge and the crazier the technology it would take.

Nitpick: 18,0000 mph.

Counter-nitpick: Are you nuts? That's faster than the fastest rocket ever built! (Helios 2: 157,000 mph.)

Better counter-nitpick: Well, I did say "or thereabouts".

Real counter-nitpick: the familiar 18,000 mph figure is calculated at sea-level. Go up a few hundred miles and the speed is lower. Real LEO satellites will go anywhere from about 16,000 to about 17,500 mph, depending on altitude. Back-of-the-envelope, 17,500 mph or more equals an orbit 87 miles high or less. You'd reenter from atmospheric drag almost immediately.

(Counter-counter-nitpick: Yes, I know, orbits are elliptical. You can certainly have an LEO orbit at a reasonable altitude that's 18,000 mph at perigee, slower at apogee.)

 

[Kharakov]

Anyone think that information transmits at different speeds at different scales? So EM or G would transmit at speeds other than c at different scales, creating interference patterns with themselves.

 

[fast]

Anyone think that information transmits at different speeds at different scales? So EM or G would transmit at speeds other than c at different scales, creating interference patterns with themselves.

Information. I tell ya, give a scientist an inch and they'll take a mile. Whatever happened to the notion that facts collected was data that later interpreted became information? Now they talk as if information is out there.

 

[humbleman]

Anyone think that information transmits at different speeds at different scales? So EM or G would transmit at speeds other than c at different scales, creating interference patterns with themselves.

That is in a different "frequency".

Actually that is one of the thoughts about God as a being in a different frequency communicating with a few thru a certain channel that majority can't turn On.

 

[Bomb#20]

Anyone think that information transmits at different speeds at different scales? So EM or G would transmit at speeds other than c at different scales, creating interference patterns with themselves.

Yes, EM transmits at all speeds. QED makes wrong predictions if you don't add in all the superposed possible paths at non-c speeds. But at macroscopic scales you get massive amounts of destructive interference at all speeds other than c, so the probability of a message being received at distance d goes down exponentially as the elapsed time gets further from d/c.

 

[humbleman]

Anyone think that information transmits at different speeds at different scales?

Screaming to a person located a block from you is one way, calling him by phone is another way.