NA Space travel basics

Let'sgoflying!

Let'sgoflying!

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Dave Taylor
So I've had some downtime and got to reading about Apollo 13 and such, and space travel questions arose...
Am I right to think that the power required to propel a mass into earth orbit is probably much, much bigger than the power required to move the same mass from the earth to the moon (even though the distance is much, much greater)?
Just looking at the rocket sizes used to defeat earth's gravity tells part of the story (I realize the mass sent into earth orbit is many times that which is sent from earth to moon).

How much greater? Ie 100 units of thrust puts us into earth orbit vs 1 unit to send a ship earth-moon? (assume the mass in each case is identical.
 
Gravity is gravity right? the Earth's gravity isn't going to decrease just because you left the atmosphere. The only thing that will go away once you're out is air resistance. I would think you would need the extra propulsion to get out of the air resistance. But once you're out, it should require a lot less effort to move you
 
Dave,

You are correct. Read up on Oberth effect, Hohmann transfers and Delta-V.

Takes about 9500-10000m/s of dV to get to low earth orbit and about half that to get into lunar orbit from low earth orbit.
 
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EdFred said:
Dave,

You are correct. Read up on Oberth effect, Hohmann transfers and Delta-V.

Takes about 9500-10000m/s of dV to get to low earth orbit and about half that to get into lunar orbit from low earth orbit.
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That's true if "from the Earth" is replaced with "from LEO." LEO speeds are pretty damn fast.
 
SkyDog58 said:
Gravity is such a drag.
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Voyager 1 and 2 disagree with you.

MAK - I don't know whose post you were reading when you quoted mine, because it's hard to replace something that isn't there.
 
That's not quite right the way to think about it. Need to think of the problem in terms of energy, not power. The incremental energy required to leave earth orbit is much smaller than the energy required to move from the surface at rest to low earth orbit.

Thrust, by itself, is meaningless unless you also talk about how long it acts.
 
SixPapaCharlie said:
Gravity is gravity right? the Earth's gravity isn't going to decrease just because you left the atmosphere. The only thing that will go away once you're out is air resistance. I would think you would need the extra propulsion to get out of the air resistance. But once you're out, it should require a lot less effort to move you
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Well technically, the effect of Earth's gravity does decrease as you leave the surface. Distance between yourself and the earth would be the quickest way to affect the force of gravity, unless you have a way to decrease mass exponentially. However, overcoming drag is the more difficult obstacle until you enter space. :)
 
Once you're in earth orbit, you don't really "leave" earth orbit to travel to the moon since the moon is also orbiting the earth. Instead, you simply change the shape of the spacecraft's orbit into a long ellipse in the same plane as the moon's orbit, thereby intersecting the moon's orbit. That requires much less energy than leaving the surface to enter earth orbit in the first place.

You time the orbit change so that the moon is there when you intersect its orbit.

Theoretically, you could do re-shape the orbit by shooting spit wads out the back of your craft, but it would take a very very long time.
 
Half Fast said:
Once you're in earth orbit, you don't really "leave" earth orbit to travel to the moon since the moon is also orbiting the earth. Instead, you simply change the shape of the spacecraft's orbit into a long ellipse in the same plane as the moon's orbit, thereby intersecting the moon's orbit. That requires much less energy than leaving the surface to enter earth orbit in the first place.

You time the orbit change so that the moon is there when you intersect its orbit.

Theoretically, you could do re-shape the orbit by shooti ng spit wads out the back of your craft, but it would take a very very long time.
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re-shaping your orbit in that way would be no more efficient than simply venting
your life sustaining cabin atmosphere out into space!!!
 
robert lomax said:
re-shaping your orbit in that way would be no more efficient than simply venting
your life sustaining cabin atmosphere out into space!!!
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It can be VERY efficient. That's essentially what ion engines do.

Unfortunately, efficiency often doesn't mean what people want it to mean. It only means using the least fuel to accomplish a given change in energy. That can mean taking a very long time.
 
So if I launch an Estes rocket from surface it goes X up. If I launch it from 17,000 feet does it go up more?
 
N659HB said:
Yes, but did they have a metal landing calculator on board? :D
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Haha. I'm not sure about that ... though there was a second computer in the Service Module. And, of course, a sock, a plastic bag, the cover of a flight manual, and lots of duct tape. :)
 
yetti said:
So if I launch an Estes rocket from surface it goes X up. If I launch it from 17,000 feet does it go up more?
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Less drag, it should. At some point, though, you'll lose some nozzle efficiency, but I suspect Estes motors are so inefficient it probably won't matter.

Ron Wanttaja
 
Once in orbit, you are halfway to anywhere!

Kind of like a mile of road can take you a mile, a mile of runway can take you anywhere in the world.
 
Dennis McKim said:
Much of the orbital mechanics calculating was done via slide rule, though there was a flight computer in the command module.
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I believe on the ground they also used analog computers quite a bit.

As late as the mid 80s we were using a hybrid analog/digital computer at work (a Perkin-Elmer, IIRC). Basically the analog portion worked like a math coprocessor called by the digital main processor. Dif eq's could be worked much faster in the analog realm than in the digital back then.
 
Half Fast said:
I believe on the ground they also used analog computers quite a bit.

As late as the mid 80s we were using a hybrid analog/digital computer at work (a Perkin-Elmer, IIRC). Basically the analog portion worked like a math coprocessor called by the digital main processor. Dif eq's could be worked much faster in the analog realm than in the digital back then.
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I don't recall the use of analog computers for Apollo, though I don't doubt it. I know that Katherine Johnson did the initial orbital calculations (launch windows and initial trajectories) at least up through Apollo 11, and that's the slide rule use to which I was referring. Later in Apollo much of the heavy lifting was done on an IBM 360/75 in Houston but, iirc, there was still a Pickett 6" slide rule on many of the Apollo missions.
Although I had an acute interest in the Apollo missions, much of my knowledge of them is second-hand; my father was a flight test engineer for North American Aviation and led the two teams performing deactivation and post-flight testing for all of the Command Modules.
How to impress a girlfriend: "Hey, you want to go climb into an Apollo capsule?" :)
 
My knowledge is second hand as well. Early in my career I worked with a few engineers who had worked Apollo (I'm in FL - when Apollo ended quite a few engineers ended up at my company). I'm certain analog computers were used during earlier programs like Gemini, but I'm not sure how much continued into Apollo. I suspect a fair amount, though. Hybrid machines were quite powerful for the period.
 
Download the Orbiter simulator. It includes actual spacecraft including the shuttle but you'll have more fun with fictional craft that are more capable and thus more forgiving of your experiments in orbital dynamics. Go anywhere in the solar system and learn how a little dV applied in the right direction will change the shape of your orbit.

As far as launches go, they start vertical because you want to get out of the thick lower atmosphere as quickly as possible but then turn horizontal because that's the direction of your orbit. The heading relative to the surface you launch is determined by a combination of your latitude, time of day, and desired orbital inclination.

Geostationary orbits are just those at an altitude where your orbital period is equal to the length of the day. That's significantly higher than most manned flights. I can't remember the numbers. Something like 130km for typical manned flight and 400km for geostationary.
 
More like 40,000 km for GEO. No manned mission comes anywhere close. Except Apollo, which far exceeded GEO.
 
SoonerAviator said:
GlennAB1 said:
Gravity decreases with altitude. (Period)
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Lol, correct. Any time you get further from the center of mass anyway.
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Completely true, but it requires emphasis: The amount of pull depends on the distance from the Center of the Earth, not from the surface. The gravitational pull at 200 miles above the Earth's surface is NOT half that at 100 miles....

The plant where I worked used to have a couple of big vacuum chambers, which were used for many vehicles in the Apollo era. A visiting politician asked a technician how much they had to strap down the spacecraft they tested inside. He assumed the gravity disappeared because the air was gone.

Ron Wanttaja
 
DavidWhite said:
Dave you should download kerbal space program.
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RileyMitchum said:
I took a course in astrodynamics, and that game really does put some of that information to practice!
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vontresc said:
And lose all productivity for a week and a half... :D
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More than a week and a half...

Add in watching hours of Scott Manley YouTube videos of it, too...

I can hear that Scottsman now saying, "I'm Scott Manley. Fly safe!"

Let'sgoflying! said:
Launch direction: not vertical, except for geostationary orbits?
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Not sure what you're asking but most launches are nearly vertical for a short time to beat the aerodynamic effects of thicker lower atmosphere as quickly as possible, and then "tilted over" once much of the atmosphere is below the flight path to aim for whatever is desired next... Direct transfer, or more often, a parking orbit around Earth to deploy and check systems in zero-G and then a secondary burn to go somewhere. Often with a different engine/stage than got you to Earth orbit.

......

And then there's times when your upper stage fails on your rocket...

And some brilliant person figures out how to get your satellite where it needed to go by sending it around the moon using its onboard thrusters...

And your competitor claims you can't do that because they have a PATENT on orbital mechanics... (Seriously Boeing?)

And then you realize you'd make more money splashing the thing and taking the insurance check because sending it around the moon would use up 11 years worth of station keeping propellant out of roughly 15 expected...

And then you put out a bid asking if anyone wants the thing in the wrong orbit...

And many people say they do, but DoD comes knocking and is the only taker who wants to keep it in an inclined orbit that won't require sending it around the moon and killing your insurance claim that the rocket failure made the satellite unusable...

So you maneuver for six months and DoD takes over using the thing...

https://en.m.wikipedia.org/wiki/AMC-14

You couldn't write better fiction if you tried. And it's not fiction. :)
 
EdFred said:
Launch direction is never vertical even for GSO. Because for GSO you still need a horizontal velocity vector.
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Ah, is that what he was asking? Yeah... You get a tiny bit of "help" by launching stuff nearer the equator via rotation of the planet, if the orbit isn't inclined much with relation to the equator, but it ain't enough to give you orbital velocity...

And then of course there's the whole "are there squishy humans on board who can't handle G forces as well as the non-squishy non-living hardware stuff"... So you change the trajectory and/or throttle the engines back some for a while, to not squish the wimpy humans... :)
 
wanttaja said:
The plant where I worked used to have a couple of big vacuum chambers, which were used for many vehicles in the Apollo era. A visiting politician asked a technician how much they had to strap down the spacecraft they tested inside. He assumed the gravity disappeared because the air was gone.
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The sad thing is how many students taking physics today have no idea when "zero gravity" begins during space flight, or why it happens when you're in orbit. I've heard the "air is gone" theory but the commonest misconception is that it's because you're out of Earth's gravitational field. Even sadder: after you give them the correct explanation and have them do the calculation and see that the gravitational force at 200 mi above the surface is about 91% what it is on the surface, many of them still don't get it... :(
 
All comes back to Newton's Cannon on a mountain:
Newton's%20Cannon.jpg
 
My big brother told me their is no gravity in space,
I tried to explain their was , but you have this orbital velocity thing holding thing up there,
he said BS,
I bounced this all off my science teacher,
he thought my brother was right!
 
robert lomax said:
My big brother told me their is no gravity in space,
I tried to explain their was , but you have this orbital velocity thing holding thing up there,
he said BS,
I bounced this all off my science teacher,
he thought my brother was right!
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There are a lot of dumb science teachers out there. Had one HS science teacher tell me that the sky was blue because water was blue. When over to the sink, filled up a glass of water, put it in front of her and asked if she wanted to change her answer.
 
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