(SPOILERS) The reasons for the Archangel's return

OrlandoNative wrote:

Keith_w wrote:Would you mind explaining WHY I couldn't get a KEW to another planet? I am glad that you are a planetary scientist, which I am not, so you should be aware that Safehold is a planet, which could be used to provide the initial gravity boost. We don't even know if there are other celestial objects in the solar system. There has been AFAIK no mention of astronomy on Safehold.

As for the rest, I didn't say that it would be particularly effective, just that it could be done. As it was pointed out in OAR, coasting missiles are easy to avoid for spaceships, but not for planets.

It's not so much that you possibly couldn't get a KEW to another planet, it's that a orbital bombardment system wouldn't be the *kind* of KEW one would use to do so. Such a system has minimal capabilities for flight. Basically a way to slow down (so the projectile falls from orbit) and possibly some steering capability while still in space.

For a projectile to travel to another planet, it would need the capability to boost itself out of the gravity field of the planet it's orbiting. That would require significant thrust. For example, the moon trips couldn't just "slingshot" themselves from Earth to Moon, though they could from Moon to Earth if they needed to; due to the Moon as a starting point being "higher" in the gravity well than low Earth orbit was.

"Slingshot" maneuvers require either (1) the projectile *not* being part of the planetary "system" being used for the slingshot (ie, having more than escape velocity to begin with) or (2) significant propulsive assist at some point during the trajectory. Otherwise, the velocity gained diving into the gravity well is lost again as the projectile climbs back out. Conservation of energy - you can't get something for nothing. It's really more of a course *correction* implementation; where one can alter course with minimal fuel usage.

It would not need to boost itself. It could be boosted by the launcher, and when it gets to the other planet, it doesn't need to boost itself out of the gravity well, the vector of transit will do that for it. After all, we do that for our planetary probes and they sure don't carry enough fuel to do that - if they did, they could boost themselves to decent speeds and not worry about slingshotting.

Keith_w wrote:

OrlandoNative wrote:
It's not so much that you possibly couldn't get a KEW to another planet, it's that a orbital bombardment system wouldn't be the *kind* of KEW one would use to do so. Such a system has minimal capabilities for flight. Basically a way to slow down (so the projectile falls from orbit) and possibly some steering capability while still in space.

For a projectile to travel to another planet, it would need the capability to boost itself out of the gravity field of the planet it's orbiting. That would require significant thrust. For example, the moon trips couldn't just "slingshot" themselves from Earth to Moon, though they could from Moon to Earth if they needed to; due to the Moon as a starting point being "higher" in the gravity well than low Earth orbit was.

"Slingshot" maneuvers require either (1) the projectile *not* being part of the planetary "system" being used for the slingshot (ie, having more than escape velocity to begin with) or (2) significant propulsive assist at some point during the trajectory. Otherwise, the velocity gained diving into the gravity well is lost again as the projectile climbs back out. Conservation of energy - you can't get something for nothing. It's really more of a course *correction* implementation; where one can alter course with minimal fuel usage.

It would not need to boost itself. It could be boosted by the launcher, and when it gets to the other planet, it doesn't need to boost itself out of the gravity well, the vector of transit will do that for it. After all, we do that for our planetary probes and they sure don't carry enough fuel to do that - if they did, they could boost themselves to decent speeds and not worry about slingshotting.

Putting my physicist hat on....
If you are talking about a yield anything higher than a kiloton and you don't want to annihilate the target planet you do need to accelerate the projectile as you cant gain sufficient energy form a standing start (i.e. free fall) without dropping a mountain on the target. A small high velocity projectile has the advantage of a smaller launcher and less time to respond. Dropping a rock from low earth orbit under gravity first requires killing the orbital velocity - which is essentially wasted energy, this would be better used for either a higher velocity non vertical strike or scaling up the velocity (and the projectile down further) a vertical strike at relativistic velocities (if you are fast enough then orbital physics becomes irrelevant (short of strange objects like black holes). It take the same energy either way. (BTW a rock free falling from orbit will not dig a significant crater - you have to be at well above escape velocity for that).
As for firing from the ground, your biggest problem would be muzzle blast, this is substantial as a relativistic projectile (or even one at escape velocity) will radiate energy at a fierce rate as the atmospheric air ahead of it is either compressed and moved aside or at relativistic velocities simply annihilated in a gamma ray burst. Think moderate sized nuclear bomb. Not un-obvious. The higher the altitude the lower the blast (less air on the projectile). NASA has done studies on this and yes its possible, even human survivable, if the muzzle is at Everest level heights, and post launch boosted, but much harder and more expensive than a simple rocket launch for one off's, for a lot of small shots it becomes cheaper but the initial infrastructure required is in GDP level numbers. BTW the entire first stage of the Saturn V was to get to 30000ft and Mach 1 and accounted for over half the launch weight.

Keith_w wrote:I never suggested that you launch things upward. However, there is no evidence that the OBS is in a stable permanent orbit. Indeed, it is probably not, even the ISS has to correct it's orbit on a regular basis. I would not be surprised to discover that the OBS also has to do so, which means it has the means to adjust it's orbit, which means that it can change its orientation, and thus, if it wanted to, launch "upward".

Apparently you have never circled something that is moving. You spend a portion of the time moving in the same direction as the direction of motion and part of the time moving in the other direction.

And exactly what evidence do you have that there aren't rockets on the weapon? Unless the OBS moves to position itself correctly to simply drop the weapon(s) on someone's head, they have to be guided there using some sort of thrusting system, if only to deorbit the weapon.

I actually believe that there are rockets on the weapon. David has hinted that the weapons are not mere dumb rocks, which suggests to me that they have guidance systems. But I also believe that they do not have rockets powerful enough to produce escape velocity. The point of these weapons is that they gain their kinetic energy relative to the planet by falling.

I misstated some details of how the gravity assist works. You actually want to approach the planet from any direction except from directly behind, and you want to end up turning towards the direction of motion of the planet. The best boost you can get is to approach the planet from ahead of the planet and end up moving in the same direction as the planet. I also used loose language, because I didn't want to get you bogged down in technical details. So let's get into a little more detail to straighten this out.

Suppose weapon W and planet P are headed toward each other, relative to the local star. W is moving at velocity V relative to the star, and P is moving at velocity U relative to the star, in the opposite direction. Notice that this means the velocity of weapon W relative to planet P is V+U. W just misses the planet, swings around in a tight parabola, and ends up heading in almost the opposite direction it started in. If the planet is so massive that the loss in its velocity is negligible, then the planet P still have a velocity of approximately U relative to the star. But the weapon W now has a velocity approximately V+2U relative to the star! According to the star, the weapon has gained up to twice the velocity of the planet. It is similar to an elastic collision.

But what is the velocity of weapon W with respect to planet P? Before the close approach, the relative velocity of W to P was V+U. After the close approach, the velocity of W to P is V+2U-U, which equals V+U. The velocity of the weapon with respect to the planet is the same before and after! The weapon does not gain any velocity with respect to the planet.

If the weapon did not have escape velocity before the close approach, it will still not have escape velocity after the close approach. Sure, you gain velocity relative to the star, but that doesn't help you escape the planet. Your velocity relative to the star will oscillate up and down, as you continue to orbit around the planet. That's what happens in any orbit. You never escape the planet.

Gravity assist never changes your velocity relative to the planet; it only changes your velocity relative to other objects which are moving relative to the planet. If you are in orbit around the planet, you have to use some other means to get escape velocity. And as I said earlier, I don't believe that the guidance rockets on the OBS are strong enough to give escape velocity.

Sorry to be so long responding, we are away from home for the weekend visiting family. I hope everyone has a good Easter.
SWM, Thank you for that paraphrase of this Wikipedia article. http://en.wikipedia.org/wiki/Gravity_assist. The fact that the object does not change velocity with regard to the planet it is using for the gravity assist is unimportant, what is important is that it changes velocity relative to the neighbourhood stellar object.

It would seem that we have made a lot of assumptions, so I thought it would attempt to detail some of the things that we know about the OBS and point out some of those that we don’t. I may get them wrong, I am, as I said, away from home and don’t have access to the books. Feel free to correct me or add to the lists, but please provide the Textev to support your changes.

Things we know:
1. Langhorne built it without the knowledge of Pei Kau-yung
2. It dropped multiple KEVs across Alexandria in multiple passes
3. It was used against the Abbey and Grave of Khody’
4. It can defend itself
Things we don’t know:
1. Can it reload itself
2. How does it maintain its orbit
3. Is it repositionable
4. What type of orbit is it in
5. How far out is its orbit
6. What do the KEVs consist of:
a. Are they objects floating in space awaiting orders to de-orbit themselves
b. Are they objects loaded into launching platforms grouped together ready to be launched
c. Do they carry thrusters or other means of changing vector both in and out of atmosphere
7. Is it independent or is it under control, ie is it active or is it passive?
8. Where is the control centre
We know that the TF had control of gravity to a certain extent (Compensators, Tractor beams)
We know that they had quite miniature fusion reactors.
We know that they were able to defeat certain sizes of Ghaba fleets
Feel free to add to the list.

1. Langhorne built it without the knowledge of Pei Kau-yung
2. It dropped multiple KEVs across Alexandria in multiple passes
3. It was used against the Abbey and Grave of Khody’
4. It can defend itself

5. It was used several times during the War Against the Fallen.

I am going to respond to things out of order here:

It would seem that we have made a lot of assumptions, so I thought it would attempt to detail some of the things that we know about the OBS and point out some of those that we don’t. I may get them wrong, I am, as I said, away from home and don’t have access to the books. Feel free to correct me or add to the lists, but please provide the Textev to support your changes.

Things we know:
1. Langhorne built it without the knowledge of Pei Kau-yung
2. It dropped multiple KEVs across Alexandria in multiple passes
3. It was used against the Abbey and Grave of Khody’
4. It can defend itself
Things we don’t know:
1. Can it reload itself
2. How does it maintain its orbit
3. Is it repositionable
4. What type of orbit is it in
5. How far out is its orbit
6. What do the KEVs consist of:
a. Are they objects floating in space awaiting orders to de-orbit themselves
b. Are they objects loaded into launching platforms grouped together ready to be launched
c. Do they carry thrusters or other means of changing vector both in and out of atmosphere
7. Is it independent or is it under control, ie is it active or is it passive?
8. Where is the control centre
We know that the TF had control of gravity to a certain extent (Compensators, Tractor beams)
We know that they had quite miniature fusion reactors.
We know that they were able to defeat certain sizes of Ghaba fleets
Feel free to add to the list.

I would say that all of this is completely accurate. It is a decent summary. It is, of course, completely irrelevant to whether you can use a gravity assist to get something in orbit out of orbit. But it is certainly a useful summary for the rest of the thread.

Keith_w wrote:SWM, Thank you for that paraphrase of this Wikipedia article. http://en.wikipedia.org/wiki/Gravity_assist. The fact that the object does not change velocity with regard to the planet it is using for the gravity assist is unimportant, what is important is that it changes velocity relative to the neighbourhood stellar object.

No, it is important. Let's go back to what you originally asked. You asked:

Keith_w wrote:Would you mind explaining WHY I couldn't get a KEW to another planet? I am glad that you are a planetary scientist, which I am not, so you should be aware that Safehold is a planet, which could be used to provide the initial gravity boost.

So you are asking whether you can use gravity assist to get away from the planet it is orbiting to another planet. I am trying to tell you why that is impossible.

If the weapon is in orbit around the Safehold, the by definition it is not traveling at escape velocity. It does not have enough energy to escape Safehold's gravity and travel to another planet. It does not matter what kind of orbit it is, or whether the orbit is stable. If it had enough energy to get out of orbit, it would not be in orbit. Do you agree with me so far?

So you are trying to use gravity assist around Safehold to boost the weapon's velocity. I have just demonstrated that gravity assist does not change the final velocity of the weapon with respect to the planet. That means that the weapon still does not have escape velocity. It cannot get away from the Safehold. It is still in orbit around Safehold.

You seem to be confused by thinking that it is only the veolocity relative to the star that matters. But consider what happens to an object in orbit around a planet. For simplicity, let us assume a circular orbit. The speed of the object relative to the planet is a constant; the direction of motion is constantly changing. Sometimes, as seen by the star, the object is traveling in the same direction as the planet, and sometimes in the opposite direction. It is just like the situation described in a gravity assist--the velocity of the object with respect to the star is greater on one side of its orbit than on the other. The formula is the same.

Gravity assist is simply the law of gravity. The velocity of the object with respect to other objects will change as the object swings around the planet. If the object was above escape velocity before it swung around the planet, then it will still be above escape velocity afterward. If the object is below escape velocity before swinging around, then it will stay below escape velocity--and it will stay in orbit around the planet.

Dilandu wrote:

1. Langhorne built it without the knowledge of Pei Kau-yung
2. It dropped multiple KEVs across Alexandria in multiple passes
3. It was used against the Abbey and Grave of Khody’
4. It can defend itself

5. It was used several times during the War Against the Fallen.

you got textev for that?

Keith_w wrote:you got textev for that?

Yes.

viewtopic.php?f=7&t=6210

When the Sisters proved . . . intransigent, the abbey — and everyone in it — was destroyed in the middle of the night in a ‘blast of holy fury,’ the last Rakurai of the War Against the Fallen. A blast which, strangely, was never recorded in any of Mother Church’s official records.”

SWM wrote:I am going to respond to things out of order here:

It would seem that we have made a lot of assumptions, so I thought it would attempt to detail some of the things that we know about the OBS and point out some of those that we don’t. I may get them wrong, I am, as I said, away from home and don’t have access to the books. Feel free to correct me or add to the lists, but please provide the Textev to support your changes.

Things we know:
1. Langhorne built it without the knowledge of Pei Kau-yung
2. It dropped multiple KEVs across Alexandria in multiple passes
3. It was used against the Abbey and Grave of Khody’
4. It can defend itself
Things we don’t know:
1. Can it reload itself
2. How does it maintain its orbit
3. Is it repositionable
4. What type of orbit is it in
5. How far out is its orbit
6. What do the KEVs consist of:
a. Are they objects floating in space awaiting orders to de-orbit themselves
b. Are they objects loaded into launching platforms grouped together ready to be launched
c. Do they carry thrusters or other means of changing vector both in and out of atmosphere
7. Is it independent or is it under control, ie is it active or is it passive?
8. Where is the control centre
We know that the TF had control of gravity to a certain extent (Compensators, Tractor beams)
We know that they had quite miniature fusion reactors.
We know that they were able to defeat certain sizes of Ghaba fleets
Feel free to add to the list.

I would say that all of this is completely accurate. It is a decent summary. It is, of course, completely irrelevant to whether you can use a gravity assist to get something in orbit out of orbit. But it is certainly a useful summary for the rest of the thread.

Keith_w wrote:SWM, Thank you for that paraphrase of this Wikipedia article. http://en.wikipedia.org/wiki/Gravity_assist. The fact that the object does not change velocity with regard to the planet it is using for the gravity assist is unimportant, what is important is that it changes velocity relative to the neighbourhood stellar object.

No, it is important. Let's go back to what you originally asked. You asked:

Keith_w wrote:Would you mind explaining WHY I couldn't get a KEW to another planet? I am glad that you are a planetary scientist, which I am not, so you should be aware that Safehold is a planet, which could be used to provide the initial gravity boost.

So you are asking whether you can use gravity assist to get away from the planet it is orbiting to another planet. I am trying to tell you why that is impossible.

If the weapon is in orbit around the Safehold, the by definition it is not traveling at escape velocity. It does not have enough energy to escape Safehold's gravity and travel to another planet. It does not matter what kind of orbit it is, or whether the orbit is stable. If it had enough energy to get out of orbit, it would not be in orbit. Do you agree with me so far?

So you are trying to use gravity assist around Safehold to boost the weapon's velocity. I have just demonstrated that gravity assist does not change the final velocity of the weapon with respect to the planet. That means that the weapon still does not have escape velocity. It cannot get away from the Safehold. It is still in orbit around Safehold.

You seem to be confused by thinking that it is only the veolocity relative to the star that matters. But consider what happens to an object in orbit around a planet. For simplicity, let us assume a circular orbit. The speed of the object relative to the planet is a constant; the direction of motion is constantly changing. Sometimes, as seen by the star, the object is traveling in the same direction as the planet, and sometimes in the opposite direction. It is just like the situation described in a gravity assist--the velocity of the object with respect to the star is greater on one side of its orbit than on the other. The formula is the same.

Gravity assist is simply the law of gravity. The velocity of the object with respect to other objects will change as the object swings around the planet. If the object was above escape velocity before it swung around the planet, then it will still be above escape velocity afterward. If the object is below escape velocity before swinging around, then it will stay below escape velocity--and it will stay in orbit around the planet.

Why do you consider it to be so important that the object not have increased it's velocity relative to the planet it is departing using the gravity effect? It is the speed relative to the remainder of the stellar neighbourhood that is important. In any case, I still have not seen a case for you that an object could not depart orbit around a planet using the gravity effect.

Dilandu wrote:

Keith_w wrote:you got textev for that?

Yes.

viewtopic.php?f=7&t=6210

When the Sisters proved . . . intransigent, the abbey — and everyone in it — was destroyed in the middle of the night in a ‘blast of holy fury,’ the last Rakurai of the War Against the Fallen. A blast which, strangely, was never recorded in any of Mother Church’s official records.”

Thanks. it is the word "last" that is important there.