I know that low earth orbit is 62.5 miles up but i dont know how much thrust and for how long is needed to get up there. Do you need to stay up there for a while for the earth to actually catch you're spacecraft and let you orbit the earth.
Banned
I know that low earth orbit is 62.5 miles up but i dont know how much thrust and for how long is needed to get up there. Do you need to stay up there for a while for the earth to actually catch you're spacecraft and let you orbit the earth.
Banned
comon people
Order of Kilopi
62.5 miles or 100 km is an arbitrary altitude defined as the beginning of space. It has nothing to do with being in orbit.Originally Posted by VenusROVER
Here is a page on orbits:
http://en.wikipedia.org/wiki/Orbit
As for getting into LEO, the issue is the change of velocity, called "Delta-v" (and there's a nice symbol for it, that's the written form).
You need (roughly) a delta-v of around 9.7 km/s. Here is a page on delta-v requirements for LEO and other destinations:
http://en.wikipedia.org/wiki/Delta-v_budget
There is no answer to your "thrust" question unless you are talking about a specific rocket.
I would suggest you read up (or at least google search) on "orbits", "orbital mechanics", and "rockets". You have some pretty fundamental misunderstandings and it would help if you tried to study the subject some yourself before asking specific questions.
You waited, what, two hours before saying that? After posting a whole series of questions in different threads? I would also suggest a better attitude unless you want your questions to be ignored.comon people
I say there is an invisible elf in my backyard. How do you prove that I am wrong?
The Leif Ericson Cruiser
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The low end of the range is closer to 8,900 m/s. I think the 9,300 m/s figure quoted in the Wikipedia page hasn't taken into account the velocity that can be gained from Earth's rotation. As much a 465 m/s can be gained if launching due east from the equator. Launches from greater latitudes or along azimuths other than eastward require more delta-v. Higher orbits also require more delta-v.
You might find some useful information at my Web site (linked to in my signature). Some day I hope to add an entire page to discuss this specific topic in detail, but as yet I haven't found the time.
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Regarding thrust,
At liftoff a rocket generally produces a thrust of about 120% or greater its own weight (less than 100% and it wouldn’t leave the ground). As the rocket climbs to higher altitude the thrust will increase slightly because of lower ambient air pressure, however each subsequent stage of a rocket produces less thrust. This is done to keep the acceleration within manageable levels – if thrust wasn’t reduced the payload would be experiencing about 12g or more at engine burnout. A two-stage rocket can usually limit maximum accelerations to about 6-7g and a three-stage rocket to about 4-5g. The maximum acceleration experienced by the Space Shuttle is around 3g, which is accomplished by staging and by throttling the main engines.
Order of Kilopi
Are we talking about free-fall orbits. We coould confuse the issue further with fast or slow forced orbits.
Et tu BAUT? Quantum mutatus ab illo.
Banned
no i mean like actual orbit around the earth
Order of Kilopi
Yes, I know. There are many ways to orbit the earth.
Et tu BAUT? Quantum mutatus ab illo.
Order of Kilopi
I think you mean free fall orbit (it's not falling back, it's "floating" around the earth like the shuttle does).
But as said, there are many ways to orbit the earth. Direction, altitude, orientation wrt the equator, launch position...all determine the required "delta v" (speed change) to launch into that orbit.
The required thrust can be calculated from the mass of the rocket, the launch trajectory (the speed through the atmosphere for instance is of importance, as is the maximum allowable acceleration) and the required "delta v".
To show you why we can't just say how much thrust you need as a general answer:
take a mini satellite of 5 kg floating 150 km up and the moon. Both are orbiting the earth. Thrust is the power a rocket engine gives. I think you feel that you'd need a quite small engine (so not so much thrust) to launch the mini satellite, but that you'd need a ridicuously enormous engine (so a terribly large amount of thrust) if you wanted to launch the moon from earth into its orbit (of course that's just an example, the moon got there by a planetary collision, we did not launch it).
The most general answer I can think of, is that current rockets usually start with a thrust equal to about 1.2 times their weight. THat is chosen to limit the acceleration loads of the rocket while making an efficient ascent through the atmosphere and towards the orbit. The speed at which you go through the atmosphere is important as the atmosphere slows you down.
Can somebody write down the theory on going fast like a canon (limited burn time) vs slow (continuous burn) towards orbit versus required energy and required fuel? There was soemthing nice about that. Ignoring atmospheric effects.
I forgot the details (for shame!) but it is something like the required energy change is the same (potential + kinetic energy change of your rocket), but because of -insert reason here- the more impulisve the launch is, the more efficient it is.
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I think you mean free fall orbit (it's not falling back, it's "floating" around the earth like the shuttle does).
But as said, there are many ways to orbit the earth. Direction, altitude, orientation wrt the equator, launch position...all determine the required "delta v" (speed change) to launch into that orbit.
The required thrust can be calculated from the mass of the rocket, the launch trajectory (the speed through the atmosphere for instance is of importance, as is the maximum allowable acceleration) and the required "delta v".
To show you why we can't just say how much thrust you need as a general answer:
take a mini satellite of 5 kg floating 150 km up and the moon. Both are orbiting the earth. Thrust is the power a rocket engine gives. I think you feel that you'd need a quite small engine (so not so much thrust) to launch the mini satellite, but that you'd need a ridicuously enormous engine (so a terribly large amount of thrust) if you wanted to launch the moon from earth into its orbit (of course that's just an example, the moon got there by a planetary collision, we did not launch it).
The most general answer I can think of, is that current rockets usually start with a thrust equal to about 1.2 times their weight. THat is chosen to limit the acceleration loads of the rocket while making an efficient ascent through the atmosphere and towards the orbit. The speed at which you go through the atmosphere is important as the atmosphere slows you down.
Can somebody write down the theory on going fast like a canon (limited burn time) vs slow (continuous burn) towards orbit versus required energy and required fuel? There was soemthing nice about that. Ignoring atmospheric effects.
I forgot the details (for shame!) but it is something like the required energy change is the same (potential + kinetic energy change of your rocket), but because of -insert reason here
I think the reason was that with a slow acceleration, you have to bring along a lot of fuel for the later parts of the trip (I mean, you need to bring almost half your fuel along to half the height you want). With a cannon type launch, you only need to "push up" the actual orbiter, not part of the fuel.
Of course, a disadvantage of a canon-type launch is the immense g-forces.
Order of Kilopi
Carrying the fuel longer part of the trip is the answer indeed.
I told the prof it was a bad thing to let me learn this maths exam of monday, see!
It makes me "slow" on other fields of the study.
Order of Kilopi
Almost a circular description (at least in my head). Isn't it because the faster you go, the less the effect of gravity is because of the angular momentum? So the faster you can go faster, the less you need to counteract gravity. (please forgive the layman wording - I hope you see what I'm getting at)
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I'm a layman too. I was talking about the launch, not the orbit. In launch, the angular momentum shouldn't play such a role (I think).
Gravity works equally hard on you no matter if you go 1 km/h or 10,000km/h. The effects of the atmosphere may be quite different though
Order of Kilopi
That's not right, as indeed gravity has the same effect no matter how fast you go. You spend less time in gravity, but you spend more on kinetic energy. And in case of reaching orbit, you can't endlessly play around with velocity as you've got a very specific goal velocity. That's covered in the required kinetic and potential energy change of your spacecraft.
But if you get rid of everything that is only needed to go into space (fuel, bossters) as fast as possible, you do not need to increase their kinetic and potential as well because you lift it high up and accelerate it. Of course, the faster you get rid of your thrust generating things, the higher the acceleration needs to be. And that's often not good (manned spacecraft...)
Order of Kilopi
An example of this: you could launch a 5000 kg capsule out of earth's gravity by attaching a motor that simply gives a continuous thrust of 1.01*g*1000kg. You craft will slowly ascend and keep on going further away from the earth.
However, try to imagine what it would take to make a rocket burn constantly at a force enough to make it hover. Indeed, lots of fuel. Fuel is weight. So you'd need more thrust in order to lift the extra fuel. And the circle is round. You'd need extremely performant and efficient engines to be able to do this (imagine a Harrier jet having engines that are so efficient that the fuel in its wings are enough to feed the engines until it has made a hovering ascend into space).
On the other hand, consider a rocket that violently burns of all of it's fuel in the first minute, giving 4 times the weight of the rocket in thrust (that is the same acceleration as letting the rocket fall in a gravity field 3 times as strong as earth's). After a minute, the fuel is gone and the remaining (lightweight because it has no fuel) spacecraft has enough excess speed to ballistically reach orbit (in reality things are more complex).
The gain is that you needed to lift your fuel weight for only 1 minute instead of hours (Harrier to space). A disadvantage is that you suffered 4 g instead of 1.1 g.
Order of Kilopi
Exactly what I was getting at, just poorly worded.snip
The gain is that you needed to lift your fuel weight for only 1 minute instead of hours (Harrier to space
Technical details aside... the faster you reach orbit, the less time is involved in counteracting gravity.
Order of Kilopi
Polar Orbits and sun-synch. Geosynch, Molniya orbits, etc.
And it takes crap-loads of thrust to get to any of them. My guess, VENUSRover, is that you grew up watching TV, where you have mini-van sized craft, and then you look at a big rocket and are tempted to call it 'primative.'
Don't be fooled. Rockets will not go away--no matter how much people may wish them to.
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They may go away, if we can build a space elevator. Well, we'll still need rockets to go anywhere from the space elevator of course, but launches from the surface of the Earth may be a thing from the past then, and rockest that are launched from orbit can be completely different (unless they have to take off from other planets again of course).
Order of Kilopi
On which I want to elaborate: a constant mass spacecraft (so one not brurning fuel, think of a levitation craft or what have you
If the craft would take longer than ascend, indeed it would require more energy to counteract gravity (hover itself up). A clear example: *ascend in one minute straight up, then gain orbital speed to end up in a stable orbit. Compare this to: *ascend in one minute. Hang there still for an hour, then gain orbital speed to end up in a stable orbit. It is clear that in the second case (a forced description of a slower ascend) you need a lot of extra hovering energy.
Now if you'd have fuel onboard that you burn druing ascend, an extra factor in the energy equation is the energy required to counteract the time integrated effect of the gravity pull on your fuel.
The most efficiënt ascend is:
*reach stable orbit as fast as possible (to need as little "hovering" ie counteracting gravity integrated over time as possible); see part 1
*to get rid of your fuel as soon as possible, so the time integrated gravity effect on that is minimal. Basically you make the ascend time of the fuel as short as possible. ONly as the fuel has no goal in itself, it doesn't matter where it is expelled as long as it's as early as possible (a cnon is ideal).
However, both expelling fuel very soon and reaching orbit very fast demand very large accelerations. That limits the applicability. You can't shoot a human off at 20 G's.*
I know that I said things that I and others said before, but as I think about things I learned years ago I feel i can explain them better and more complete.
In fact the evacuation "tower" rocket atop SaturnV and Soyuz does give an enormous acceleration. It is meant to shoot astronauts away from a burning or exploding rocket, so you want to get out of there FAST. It was used once when a Soyuz started to burn on the pad. As the trigger cables of the antenna were burned (I thought it were those), they very quickly had to find al alternative way to give the escape tower the command to fire. They managed to do this just in time, seconds before the soyuz exploded by using an alternative (non-radio) circuit towards the tower. The astronauts were blown away with a tremendous force (I thought it was over 20 G's). Their orientation and custom seats protected them. They came down in the capsule by parachutes. Though unconscious and requiring a bit of care afterwards, they lived. That's three astronauts sitting on basically a HUGE burning bomb exploding underneath them, who survive thanks to a safety precaution that has been used only once in all the years of spaceflight. Safety is a burden, until the day it is needed. Then it's magnificent. Think of the countless pilots saved by their ejection seat from an otherwise certain death.
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This of course ignores the effect of the atmosphere. If you apply constant thrust over a period of several minutes, then the rocket climbs slowly through the thickest part of the atmosphere and then gains most of its velocity in the thin atmosphere above. This helps to mitigate the effect of drag on the vehicle. If you apply all of the energy explosively at ground level, then the vehicle is traveling fast where the air is the thickest. This maximizes the negative effect of drag.
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Let us assume a payload of one kilogram. To make it orbit the Earth at a distance r from the center of the Earth, it must have a certain speed, v, such that v2/r (the so-called centripetal force) is equal to Earth's gravity at that distance.
At Lower Earth Orbit, which is just far enough above the surface to clear the atmosphere, this gravity is roughly 10 m/s2, and the distance to the center is roughly 6400 km. So v2 comes to roughly 64,000,000 m2/s2, which is the square of 8000 m/s. You need a speed of 8 kilometers per second.
To gain this speed, you must give your one kilogram payload a kinetic energy of 32,000,000 joules, or 1/2 * m * v2. To do this, you must perform work. Work is force times distance, so if you use a large distance, you can use a small net force, and vice versa. However, this net force is added to other forces, needed to overcome friction and/or gravity. Which makes it more advantageous to apply a large force over a small distance. Let us use a thrust of 9 gees, or 90 m/s2, and assume that 1 gee is needed to compensate for gravity and friction. Then we have a net acceleration of 80 m/s2, which exerts a force of 80 Newtons on our one kilogram payload. This force must be applied while it covers a distance of 400,000 m or 400 kilometers.
As these 400 kilometers will be traversed with a mean speed of 4 kilometers per second, the thrust must be maintained for 100 seconds.
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But to maintain a thrust, you need fuel. This fuel must be lifted as well. To do this, you need more thrust, and more fuel. Etcetera... The calculations aren't that easy, except when you use a cannon or so.
Order of Kilopi
D'oh I fogot to put that part in my post! I thought I had covered it all, but I simply forgot to write the no atmosphere assumption down.
Your explanation is absolutely correct (unless I misunderstood things thought to me
Order of Kilopi
There are some nice equations on required delta_v, attainable delta_v etc. Check Tsiolkowski and company.
But it doesn't cover all details.
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Unfortunately there is no simply way the calculate a solution to the problem since all the variables are constantly changing throughout the ascent. I usually simulated launches using a Excel spreadsheet where each row represents a small step in time. The changes that occur over the time increment are calculated with the iteration function turn on. I've derived equations to estimate such things as air density as a function of altitude, drag coefficient as a function of Mach number, etc. The spreadsheets seem to yield pretty good results, but they aren't very user friendly. The hardest part is finding a trajectory that works, which I derive by trial and error.
Order of Kilopi
you can find the atmospheric derivatives in ISA tables if you want the official ones.
Making a computer program from your spreadhseet would be more user friendly if done right, but it's very nice you've done it!
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I used a table of properties for "standard atmosphere", which may be what you're referring to. I then took the data in the table and converted it to a polynomial expression.
Not to mention more stable. The Excel spreadsheet has a tendency to go nutty on me every once and a while.
I started out writing a program in BASIC about ten years ago. It worked pretty well but it was difficult to change the launch vehicle configurations and specifications. Take a Delta II for instance, it has nine solid strap-ons with six ground lit and three air lit after burnout of the first six. Every other rocket I know of ignites all its strap-ons at liftoff, so how do I easily tell the computer program to do something completely different for this one unique vehicle? It's a programming problem I think is solvable had I planned to design that much flexibility into the program from the beginning; unfortunately I didn't so re-programming became difficult. I found it much easier to write and modify an Excel spreadsheet for different launch vehicles than to re-write the computer program.
It is in my future plans to try writing a new computer program with the required flexibility for entering different vehicle types and specs. I'd also like to include an option for designing your own launch vehicle -- perhaps a wizard that would walk the user through a step by step selection process; i.e. how many stages, how many strap-ons, what type of propellant, etc. I’d also like to have the program derive an ascent trajectory so I don’t have to go through the tedious trial and error process myself. I also need to learn a better computer language (perhaps C++). Someday I'll find the time for all this.
Order of Kilopi
Unless you're going nuttynutty with the features or want really tremendous calculation speeds, Basic will od just fine if you feel comfortable programming in it.
Add a plugin like the 3DState 3D engine (I'm sorry for this minor advertising
Order of Kilopi
While we're on the subject... what , if any, gain would we get from using air-breathing SCRAMJET strap-ons? Ignoring cost and complexity, would any mass decrease in oxidizer be beneficial or would we lose thrust too much thrust?
Et tu BAUT? Quantum mutatus ab illo.
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Since it is air breathing, a scramjet can deliver a specific impulse far greater than a rocket that must carry its supply of oxidizer. I haven’t studied scramjets much but they seem to have the potential to significantly reduce the amount of propellant necessary to reach space. Unfortunately they need to be moving at hypersonic speed to work, therefore a conventional rocket is still needed to launch the vehicle and get it moving to Mach 5 or more before the scramjet can kick in. It seems to me scramjets would be limited to the role of a second stage. The high thrust needed for liftoff will have to come from elsewhere.
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