The dumb questions thread

[Jokodo]

Some may confuse orbital velocity with gravitational escape.

If you launch straight up it does not matter how fast you go, as long as thrust is greater than gravity, which diminishes with distance. At some point Earth's gravity is negligible and you have escaped.

Achieving orbit and staying in orbit balistically requires a velocity,. I expect a kinetic energy 0.5mv^2 of less than the gravitational energy at the desired orbit. Which I assume is why orbits decay over time. In orbit you are always falling back to Earth.

Orbits decay over time because of atmospheric friction.

The atmosphere doesn't have a strict upper limit, it just gets thinner and thinner until it becomes indistinguishable from interplanetary space and continuously looses (a small amount of) matter at the "top" where some of the molecules achieve escape velocity.

At near earth orbit altitudes, there may be a vacuum by our standards, but there's still enough left for friction to be a significant factor on the timescale of years.

 

[steve_bank]

Some may confuse orbital velocity with gravitational escape.

If you launch straight up it does not matter how fast you go, as long as thrust is greater than gravity, which diminishes with distance. At some point Earth's gravity is negligible and you have escaped.

Achieving orbit and staying in orbit balistically requires a velocity,. I expect a kinetic energy 0.5mv^2 of less than the gravitational energy at the desired orbit. Which I assume is why orbits decay over time. In orbit you are always falling back to Earth.

Orbits decay over time because of atmospheric friction.

The atmosphere doesn't have a strict upper limit, it just gets thinner and thinner until it becomes indistinguishable from interplanetary space and continuously looses (a small amount of) matter at the "top" where some of the molecules achieve escape velocity.

At near earth orbit altitudes, there may be a vacuum by our standards, but there's still enough left for friction to be a significant factor on the timescale of years.

Air adds to decay, but if KE is less that gravitational energy I assume the orbit decays regardless of friction.

If I shoot a bullet tangent on the surface ill will fall in a parabolic trajectory regardless of velocity or air resistance.

At the distance of comm satellites gravity is so low that the rate of fall back to Earth is small.



Fire a bullet and drop a bullet at the same time and both will hit the ground at the same time.

 

[bilby]

Some may confuse orbital velocity with gravitational escape.

If you launch straight up it does not matter how fast you go, as long as thrust is greater than gravity, which diminishes with distance. At some point Earth's gravity is negligible and you have escaped.

Achieving orbit and staying in orbit balistically requires a velocity,. I expect a kinetic energy 0.5mv^2 of less than the gravitational energy at the desired orbit. Which I assume is why orbits decay over time. In orbit you are always falling back to Earth.

Orbits decay over time because of atmospheric friction.

The atmosphere doesn't have a strict upper limit, it just gets thinner and thinner until it becomes indistinguishable from interplanetary space and continuously looses (a small amount of) matter at the "top" where some of the molecules achieve escape velocity.

At near earth orbit altitudes, there may be a vacuum by our standards, but there's still enough left for friction to be a significant factor on the timescale of years.

Air adds to decay, but if KE is less that gravitational energy I assume the orbit decays regardless of friction.

If I shoot a bullet tangent on the surface ill will fall in a parabolic trajectory regardless of velocity or air resistance.

At the distance of comm satellites gravity is so low that the rate of fall back to Earth is small.



Fire a bullet and drop a bullet at the same time and both will hit the ground at the same time.

You assume wrong.

By definition the horizontal component of orbital velocity is sufficient that the orbit doesn't decay; The rate of fall is equal to the rate at which the Earth's surface curves away.

In the absence of friction from the atmosphere, the orbit would never decay.

If the kinetic energy is lower, then you no longer have orbital velocity, and your spacecraft follows a sub orbital trajectory. That's not orbital decay, it's just insufficient velocity to reach orbit.

 

[Jokodo]

Some may confuse orbital velocity with gravitational escape.

If you launch straight up it does not matter how fast you go, as long as thrust is greater than gravity, which diminishes with distance. At some point Earth's gravity is negligible and you have escaped.

Achieving orbit and staying in orbit balistically requires a velocity,. I expect a kinetic energy 0.5mv^2 of less than the gravitational energy at the desired orbit. Which I assume is why orbits decay over time. In orbit you are always falling back to Earth.

Orbits decay over time because of atmospheric friction.

The atmosphere doesn't have a strict upper limit, it just gets thinner and thinner until it becomes indistinguishable from interplanetary space and continuously looses (a small amount of) matter at the "top" where some of the molecules achieve escape velocity.

At near earth orbit altitudes, there may be a vacuum by our standards, but there's still enough left for friction to be a significant factor on the timescale of years.

Air adds to decay, but if KE is less that gravitational energy I assume the orbit decays regardless of friction.

If kinetic energy is less than gravitational energy, than the sattelite does not have orbital velocity.

IF the earth were a point mass with no atmosphere, that wouldn't be so bad for the satellite - it would still assume an eccentric orbit with its current position at the apigee, that is, as its trajectory curves inward towards the earth, it gains kinetic energy from gravitational acceleration until the point where it reaches and exceeds orbital velocity and thus start to move out again, but since it doesn't have escape velocity, it'll be orbiting in an ellipse.

Fortunately for us and unfortunately for the satellite, the earth is not a point mass and does have an atmosphere, so such an orbit is likely to bring the satellite into an unhealthy proximity of 0.0 meters with the latter or even the planet's surface.

At the distance of comm satellites gravity is so low that the rate of fall back to Earth is small.

At the distance of satellites in low earth orbit (which is not a precisely defined altitude but a rather arbitrary range), gravity is anywhere between 60 and >90% of what it is at the surface. The International space station at an altitude of 408km experiences about 88% of it, a sattelite at 2000 km above ground (the conventional limit for what we call "LEO" still about 58%, and even a geostationary satellite 2.3%.

Even pretending that that acceleration remained constant during the course of the fall, that is calculating a free fall over 35,800 km with a constant acceleration equal that at a geosynchronous orbit, an object dropped from there that is stationary relative to the earth would hit ground in less than 5 hours.

Fire a bullet ~and drop a bullet at the same time and both will hit the ground at the same time.

If I shoot a bullet tangent on the surface ill will fall in a parabolic trajectory regardless of velocity or air resistance.

(I took the liberty two group these two statements as they make more sense together)

Actually the two bullets won't hit the ground at the exact same time. Ignoring air friction, a bullet dropped from an altitude of 1.5 meters will hit the ground after 553 milliseconds (you can use this handy calculator). Again ignoring air friction and assuming a perfectly even ground and a muzzle velocity of 1000m/s (approximately what you get for a high-end rifle), the bullet will have travelled 553 meters horizontally in that time. Due to the earth's curvature, the ground is 2.4 cm "lower" there than it is at you position. There are probably more elegant ways to calculate this, but I used cosine(arctan(553m/6371000m))) *6371000m -- that is, the difference between your position's distance from the center of the earth and the bullet's position at 553m from you if it were unaffected by gravity and did fly on a strictly horizontal trajectory, derived via the cosine of the angle at the center of the earth of a triangle with the center, your position, and the target position as the corners.

It takes the bullet 4.4 milliseconds to traverse that extra distance (of course in reality this is small enough for noise from air drift, misalignment of the rifle by a few arcseconds, and bumps in even the flattest to drown the signal). That's how much it's take the bullet you shot longer. That doesn't seem like a lot, but that's only because the bullet is so fucking slow compared to the speeds we need for orbit. So no, the trajectory's shape is not independent of velocity.

If you could fire the bullet with a muzzle velocity of 7.9km/s (again ignoring air friction and, unless you're at the pole (or equator and shooting due east or west), the rotation of the earth), which is the orbital velocity at our distance of the center of the earth, it would hit you in the back of the head after about 1 and a half hours 5060 seconds (slightly more when shooting west at the equator as the bullet has to catch up with you, slightly less when shooting east at the equator).

 

[humbleman]

Is it physically possible to launch a manned craft from earths surface and travel to the moon and do another moon landing and return alive and never once in speed go faster than half the earths escape velocity at any point from launch from earth to touch down back on earth? I think the answer is yes. What say you?

Half the earth's escape velocity at what distance from the earth? Escape velocity is a radius-dependent phenomenon.

If you mean half of 25,000 mph, escape velocity at the surface, then probably so. I can think of some ways it might be done. A space elevator dangling from the moon down to low earth orbit should do the trick. If you mean half of escape velocity at the craft's actual distance from the earth at each point on its trajectory, then no, it's impossible regardless of the path. When it's sitting still, landed on the moon, it's already going faster than half of the earth's escape velocity at that distance.

What I had in mind is taking off from the earths surface much like a helicopter would lift up off the ground. Then, I'd slowly make an ascent up into the upper atmosphere. Next, I would use my propulsion system (that remains intact with my vessel), pick up some speed and enter into space. Not in orbit, just into space.

Then, I'd take off around cruisin speed, 10,000MPH to 12,000 MPH and try to get near the moons path. I might, depending on the speed of the moon have to kick the speed up a notch. The point wasn't necessarily that I didn't have to reach 25,000 MPH to land on the moon so much more than I don't necessarily have to go 25,000 MPH to reach it.

I should be able to get to the moon and never go faster than a mere 1,000 MPH. Granted, it might fly past me like a superbullet once I actually got there, but I just want to get there without having to travel past this speed we call escape velocity. And no, no elevators allowed. I have to use my own shuttle craft looking vehicle with a supremely futuristic state of the art propulsion system that harnesses the full power and potential of no more than a few of our universe's most abundant elements with such efficiency that it would be the envy of NASA in any timeline.

Oh man, save all that trouble. Just do like other cartoons similar to yours, let someone pinch you in the butt and you will jump up to the moon in a second.

Dumb question.

Satellites perceived a solar wind which was strong enough to cut off the top part of the atmosphere and taken it away into outer space. The atmosphere itself filled up the missing part and became spherical all around again.

Having another strong solar wind cut off a greater part of the atmosphere, but after the rotation of earth makes it recover the spherical form of it, if the atmosphere ends as half of what it was before, what should be the consequences for life on earth? What should be the consequences in humans?

 

[Jokodo]

Dumb question.

Satellites perceived a solar wind which was strong enough to cut off the top part of the atmosphere and taken it away into outer space. The atmosphere itself filled up the missing part and became spherical all around again.

Having another strong solar wind cut off a greater part of the atmosphere, but after the rotation of earth makes it recover the spherical form of it, if the atmosphere ends as half of what it was before, what should be the consequences for life on earth? What should be the consequences in humans?

With a solar wind strong enough to accomplish that, the thinning atmosphere would be the least our problems - we'd be dead from radiation by the time we took notice, at the very least those of us who happen to be on the dayside of the planet. An

Assuming instead that half of the atmosphere by mass magically disappeared: The atmosphere is much denser towards the bottom than towards its fuzzy border with space. Approximately half of it's volume is located below 5,500 meters above sea level. That's also the altitude at which air pressure reaches half the value it has at sea level. So in short, places on sea level would get air as thin as we now have on a peak of over 5000m. Altitude sickness would become a global epidemic, people and most animals would become much slower at everything we do at least for a while, but most would survive.

(ETA: on second thought, the instant drop of the air pressure at sea level would lead to the evaporation of a huge amount of water, and thus to a considerable chill effect; I don't know how to calculate the size of that effect, but it seems initially plausible to me that it would be big enough to freeze many of us who happen to be outside even when we pretend away the getting roasted part; but I might be overestimating the effect by a couple orders of magnitude.)

But in order for a solar wind to achieve that feat, I guess it's safe to say it would have to be massive enough to roast our planet. The atmosphere gets very thin indeed at the "top part", skimming off a huge volume off the "top part" would only have a minor impact on its total mass. To reduce the mass by half you'd have to shave off every bit of it that's currently higher than 5.5 km above sea level (or possibly more likely: shave off all of the atmosphere on the dayside).

 

[Jokodo]

(slightly more when shooting west at the equator as the bullet has to catch up with you, slightly less when shooting east at the equator).

Or possibly the other way round...

 

[Bomb#20]

Is it physically possible to launch a manned craft from earths surface and travel to the moon and do another moon landing and return alive and never once in speed go faster than half the earths escape velocity at any point from launch from earth to touch down back on earth? I think the answer is yes. What say you?

Half the earth's escape velocity at what distance from the earth? Escape velocity is a radius-dependent phenomenon.

If you mean half of 25,000 mph, escape velocity at the surface, then probably so. I can think of some ways it might be done. A space elevator dangling from the moon down to low earth orbit should do the trick. If you mean half of escape velocity at the craft's actual distance from the earth at each point on its trajectory, then no, it's impossible regardless of the path. When it's sitting still, landed on the moon, it's already going faster than half of the earth's escape velocity at that distance.

What I had in mind is taking off from the earths surface much like a helicopter would lift up off the ground. Then, I'd slowly make an ascent up into the upper atmosphere. Next, I would use my propulsion system (that remains intact with my vessel), pick up some speed and enter into space. Not in orbit, just into space.

Then, I'd take off around cruisin speed, 10,000MPH to 12,000 MPH and try to get near the moons path. I might, depending on the speed of the moon have to kick the speed up a notch. The point wasn't necessarily that I didn't have to reach 25,000 MPH to land on the moon so much more than I don't necessarily have to go 25,000 MPH to reach it.

I should be able to get to the moon and never go faster than a mere 1,000 MPH. Granted, it might fly past me like a superbullet once I actually got there, but I just want to get there without having to travel past this speed we call escape velocity. And no, no elevators allowed. I have to use my own shuttle craft looking vehicle with a supremely futuristic state of the art propulsion system that harnesses the full power and potential of no more than a few of our universe's most abundant elements with such efficiency that it would be the envy of NASA in any timeline.

So your problem is "specific impulse". A 12,000 MPH orbit is about 4000 miles up. So to get to deep space without going over 12,000 MPH is going to take you 4000/12000 = 1/3 hour = 20 minutes = 1200 seconds. So you'd need a rocket engine with a specific impulse of 1200 seconds, if gravity stayed constant, but of course it gets weaker as you go up, so you don't actually need that much. But chemical rockets aren't even even in the ballpark -- 200 to 400 is typical. You can make a nuclear rocket with a specific impulse of 900 seconds; that's probably enough to make your plan work. (You can also get very high specific impulses with stuff like ion drives, but then you have a thrust problem -- you'll never get off the ground.)

To do it at 1000 MPH, you'd have to hold yourself up against gravity all the way to the moon, 239 hours. The only high-thrust rocket we've thought with a specific impulse that high is an antimatter rocket.

 

[Loren Pechtel]

Square-cube law is going to be very nasty at this point, I don't believe it could be built.

In-flight fueling, perhaps. The question isn't about practicality or safety, just possibility.

In-flight fueling from what?? Anything that's going to deliver fuel has to match velocities with the target and thus is subject to most of the same limits that the rocket itself is.

 

[humbleman]

Assuming instead that half of the atmosphere by mass magically disappeared: The atmosphere is much denser towards the bottom than towards its fuzzy border with space. Approximately half of it's volume is located below 5,500 meters above sea level. That's also the altitude at which air pressure reaches half the value it has at sea level. So in short, places on sea level would get air as thin as we now have on a peak of over 5000m. Altitude sickness would become a global epidemic, people and most animals would become much slower at everything we do at least for a while, but most would survive.

If the atmospheric problem persists for generations, construction might be no more than five floor buildings... hmm

(ETA: on second thought, the instant drop of the air pressure at sea level would lead to the evaporation of a huge amount of water, and thus to a considerable chill effect; I don't know how to calculate the size of that effect, but it seems initially plausible to me that it would be big enough to freeze many of us who happen to be outside even when we pretend away the getting roasted part; but I might be overestimating the effect by a couple orders of magnitude.)

What about creating a green house environment instead? Vapors might invaded the low sky but the heat from the Sun might cause no rain but maintain it as vapor before chances for condensation on top and rain. Vapors in the atmosphere might prevent fast evaporation after a while. I guess.

But in order for a solar wind to achieve that feat, I guess it's safe to say it would have to be massive enough to roast our planet. The atmosphere gets very thin indeed at the "top part", skimming off a huge volume off the "top part" would only have a minor impact on its total mass. To reduce the mass by half you'd have to shave off every bit of it that's currently higher than 5.5 km above sea level (or possibly more likely: shave off all of the atmosphere on the dayside).

Yup. It was a dumb question, however the effects of a half size atmosphere it was my main interest.

If it will affect your mobility and going up high hills or mountains, if the atmosphere takes thousands of years to recover its former status, will this affect the size of the species? Will species be taller or shorter?

 

[fast]

What I had in mind is taking off from the earths surface much like a helicopter would lift up off the ground. Then, I'd slowly make an ascent up into the upper atmosphere. Next, I would use my propulsion system (that remains intact with my vessel), pick up some speed and enter into space. Not in orbit, just into space.

Then, I'd take off around cruisin speed, 10,000MPH to 12,000 MPH and try to get near the moons path. I might, depending on the speed of the moon have to kick the speed up a notch. The point wasn't necessarily that I didn't have to reach 25,000 MPH to land on the moon so much more than I don't necessarily have to go 25,000 MPH to reach it.

I should be able to get to the moon and never go faster than a mere 1,000 MPH. Granted, it might fly past me like a superbullet once I actually got there, but I just want to get there without having to travel past this speed we call escape velocity. And no, no elevators allowed. I have to use my own shuttle craft looking vehicle with a supremely futuristic state of the art propulsion system that harnesses the full power and potential of no more than a few of our universe's most abundant elements with such efficiency that it would be the envy of NASA in any timeline.

So your problem is "specific impulse". A 12,000 MPH orbit is about 4000 miles up. So to get to deep space without going over 12,000 MPH is going to take you 4000/12000 = 1/3 hour = 20 minutes = 1200 seconds. So you'd need a rocket engine with a specific impulse of 1200 seconds, if gravity stayed constant, but of course it gets weaker as you go up, so you don't actually need that much. But chemical rockets aren't even even in the ballpark -- 200 to 400 is typical. You can make a nuclear rocket with a specific impulse of 900 seconds; that's probably enough to make your plan work. (You can also get very high specific impulses with stuff like ion drives, but then you have a thrust problem -- you'll never get off the ground.)

To do it at 1000 MPH, you'd have to hold yourself up against gravity all the way to the moon, 239 hours. The only high-thrust rocket we've thought with a specific impulse that high is an antimatter rocket.

What's the thrust impulse distinction?

 

[humbleman]

What I had in mind is taking off from the earths surface much like a helicopter would lift up off the ground. Then, I'd slowly make an ascent up into the upper atmosphere. Next, I would use my propulsion system (that remains intact with my vessel), pick up some speed and enter into space. Not in orbit, just into space.

Then, I'd take off around cruisin speed, 10,000MPH to 12,000 MPH and try to get near the moons path. I might, depending on the speed of the moon have to kick the speed up a notch. The point wasn't necessarily that I didn't have to reach 25,000 MPH to land on the moon so much more than I don't necessarily have to go 25,000 MPH to reach it.

I should be able to get to the moon and never go faster than a mere 1,000 MPH. Granted, it might fly past me like a superbullet once I actually got there, but I just want to get there without having to travel past this speed we call escape velocity. And no, no elevators allowed. I have to use my own shuttle craft looking vehicle with a supremely futuristic state of the art propulsion system that harnesses the full power and potential of no more than a few of our universe's most abundant elements with such efficiency that it would be the envy of NASA in any timeline.

So your problem is "specific impulse". A 12,000 MPH orbit is about 4000 miles up. So to get to deep space without going over 12,000 MPH is going to take you 4000/12000 = 1/3 hour = 20 minutes = 1200 seconds. So you'd need a rocket engine with a specific impulse of 1200 seconds, if gravity stayed constant, but of course it gets weaker as you go up, so you don't actually need that much. But chemical rockets aren't even even in the ballpark -- 200 to 400 is typical. You can make a nuclear rocket with a specific impulse of 900 seconds; that's probably enough to make your plan work. (You can also get very high specific impulses with stuff like ion drives, but then you have a thrust problem -- you'll never get off the ground.)

To do it at 1000 MPH, you'd have to hold yourself up against gravity all the way to the moon, 239 hours. The only high-thrust rocket we've thought with a specific impulse that high is an antimatter rocket.

What's the thrust impulse distinction?

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

https://www.space.com/12957-nasa-giant-rocket-space-launch-system-infographic.html

NASA unveiled its new rocket for deep space exploration - the Space Launch System - on Sept. 14, 2011. The rocket will launch astronauts into space on NASA's Orion Multi-Purpose Crew Vehicle, and serve as the go-to booster for U.S. missions to explore asteroids and, eventually, Mars.

This infographic above shows how the Space Launch System will work. The first test flight of the new rocket, which will be more powerful than NASA's mighty Saturn V moon rocket, is set for 2017.

Saturn rocket? Lets see that 50 years old rocket.

https://www.space.com/18422-apollo-saturn-v-moon-rocket-nasa-infographic.html

NASA's Mighty Saturn V Moon Rocket Explained (Infographic)
The rocket that launched men to the moon was first tested in 1967.
Credit: Karl Tate, SPACE.com contributor

Designed to fly three Apollo astronauts to the moon and back, the Saturn V made its first unmanned test flight in 1967. A total of 13 Saturn V rockets were launched from 1967 until 1973, carrying Apollo missions as well as the Skylab space station. Every part of the giant rocket is used and then discarded during a mission. Only the tiny command module survives to return to Earth.

The Saturn rocket needed 203,400 gallons (770,000 liters) of kerosene fuel and 318,000 gallons (1.2 million liters) of liquid oxygen needed for combustion. For going up the stage’s five F-1 rocket engines ignite and produce 7.5 million pounds of thrust. After starts going out of our atmosphere at an altitude of 42 miles (67 kilometers), the F-1 engines shut down. Explosive bolts fire, and the severed first stage falls into the Atlantic Ocean.

The Saturn V rocket’s first stage carries 203,400 gallons (770,000 liters) of kerosene fuel and 318,000 gallons (1.2 million liters) of liquid oxygen needed for combustion. At liftoff, the stage’s five F-1 rocket engines ignite and produce 7.5 million pounds of thrust.

At an altitude of 42 miles (67 kilometers), the F-1 engines shut down. Explosive bolts fire, and the severed first stage falls into the Atlantic Ocean.

The second stage carries 260,000 gallons (984,000 liters) of liquid hydrogen fuel and 80,000 gallons (303,000 liters) of liquid oxygen.

A few seconds after the second stage’s five rocket engines are ignited, an interstage skirt at the bottom end of the second stage is jettisoned. Shortly after that, the emergency escape rocket on top of the vehicle, only usable below 19 miles altitude, is fired off and discarded.

At 9 minutes and 9 seconds after launch, the second stage is discarded and the third stage’s rocket engine is fired. The third stage carries 66,700 gallons (252,750 liters) of liquid hydrogen fuel and 19,359 gallons (73,280 liters) of liquid oxygen

The third stage’s single rocket engine is fired until 11 minutes and 39 seconds after launch, when the vehicle has attained sufficient speed to reach Earth orbit. About two and a half hours later, the third stage engine is restarted to send the Apollo spacecraft out of Earth orbit and toward the moon.

After the astronauts in Apollo dock with the lunar landing module and pull away from the now-useless third stage, this last remaining part of the Saturn V coasts away into deep space or is commanded to fly to a crash landing on the moon.

You might be right... how the hell the moon can be reached with rockets carrying tanks full with kerosene? ha ha ha...

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?

The rest will come from the core stage, which Boeing developed especially for SLS. Even this new component, though, will have the same 27.6-foot diameter as the Space Shuttle external tank, all the way through the 130- ton version, to capitalize on existing manufacturing and processing infrastructure. Like the Shuttle’s external tank, the core stage will be filled with a liquid hydrogen fuel tank and a tank of liquid oxygen to oxidize the fuel as it’s burnt. But the Block 1 rocket will stand 212 feet tall and carry about 25 percent more propellant than the Shuttle’s 143-foot tank.

Converting that fuel to lift will be modified versions of the same RS-25 rocket engines that powered the Shuttle, although SLS will use four engines rather than three. And Aerojet Rocketdyne is optimizing the engines to fire at 512,000 pounds of thrust each, up from 491,000 pounds on the Shuttle, and replacing the electronic engine control with a new version.

All this will serve to put the capsule in a higher orbit than the Shuttle inhabited, with enough fuel left over in the upper stage to get to the Moon and back, even with the initial Block 1 vehicle.

 

[Wiploc]

Square-cube law is going to be very nasty at this point, I don't believe it could be built.

In-flight fueling, perhaps. The question isn't about practicality or safety, just possibility.

In-flight fueling from what?? Anything that's going to deliver fuel has to match velocities with the target and thus is subject to most of the same limits that the rocket itself is.

It'd be crazy. But the thread is about whether it could theoretically work. If the rocket can't carry enough fuel to stay below escape velocity, then maybe, theoretically speaking, it would be possible for several rockets to carry the fuel. Think of them as self-powered drop tanks.

 

[bilby]

In-flight fueling from what?? Anything that's going to deliver fuel has to match velocities with the target and thus is subject to most of the same limits that the rocket itself is.

It'd be crazy. But the thread is about whether it could theoretically work. If the rocket can't carry enough fuel to stay below escape velocity, then maybe, theoretically speaking, it would be possible for several rockets to carry the fuel. Think of them as self-powered drop tanks.

Self-powered drop tanks? I think you just invented the multi-stage rocket.

 

[Wiploc]

In-flight fueling from what?? Anything that's going to deliver fuel has to match velocities with the target and thus is subject to most of the same limits that the rocket itself is.

It'd be crazy. But the thread is about whether it could theoretically work. If the rocket can't carry enough fuel to stay below escape velocity, then maybe, theoretically speaking, it would be possible for several rockets to carry the fuel. Think of them as self-powered drop tanks.

Self-powered drop tanks? I think you just invented the multi-stage rocket.

So, you want to walk across this desert. It will take four days, but you can carry water enough for only three days. Are you defeated? No, because you can have a friend come with you. After one day, he can give you enough water for the fourth day, and he'll still have a one day supply he can use to walk back to the start.

If we want to make the analogy better, then you can carry enough water for four days, but that would slow you down enough that you'd take five days to cross the desert. A multi stage rocket would have to carry you and your water, and it's own water, which would be hopeless. But a self powered drop tank can get well up into the atmosphere without carrying you--and without you carrying it--and then transfer fuel to you.

 

[humbleman]

Self-powered drop tanks? I think you just invented the multi-stage rocket.

So, you want to walk across this desert. It will take four days, but you can carry water enough for only three days. Are you defeated? No, because you can have a friend come with you. After one day, he can give you enough water for the fourth day, and he'll still have a one day supply he can use to walk back to the start.

If we want to make the analogy better, then you can carry enough water for four days, but that would slow you down enough that you'd take five days to cross the desert. A multi stage rocket would have to carry you and your water, and it's own water, which would be hopeless. But a self powered drop tank can get well up into the atmosphere without carrying you--and without you carrying it--and then transfer fuel to you.

Why not planning everything from the very beginning with established conditions? In your analogy you added a friend giving water to the another. The theoretical started with one person.

The risk of transferring fuel at such speed is a great explosion if any little mistake is made.

 

[Loren Pechtel]

So your problem is "specific impulse". A 12,000 MPH orbit is about 4000 miles up. So to get to deep space without going over 12,000 MPH is going to take you 4000/12000 = 1/3 hour = 20 minutes = 1200 seconds. So you'd need a rocket engine with a specific impulse of 1200 seconds, if gravity stayed constant, but of course it gets weaker as you go up, so you don't actually need that much. But chemical rockets aren't even even in the ballpark -- 200 to 400 is typical. You can make a nuclear rocket with a specific impulse of 900 seconds; that's probably enough to make your plan work. (You can also get very high specific impulses with stuff like ion drives, but then you have a thrust problem -- you'll never get off the ground.)

You need almost 1,200 seconds to counter gravity. You need another 530 to get up to 12,000 mph that you'll need to coast to the moon. And you'll need another nearly 1,200 seconds on the way back down. You'll also need a minimum of another 28 seconds at the moon. Thus you need about 2,950. The only way you're going to get that kind of ISP is the direct use of nuclear power (not merely nuclear thermal systems.) We have no ideas on something that can do this without being dirty.

To do it at 1000 MPH, you'd have to hold yourself up against gravity all the way to the moon, 239 hours. The only high-thrust rocket we've thought with a specific impulse that high is an antimatter rocket.

 

[Loren Pechtel]

What's the thrust impulse distinction?

The term is "specific impulse", or in equations I_sp, or ISP if subscripts aren't available (Impulse, specific.) Rockets are basically measured by two things:

Thrust--how hard it can push.

Specific impulse--how long it can push itself against Earth's gravity. (Note that we still use that measure for things like ion drives that can't lift themselves against Earth's gravity.)

The best we can do with chemical rockets is liquid hydrogen/liquid oxygen which tops out at about 450 seconds. (Note that no system will ever actually attain these numbers because that leaves no weight for the engine and tanks.)

In the real world there are other issues that matter:

1) Solid boosters are reliable, simple and thus cheap. However, they can't be controlled. They have no throttle and no off switch--once you light them you have to ride them.

2) Liquid hydrogen is hard to handle and very, very cold--necessitating things like using helium to purge your plumbing because anything else could freeze solid and block your fuel feed. This is why SpaceX uses RP-1/liquid oxygen even though the ISP is only about 350. That means they're carrying a lot more fuel than if they used liquid hydrogen but they are after the cheapest rocket, not the most high tech one.

3) All cryogenic propellants are problematic to store. It's not practical to put a powerful enough refrigerator on a spacecraft, they simply accept that it boils. For the duration of a launch this isn't a problem. For the Apollo missions they used the stuff that boiled off to power the spacecraft--but the lunar lander used weaker stuff that wouldn't boil.

4) Hypergolic propellants avoid any issues of lighting the engine. Turn on the pumps, it burns. Less to go wrong. (The recent crash of the falcon heavy central core was caused by running out of fuel for the engine lighter.) They're also nasty, nasty things to deal with. For extended times in space you have to give up the power to get something you can store anyway, thus hypergolics are common. (While the shuttle rode on solids + LH/LOX the orbital engines were hypergolic.)

 

[Loren Pechtel]

What's the thrust impulse distinction?

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!

Do you even understand what you're replying to?

The Saturn V that you are using to try to rebut him accelerated to 25,000 mph. He's talking about going to the moon without exceeding 1,000 mph. That requires antimatter.

 

[Bomb#20]

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!