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SLS launches likely delayed
JF Mezei wrote:
On 2017-04-17 21:45, Jeff Findley wrote: Those are all tests of individual components. The first "all up" test will be EM-1. Once the thing has been assembled and perhaps at the pad, don't they spend mucho time doing integration tests and simulations, test engine firings etc? No, they do none of that. The 'take the vehicle to launch ready' that SpaceX does is very unusual. Usually the first time the vehicle gets fueled is when you're going to light it off and fly. Once it gets to EM-1 with an actual launch day/time announced and it gets to that date, doesn't that mean NASA is confident it has worked out the bugs and that the thing won't blow up in its face? They certainly hope they have, but there is only one way to be sure that the all up vehicle works and that is to fly it. With modern computer simulations, doesn't that allow NASA to perform a lot of validation before first first flight? You can only simulate the stuff you know about. snip trip to the weeds -- "Some people get lost in thought because it's such unfamiliar territory." --G. Behn |
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SLS launches likely delayed
On Apr/17/2017 at 9:48 PM, Jeff Findley wrote :
In article , says... On Apr/17/2017 à 12:07 PM, JF Mezei wrote : On 2017-04-16 22:46, Fred J. McCall wrote: Looks like NASA's first two launches of the SLS for their lunar tests will be delayed by a year or more. That means SpaceX will almost certainly be there before them. The announcement of the first flight being manned may have more to do with the delay than budgets. That article had a link to a NASA web page which describes its concept for Mars. That page does not paint Orion/SLS as sending man to Mars. NASA wants to build ISS-2 in lunar orbit to test the transit ship there. So SLS/Orion act as shuttles to/from the vehicle in lunar orbit. NASA admits Orion isn't big enough to being crews on months long mission to Mars and back. If you will assemble a transit ship in lunar orbit, you might need something like SLS to bring modules up there. Assembling in LEO costs less in module launches, but more in fuel to escape from Earth. Assembling in Lunar orbit costs more in launches of modules, but less to escape earth/moon orbit. Does the balance tip heavily on one of those or is it more or less even ? It is cheaper to do most of your acceleration low in the gravity well. You can read on the Oberth Effect, for instance: https://en.wikipedia.org/wiki/Oberth_effect How does a fly-by maneuver fit into this? Jeff I'm not sure of what you mean here. A gravity assist from the Moon is too small to be important. If the Moon happens to be at the right place at the right time, sure use it, but it isn't worth delaying the mission for a lunar gravity assist. A gravity assist from Earth gives a bigger push, but that would imply an impossibly long journey for a manned mission. You would have to go into solar orbit then, maybe two years later, meet Earth again and use the push to get to Mars. You do save fuel, but it really isn't worth it. Alain |
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SLS launches likely delayed
On Apr/18/2017 at 7:08 AM, Alain Fournier wrote :
On Apr/17/2017 at 9:48 PM, Jeff Findley wrote : In article , says... On Apr/17/2017 à 12:07 PM, JF Mezei wrote : On 2017-04-16 22:46, Fred J. McCall wrote: Looks like NASA's first two launches of the SLS for their lunar tests will be delayed by a year or more. That means SpaceX will almost certainly be there before them. The announcement of the first flight being manned may have more to do with the delay than budgets. That article had a link to a NASA web page which describes its concept for Mars. That page does not paint Orion/SLS as sending man to Mars. NASA wants to build ISS-2 in lunar orbit to test the transit ship there. So SLS/Orion act as shuttles to/from the vehicle in lunar orbit. NASA admits Orion isn't big enough to being crews on months long mission to Mars and back. If you will assemble a transit ship in lunar orbit, you might need something like SLS to bring modules up there. Assembling in LEO costs less in module launches, but more in fuel to escape from Earth. Assembling in Lunar orbit costs more in launches of modules, but less to escape earth/moon orbit. Does the balance tip heavily on one of those or is it more or less even ? It is cheaper to do most of your acceleration low in the gravity well. You can read on the Oberth Effect, for instance: https://en.wikipedia.org/wiki/Oberth_effect How does a fly-by maneuver fit into this? Jeff I'm not sure of what you mean here. A gravity assist from the Moon is too small to be important. If the Moon happens to be at the right place at the right time, sure use it, but it isn't worth delaying the mission for a lunar gravity assist. A gravity assist from Earth gives a bigger push, but that would imply an impossibly long journey for a manned mission. You would have to go into solar orbit then, maybe two years later, meet Earth again and use the push to get to Mars. You do save fuel, but it really isn't worth it. I should add that you can use Earth's gravity to help get to Mars from lunar orbit if you are using a high impulse rocket such as a chemical rocket. From Lunar orbit, you lower your perigee to about 100 km above Earth's surface. Then near perigee, you do your main rocket burn. You get to Mars with less energy than if you would make a direct burn from Lunar orbit directly to Mars. This is not quite what is normally called a fly-by manoeuvre or a gravity assist. But you still improve your efficiency by the Oberth effect. If you are using a low impulse propulsion such as an ion-drive. Such a manoeuvre would be counter productive. Alain Fournier |
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SLS launches likely delayed
In article , says...
On Apr/17/2017 at 9:48 PM, Jeff Findley wrote : In article , says... On Apr/17/2017 à 12:07 PM, JF Mezei wrote : On 2017-04-16 22:46, Fred J. McCall wrote: Looks like NASA's first two launches of the SLS for their lunar tests will be delayed by a year or more. That means SpaceX will almost certainly be there before them. The announcement of the first flight being manned may have more to do with the delay than budgets. That article had a link to a NASA web page which describes its concept for Mars. That page does not paint Orion/SLS as sending man to Mars. NASA wants to build ISS-2 in lunar orbit to test the transit ship there. So SLS/Orion act as shuttles to/from the vehicle in lunar orbit. NASA admits Orion isn't big enough to being crews on months long mission to Mars and back. If you will assemble a transit ship in lunar orbit, you might need something like SLS to bring modules up there. Assembling in LEO costs less in module launches, but more in fuel to escape from Earth. Assembling in Lunar orbit costs more in launches of modules, but less to escape earth/moon orbit. Does the balance tip heavily on one of those or is it more or less even ? It is cheaper to do most of your acceleration low in the gravity well. You can read on the Oberth Effect, for instance: https://en.wikipedia.org/wiki/Oberth_effect How does a fly-by maneuver fit into this? Jeff I'm not sure of what you mean here. You're the one who brought up the Oberth effect. It's only applicable if you're doing a gravity-assist fly-by maneuver. Otherwise, your statement "It is cheaper to do most of your acceleration low in the gravity well." makes no sense. From the page you cite: In astronautics, a powered flyby, or Oberth maneuver, is a maneuver in which a rocket falls into a gravitational well, and then accelerates when its fall reaches maximum speed. A gravity assist from the Moon is too small to be important. If the Moon happens to be at the right place at the right time, sure use it, but it isn't worth delaying the mission for a lunar gravity assist. A gravity assist from Earth gives a bigger push, but that would imply an impossibly long journey for a manned mission. You would have to go into solar orbit then, maybe two years later, meet Earth again and use the push to get to Mars. You do save fuel, but it really isn't worth it. Okay, then I really don't know why you brought it up. Jeff -- All opinions posted by me on Usenet News are mine, and mine alone. These posts do not reflect the opinions of my family, friends, employer, or any organization that I am a member of. |
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SLS launches likely delayed
On Apr/19/2017 at 6:02 AM, Jeff Findley wrote :
In article m, says... Reality check question: If you spend X energy from low Earth altitude/orbit to Moon orbit, does Oberth effect claim you will get a value of energy greater than X falling back from Moon to low earth altitude? If you don't leave the earth/moon system, I don't think there is a "free lunch" to be had here. In order for the Oberth effect to be present, you have to do a parabolic fly-by maneuver. As in your velocity is higher than escape going in and much higher than escape when going out. The extra velocity you gain is "stolen" from the planet which you fly- by. No fly-by, no Oberth effect. No that is not what the Obert effect is. The Obert effect is due to greater efficiency of a rocket burn deep in a gravity well than higher in the gravity well. Suppose you are in an elliptic orbit with let's say perigee at 200 km and apogee at 40,000 km. While you go up from 200 km to 40,000 km you lose speed. Then when you go down from 40,000 km to 200 km you gain speed. If you do a rocket burn at 200 km, you gain yet more speed. After that burn, when you go back up, you will lose less speed between 200 km and 40,000 km than on previous orbits because you are going faster and therefore, you reach 40,000 km in less time so gravity has less time to slow you down. So when you reach 40,000 km you have the speed you had on previous orbits plus the additional speed of your rocket burn at 200 km plus the additional speed due to the Oberth effect, that is the additional speed due to you slowing down less while going up. If you had done your rocket burn at 40,000 km you would only get your speed plus your delta-v due to the rocket burn. You get that even if the planet was a rogue planet not around a star. The fly-by gravity assist is a different thing. If you don't do a rocket burn low in the gravity field of a lonely planet you don't really get an extra push from going into the gravity field of that planet. You come back out with the same speed you went in, just in another direction. If the planet is around a star, you again get out with the same speed relative to the planet, just in another direction. But that can mean a greater speed relative to the star. For example, if you had zero speed relative to the star, you had a large speed relative to the planet. Now changing the direction of that large speed gives you a large speed relative to the star. So you go from no speed relative to the star to a large speed relative to the star with no rocket burn. There is also another component to gravity assist, in that you actually change very slightly the orbit of the planet, either stealing energy from the planet or giving it some energy. That energy goes into the spacecraft. This doesn't need any rocket burn at all. The Oberth effect does require an acceleration deep in the gravity well. Alain Fournier |
#19
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SLS launches likely delayed
Alain Fournier wrote:
On Apr/19/2017 at 6:02 AM, Jeff Findley wrote : In article m, says... Reality check question: If you spend X energy from low Earth altitude/orbit to Moon orbit, does Oberth effect claim you will get a value of energy greater than X falling back from Moon to low earth altitude? If you don't leave the earth/moon system, I don't think there is a "free lunch" to be had here. In order for the Oberth effect to be present, you have to do a parabolic fly-by maneuver. As in your velocity is higher than escape going in and much higher than escape when going out. The extra velocity you gain is "stolen" from the planet which you fly- by. No fly-by, no Oberth effect. No that is not what the Obert effect is. The Obert effect is due to greater efficiency of a rocket burn deep in a gravity well than higher in the gravity well. Suppose you are in an elliptic orbit with let's say perigee at 200 km and apogee at 40,000 km. While you go up from 200 km to 40,000 km you lose speed. Then when you go down from 40,000 km to 200 km you gain speed. If you do a rocket burn at 200 km, you gain yet more speed. After that burn, when you go back up, you will lose less speed between 200 km and 40,000 km than on previous orbits because you are going faster and therefore, you reach 40,000 km in less time so gravity has less time to slow you down. So when you reach 40,000 km you have the speed you had on previous orbits plus the additional speed of your rocket burn at 200 km plus the additional speed due to the Oberth effect, that is the additional speed due to you slowing down less while going up. If you had done your rocket burn at 40,000 km you would only get your speed plus your delta-v due to the rocket burn. You get that even if the planet was a rogue planet not around a star. The fly-by gravity assist is a different thing. If you don't do a rocket burn low in the gravity field of a lonely planet you don't really get an extra push from going into the gravity field of that planet. You come back out with the same speed you went in, just in another direction. If the planet is around a star, you again get out with the same speed relative to the planet, just in another direction. But that can mean a greater speed relative to the star. For example, if you had zero speed relative to the star, you had a large speed relative to the planet. Now changing the direction of that large speed gives you a large speed relative to the star. So you go from no speed relative to the star to a large speed relative to the star with no rocket burn. There is also another component to gravity assist, in that you actually change very slightly the orbit of the planet, either stealing energy from the planet or giving it some energy. That energy goes into the spacecraft. This doesn't need any rocket burn at all. The Oberth effect does require an acceleration deep in the gravity well. Perhaps I'm all screwed up here (hey, I'm retired and it's early morning), but I'm missing some things in this discussion. 1) 'Gravity slingshot' isn't just about changing the direction of your velocity vector, is it? In a lot of cases these are designed to 'steal' orbital velocity from the planet by 'falling' from the back side of the planet's velocity vector so you get 'dragged' along as you fall inward, aren't they? So you get increased velocity 'free'. 2) Oberth Effect really only applies for orbits in the same plane, doesn't it? If you want to do something like an orbital plane change, those are actually 'easier' and 'more efficient' if you are higher up (the opposite of Oberth Effect) and they will actually raise the apoapsis of the orbit and do the plane change burn at maximum distance from the planet and then recircularize. Am I wrong? -- "The reasonable man adapts himself to the world; the unreasonable man persists in trying to adapt the world to himself. Therefore, all progress depends on the unreasonable man." --George Bernard Shaw |
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SLS launches likely delayed
Le Apr/19/2017 à 2:07 PM, Fred J. McCall a écrit :
Alain Fournier wrote: On Apr/19/2017 at 6:02 AM, Jeff Findley wrote : In article m, says... Reality check question: If you spend X energy from low Earth altitude/orbit to Moon orbit, does Oberth effect claim you will get a value of energy greater than X falling back from Moon to low earth altitude? If you don't leave the earth/moon system, I don't think there is a "free lunch" to be had here. In order for the Oberth effect to be present, you have to do a parabolic fly-by maneuver. As in your velocity is higher than escape going in and much higher than escape when going out. The extra velocity you gain is "stolen" from the planet which you fly- by. No fly-by, no Oberth effect. No that is not what the Obert effect is. The Obert effect is due to greater efficiency of a rocket burn deep in a gravity well than higher in the gravity well. Suppose you are in an elliptic orbit with let's say perigee at 200 km and apogee at 40,000 km. While you go up from 200 km to 40,000 km you lose speed. Then when you go down from 40,000 km to 200 km you gain speed. If you do a rocket burn at 200 km, you gain yet more speed. After that burn, when you go back up, you will lose less speed between 200 km and 40,000 km than on previous orbits because you are going faster and therefore, you reach 40,000 km in less time so gravity has less time to slow you down. So when you reach 40,000 km you have the speed you had on previous orbits plus the additional speed of your rocket burn at 200 km plus the additional speed due to the Oberth effect, that is the additional speed due to you slowing down less while going up. If you had done your rocket burn at 40,000 km you would only get your speed plus your delta-v due to the rocket burn. You get that even if the planet was a rogue planet not around a star. The fly-by gravity assist is a different thing. If you don't do a rocket burn low in the gravity field of a lonely planet you don't really get an extra push from going into the gravity field of that planet. You come back out with the same speed you went in, just in another direction. If the planet is around a star, you again get out with the same speed relative to the planet, just in another direction. But that can mean a greater speed relative to the star. For example, if you had zero speed relative to the star, you had a large speed relative to the planet. Now changing the direction of that large speed gives you a large speed relative to the star. So you go from no speed relative to the star to a large speed relative to the star with no rocket burn. There is also another component to gravity assist, in that you actually change very slightly the orbit of the planet, either stealing energy from the planet or giving it some energy. That energy goes into the spacecraft. This doesn't need any rocket burn at all. The Oberth effect does require an acceleration deep in the gravity well. Perhaps I'm all screwed up here (hey, I'm retired and it's early morning), but I'm missing some things in this discussion. 1) 'Gravity slingshot' isn't just about changing the direction of your velocity vector, is it? In a lot of cases these are designed to 'steal' orbital velocity from the planet by 'falling' from the back side of the planet's velocity vector so you get 'dragged' along as you fall inward, aren't they? So you get increased velocity 'free'. Changing the direction of your velocity vector relatively to one body (let's say a planet) can be very much the same thing as increasing your orbital velocity relatively to another body (let's say the star). And changing the velocity vector is done by changing (so very slightly) the velocity vector of the planet. So the two are kind of the same thing. (In a previous post, I was implying they are different, I was wrong.) If you have zero velocity relatively to a star, you can have a high velocity relatively to a planet orbiting that star in the direction opposite to the orbital motion of the planet. If you change that to a high velocity relatively to the planet in the same direction as the orbital motion of the planet, you went from zero velocity relatively to the star to twice the orbital velocity of the planet relatively to the star. And you are on your way to being ejected from the solar system. 2) Oberth Effect really only applies for orbits in the same plane, doesn't it? If you want to do something like an orbital plane change, those are actually 'easier' and 'more efficient' if you are higher up (the opposite of Oberth Effect) and they will actually raise the apoapsis of the orbit and do the plane change burn at maximum distance from the planet and then recircularize. Yes and no, depends on your point of view. In a previous post I explained how you can use the Oberth Effect while staying in orbit in the gravity well. I was giving that example only to show the difference between the Oberth Effect and gravity slingshot. That is not the classical example of using the Oberth Effect. Usually when you use the Oberth Effect, you start outside a gravity well, go deep into it, fire your rockets and comeback out much faster than if you had simply fired your rockets without a gravity well. You could do that to do a plane change. Let's say you want to go into a polar orbit around the Sun at a distance similar to Venus' orbital distance. You can go deep into Venus' gravity well, fire your rockets deep in the gravity well, and use that to make your plane change. I'm not saying that is the best way to do it. One would have to evaluate different scenarios to know what is best. A good candidate would be to go to Jupiter, use a gravity assist from Jupiter to get into a polar orbit with perigee near Venus (no need for a rocket burn here, Jupiter is massive enough to do the plane change without using the Oberth Effect). Then use the Oberth effect at Venus to circularize your orbit. So, yes you are correct. If you want to do a plane change in a simple gravity well, you do your burn high up, not low in the gravity well as when using the Oberth Effect. But if you are in a gravity well with secondary gravity wells, you can very well use the Oberth Effect in those secondary gravity wells to help you in your plane change. Alain Fournier |
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