Author Topic: Orbit Raising Drive, Old Thought  (Read 4005 times)

Offline redneck

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Orbit Raising Drive, Old Thought
« on: 01/01/2023 08:08 pm »
I had an idea over 30 years ago (pre-internet) inspired by something I read, probably in Analog. I scanned a few discussions here and decided it probably wasn't worse than some of the things kicked around.   As best I can tell, it doesn't work based on information I have been able to find. But if there happens to be some loophole that I have missed, it would be nice if someone can make something of it.  Also, crackpot ideas have a way of repeating by different authors at different times. There is likely a discussion or several about this somewhere in archives given the right search terms.

The idea is based on the idea that a mass at a given orbital height that is moving faster than circular orbit for that height has an energy that is the square of the difference. Similarly, a mass moving at less than orbital velocity for that altitude has an energy the square root of the different. So two masses tethered together spinning at half orbital velocity for that altitude have 2.25+0.25/2 of the energy required to stay in that orbit. Or net about 1.25 times times the energy required for that orbit and should translate to higher orbits working against the gravity field of the Earth. No expelled propellant, just energy to keep the tether and masses spun up.

I can't remember the original publication that triggered the thought. The various planet/moon pairs slowly translating into higher solar orbits seemed to lend weight to the idea. My other check was the gyroscopes and reaction wheels in orbit. If there were any validity to the concept, there should be a small but measurable tendency for them to exert an upward force on the spacecraft. Apparently that doesn't happen from any information that I have seen.

It seems to me that gravity  would be operating on the center of mass to bust the concept. Also the lack of corroborating data from gyroscopes and reaction wheels should bust it as well. Still, it would be interesting to find previous discussions on this idea as "pre-internet for me" meant pre-discussion with informed people.

Offline Jim

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Re: Orbit Raising Drive, Old Thought
« Reply #1 on: 01/06/2023 05:59 pm »

It seems to me that gravity  would be operating on the center of mass to bust the concept.

It is.  That is also why it was called center of gravity.

Offline redneck

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Re: Orbit Raising Drive, Old Thought
« Reply #2 on: 01/06/2023 07:02 pm »
My wasted months and dollars chasing that answer were not completely wasted.  In the process I figured out how to use powered tethers combined with chemical rockets to get an engine with 650 Isp. Low T/W and only good in vacuum in addition to being somewhat complex.   Might be useful at some point, or may not depending on available alternatives   

Offline 1

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Re: Orbit Raising Drive, Old Thought
« Reply #3 on: 01/07/2023 12:06 am »
There are a few common traps that folks fall into when trying to apply everyday intuition to not-everyday physical systems. The main issue with your proposal is that it's simply incomplete.

Main issue, understand that kinetic energy by itself is nowhere near sufficient to completely describe the behavior of a system. Even before getting into the details of things like conserved quantities and invariance, remember that simply spinning an object gives it kinetic energy. KE does not require any linear motion as a whole. A spinning top would have more kinetic energy than a stationary top; even though the spinning one doesn't 'go' anywhere. So transitioning your once-stationary satellite into a spinning bola-like configuration need not lead to any change in orbital trajectory, no matter how much spin (and thereby KE) you wish to add.

Second issue,
... a mass at a given orbital height that is moving faster than circular orbit for that height ...Similarly, a mass moving at less than orbital velocity for that altitude.... No expelled propellant, just energy to keep the tether and masses spun up.

You've immediately declared, and then promptly forgotten that you have a tether in your system. It does not matter if a mass is moving too quickly, or too slowly for a given orbit if gravity is no longer the only force acting on those masses. The tether will quite literally hold the 'too fast' part back, and pull the 'too slow' part forwards. And this, of course, is all before getting into the details of HOW you spin up your bola-lite. If there are no propellants used, then you need a counterweight/flywheel/two more bola balls/something.

So yeah, incomplete system characterization. And I suspect your 650s ISP engine falls into a similar trap as well.

I will, however, also say that it was good for you to immediately recognize that your proposal likely doesn't work and compare it to reaction wheels and the like. The tone of the opening post was a breath of fresh air compared to how these threads sometimes start.

Offline Asteroza

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Re: Orbit Raising Drive, Old Thought
« Reply #4 on: 01/07/2023 02:33 am »
Isn't this tidal force manipulation through rotating tether rotation speed and tether length?
AKA tether winching your way to higher orbits?

This paper and it's references seem like a start?

https://arxiv.org/abs/1403.2221
« Last Edit: 01/07/2023 02:34 am by Asteroza »

Offline 1

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Re: Orbit Raising Drive, Old Thought
« Reply #5 on: 01/07/2023 03:39 am »
Tidal force manipulation requires continually changing the length of the tether such that a chosen part of your system interacts slightly more strongly than the other parts with an orbited body. This is perfectly allowable, but it ultimately requires a momentum and energy exchange with that orbited body. It cannot be done by "only" imparting spin onto an object and analyzing the motion of the spinning object.

The paper mentioned makes the need for variable tether lengths quite clear from the get go, but probably expresses the statement most strongly in the sentences between equations 6 and 7:

Quote
Considering the system angular momentum is always conserved, and the mechanical energy is
conserved only with invariant tether length
...

As worded, this phrase is intended to support viability of the idea by reminding the reader that the authors intend to vary the tether length; but it also acknowledges that a static length tether will not work. At least, not with the usual assumptions and approximations.

Offline redneck

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Re: Orbit Raising Drive, Old Thought
« Reply #6 on: 01/07/2023 09:26 am »


So yeah, incomplete system characterization. And I suspect your 650s ISP engine falls into a similar trap as well.



https://selenianboondocks.com/2008/10/tetherocket/

I'll bite on this one. With the caveat that theoretically possible is not the same as technically and financially feasible.

Offline 1

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Re: Orbit Raising Drive, Old Thought
« Reply #7 on: 01/07/2023 09:10 pm »


So yeah, incomplete system characterization. And I suspect your 650s ISP engine falls into a similar trap as well.



https://selenianboondocks.com/2008/10/tetherocket/

I'll bite on this one. With the caveat that theoretically possible is not the same as technically and financially feasible.

Okay, now I see what you're after. That will make a difference of sorts, but not a useful one. For similar reasons as above, unfortunately.

To see this easier, let's change the firing sequence slightly, and ignore the yellow armature altogether. Let's pretend that both rockets on the blue armature fire simultaneously. That is to say, both rockets fire from 8 to 10 o'clock positions, and both from 2 to 4 o'clock positions.

If the device were not moving at all, it is (I hope) intuitively obvious that the device as a whole will not go anywhere, and will simply spin in circles like a top. So, knowing that the laws of motion are identical in all reference frames, firing both rockets at the same time should not do anything different even if your system already has some velocity. The question then becomes, does the analysis presented in the blog post agree with that?

And here, we see the system break. If you fire the one blue rocket from 8 to 10 as normal, you get your exhaust velocity of 7.5km/s relative to system center distant observer as stated. However, the rocket firing from 2 to 4 only gets an exhaust velocity of 1.5km/s. So if the perceived benefits of the tether rocket were real, this system should still accelerate forward, and do so very rapidly. If all reference frames do not agree on what would happen to the system, then it's a clear indication that the system has not been fully characterized.

Pleas recognize that, thus, it is better analysis of the system that's needed, rather than a additional changes to the system itself.

To see what's going on in a bit more detail, you can watch this video if you prefer, which discusses motion of objects that are deliberately NOT driven through their center of mass.

https://www.youtube.com/embed/cBdXtSZcDsg

I kind of wish this fellow left his analysis in pure variables, but I chose this video because he discusses the effect of an impulse, which is, within limits, applicable to the effect of a rocket burn over some finite duration. A subtlety of this is that an impact can be approximated as instantaneous, which makes the effect independent of existing rotational velocity. He uses a rod instead of a bola-like object, but that functionally only changes the formula for moment of inertia "I".

Most important result begins somewhere around the 9 minute mark, where he notes that the change to linear velocity as measured by motion of the center of mass is actually independent from where the rod is struck, and it is only applied angular velocity that is dependent on position.

So what this finally, functionally means is that attaching rockets to tethers can make it easier to spin up and spin down the device, but can never be used to improve linear translational performance. All such real-world effects that do improve vehicle performance (e.g., oberth, planetary flyby) ultimately require interaction some way some how with an object external to your spacecraft.

Hope this helps.

edit. critical correction. Must be careful with my own terminology as well.
« Last Edit: 01/07/2023 09:12 pm by 1 »

Offline redneck

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Re: Orbit Raising Drive, Old Thought
« Reply #8 on: 01/07/2023 10:06 pm »

And here, we see the system break. If you fire the one blue rocket from 8 to 10 as normal, you get your exhaust velocity of 7.5km/s relative to system center distant observer as stated. However, the rocket firing from 2 to 4 only gets an exhaust velocity of 1.5km/s. So if the perceived benefits of the tether rocket were real, this system should still accelerate forward, and do so very rapidly. If all reference frames do not agree on what would happen to the system, then it's a clear indication that the system has not been fully characterized.



I think you are missing that the rocket only fires in pulses when the vectors line up. The blue never fires other than the 8 to 10 position as the yellow only fires from the 2 to 4 position. If it wasn't clear in the post, it was certainly made clear in comments. Several people had the same visualization problem with the concept.

Offline 1

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Re: Orbit Raising Drive, Old Thought
« Reply #9 on: 01/08/2023 08:26 am »
No, I get that perfectly.

What I'm trying to show with a thought experiment, and what the linked video explicitly proves mathematically, is that firing a rocket at the end of an arm does no better than simply firing it directly through the center of mass. In other words, the perceived benefit of spinning up an arm is an illusion. Addition of the yellow arm does exactly the same; for a grand total of two engines firing that do no better than two equivalent engines firing directly through the center of mass. This is in line with every other treatment of a rocket engine, where the ISP is considered constant regardless of how fast the rocket itself is traveling.

To get an improvement beyond what a system can do by itself, it must interact with something other than itself. Any other result is erroneous.

Offline redneck

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Re: Orbit Raising Drive, Old Thought
« Reply #10 on: 01/08/2023 09:35 am »
One of the problems I had in the original concept was explaining the torque down the arms. I will put together a sketch and description in the advanced concepts section when I get a bit of time. None of us need extended arguments that go nowhere as in too many threads in this section.

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Re: Orbit Raising Drive, Old Thought
« Reply #11 on: 01/09/2023 02:23 am »
Sounds good to me; I look forward to reading it.

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