ACME Test

[n0mad23]

Ok,

Here's an alpha for those of you interested. No documentation is included, so you'll have to get it here.

For information, see the ZTC ltd. thread. Any questions, please post here.

Requires Vinka's Spacecraft3.dll

The tether's in an orbit pretty much aligned with the moon, and acts as a Delta-V assist for LTO.

Release the DG around periapsis, turn prograde and burn. It's important to undock at the top of the tether's rotation!

IMFD works really well here, if not for the autoburn, certainly to show your orbit.

 

[Bullethead]

One word: Yee-hah!

This gizmo could probably pay for itself just in space tourism as the solar system's most extreme thrill ride: "Six Flags over LEO" :speakcool:.

It's an interesting ride going around a few times waiting for perapsis, watching your orbit alt going up and down with the rotation of the flinger. Looking out the window, you can really tell you're at the end of a long string instead of rotating about your own ship's axis. It's a totally different sensation than you're used to from a spacecraft, more like turning a circle in an airplane, which gives a pleasantly disconcerting feeling that you've somehow gotten out of control. You get a nice view of the Earth as you go around to enjoy your hi-res textures.

Then you undock and, POOF, you're GONE! :leaving: You leave the whole 40km-long flinger thing behind in a blink of an eye. It's something of a shock, because up until then you have no idea how much tangential velocity you've got. You can't see but the nearest part of the flinger so you don't know how big an arc you're following, your rotation doesn't look very fast, and your motion relative to the Earth below is no faster than in any other comparable orbit. Then suddenly the whole flinger fills your view and just as rapidly disappears in the distance. :woohoo:

I guess it wouldn't be so surprising in real life, because you'd be feeling the Gs during the wind-up and would know you were about to play "pop the whip". But I don't think that would be as enjoyable as the surprise zing you get in Orbiter. "Mommy, can we ride it again?"

I figure eventually, once it's long enough to achieve escape velocity, this thing will need some sort of specialized software to set up the shots. The degree of precision and the amount of automation in this software would be dependent on the potential dV of the ship in question. If it can make noticeable course corrections or even land itself, then more can be left to chance. But I think an automatic release timer would be necessary, to minimize aiming errors.

But that's an issue for another day. Right now, this is a blast to play with. Can't wait to see the next version.

 

[n0mad23]

Yeeh-Ha, indeed!

I'm glad to see it's infectious, and maybe I'm not the only "Luna-tick" around here. Every time I've played around with this idea in the last decade, that's been my reaction. "Far-ing-Out!"

I really like what you said in our other communication about it being like a "medieval trebuchet." I think it's a better image than the sling of David or Palestinian youth.

I do have this test jacked up a bit. The apsis of the tether-sling is higher than it should be, but I've done this to get the velocity up a bit.

And you're right - there's going to have to be software development for this to turn into a working transportation system. I'll post some thoughts on that on the related thread.

In the meantime, here's a little "mini-tutorial" for using the scenario I've put into this test.

ZTC Ltd. ACME Tether Test mini-tutorial.
​

This scenario requires IMFD (I used V. 5.1e).

Switch to cockpit view. Turn on the HUD and leave it in the default ‘surface’ mode.

Turn the MFDs on. Set one to ‘Orbit’, and the other to ‘Interplanetary.’ Note your distance to periapsis and keep this in mind.

To get the best sense of the change in velocity and altitude, set the ‘Orbit’ MFD ‘Target’ to ‘20k-cable’.

Switch IMFD to ‘Course’. Select ‘Target’ and enter ‘Moon’. Switch ‘Realtime’ to ‘Off-Axis’.

Now switch to ‘Surface’ MFD so you can see both the velocity, and the approach of periapsis.

When you’re nearest (either before or after) periapsis, and you’re looking directly at the ground (the apex of tether rotation), undock. Note instantaneous change in Delta-V and orbits.

Rotate into prograde orientation, and relax. You’re about 750 seconds away from your LTO burn. At this stage, you can either have IMFD do an Auto Burn (AB), or you can do it manually. IMFD’s really useful here either way, as it allows you to see a graphic representation of your orbit.

For your lunar ‘Orbital Insertion’, I find a combination of ‘Orbit’ MFD and ‘IMFD Map’ with the Moon set as its reference work well.

Finally, notice how little fuel this actually took.
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attached is a text copy of this tutorial

 

[Bullethead]

I really like what you said in our other communication about it being like a "medieval trebuchet."

I sometimes catch this show called "Whacked Out Sports" on Spike. One day, they had a clip of a chick being launched by a small trebuchet into what the narrator called the "tragically misnamed 'safety net'". She landed dead center in it but, due to some excess angular momentum, bounced out and hit the ground from a height of about 20' :clap:. This thing seems to have the same potential for disastrous consequences if mishandled. My next test will be to release when level with horizon on my way down. Wonder how high I'll bounce? :rofl:

In the meantime, here's a little "mini-tutorial" for using the scenario I've put into this test.

I'll do that after my "centrepital dV" test and will report back.

 

[Bullethead]

I'll do that after my "centrepital dV" test and will report back.

Well, here's a counter-intuitive thing for you, but it shouldn't be that surprising with a little thought. The only way to crash into Earth is to release when you're facing up (+90^), which amounts to a retrograde burn. Even if you release while being thrown dowards toward Earth, you still end up in a stable, though eccentric, Earth orbit.

I made it to the moon following your instructions. The cool thing was actually hitting the *****--good job setting the scenario up to do that, without us having any real idea from our instruments what's going on at the point of release. I just took whatever orbit inclination and alt I was dealt when I arrived, which basically means whatever was easiest (cheapest on fuel) to do. Thus, if I was aiming for a specific base to land on, I'd have had to do some expensive inclination burns in lunar orbit.

IMFD's Planet Approach program is cool for avoiding this problem, because you can input the desired lunar orbit's inclination and (circular) altitude. Then you can burn before you get too close to the moon to point you in the direction that would make you hyperbolic at these parameters, and then just to a braking burn into the desired orbit when you get close to the moon.

Anyway, with or without the flinger, you've still got to take off and get your ejection plane right. So I figure that's a wash, whether you're docking with the flinger or doing without (and it's not in the scenario anyway). On the lunar end, you've still got to tweak yourself into your desired inclination and make the insertion burn. So that's another wash. At present, therefore, the fuel savings is all in the ejection burn. Because the flinger at present won't give you escape velocity, despite the spectacular relative velocity when you leave the flinger, you still have to make an ejection burn. However, with the flinger's toss, it's not as big as it would be if you were doing it all yourself.

This leads to a (probably) stupid question... AFAIK, the bottom line in the ejection burn is achieving the required amount of tangential velocity. Because your tangential velocity is higher the lower your PeA, it would seem to me that the best way save fuel on ejection would be an "underhand" toss, instead of the present "overhand" toss. IOW, looking down from the north pole, the flinger would be rotating clockwise instead of counterclockwise, and you'd release with your back to the Earth instead of facing the Earth. Is this correct?

 

[n0mad23]

Well, here's a counter-intuitive thing for you, but it shouldn't be that surprising with a little thought.

What do you think happens if you toss at mid-rotation up orbit? How about backwards, down orbit?

That's orbital mechanics for you.

Because your tangential velocity is higher the lower your PeA, it would seem to me that the best way save fuel on ejection would be an "underhand" toss, instead of the present "overhand" toss. IOW, looking down from the north pole, the flinger would be rotating clockwise instead of counterclockwise, and you'd release with your back to the Earth instead of facing the Earth. Is this correct?

I have to go back into the documentation for this one. It's something I've deliberated with before. If memory serves (which is why I did it this way) it's got to do with the rotation in another rotation (orbit). In other words, I believe that to toss it underhand would actually have the tether be constantly 'unwinding' and loosing energy.

In terms of energy/velocity transfer, this 'test' can't demonstrate the best aspect, namely Delta-V increase from being grabbed in LEO. The tangential velocity boost can pretty much double the total boost - in the range of 3 m/s.

Yup - it's an ACME. But it's showing me that the theory's sound enough.

 

[tblaxland]

The tether needs to rotate counter-clockwise to reduce the surface relative tip velocity. This means that the tether can pick up sub-orbital payloads and fling them into orbit. See Hypersonic Airplane Space Tether Orbital Launch (HASTOL) System.

 

[NukeET]

How's this for cross thread pollination?

Larry Niven's The Descent of Anansi. Co-authored with Steven Barnes.

They use the tether trick in that story.


I really must download this Sean and give it a test ride.

 

[n0mad23]

The tether needs to rotate counter-clockwise to reduce the surface relative tip velocity. This means that the tether can pick up sub-orbital payloads and fling them into orbit. See Hypersonic Airplane Space Tether Orbital Launch (HASTOL) System.

I have this one in my collection, and Tether's Unlimited has probably informed this project more than any other. I'm convinced that NASA's MXER designs (which I'm also drawing from) in some aspects are far superior and am trying to blend the best of both.

It's rotating the way it is, because it's designed for LEO transfers. I'm drawing from MXER document here. Short response: It's got to do with rotating object systems and conservation of energy.

So while the HASTOL system is not what I actually had in mind for this, it is making me think that the tether reaching down for sub-orbital interceptions would probably be a good part of a three stage system. Hypersonic sub-orbital toss to LEO, LEO toss to LTO.

I think with a tether-sling orbiting into such a low orbit would require "faking" some things in Orbiter that I'm not sure can be done. Specifically I'm thinking of MXER designs that have anodes and cathodes set near the two masses. Ions are picked up at the bottom and spit back out at the top. The result is magnetic repulsion.

Of course this would also mean a whole lot more synchronizing and programming as well. This ACME Test is named after the cartoon anvil - medieval tech bootstrapped into LEO. The real system's going to need rotate around two masses, and reel one of the masses like a climber. The whole system needs to twist to reel prograde, retrograde, etc.

I am curious about the differences in tangential velocity enough now, that I think I'll give your suggestion a try and see what differences there are. Clockwise vs. Counter.

 

[n0mad23]

In this context, the Counter Clockwise rotation won't work. While it's a good idea for the sub-orbital toss to LEO, I don't see it working for LEO to LTO.

You can see for yourself with the attached .scn. The only variable I changed was the rotation direction. And keep in mind that there's still 3.59 g's of force pulling down at the glider.

 

[n0mad23]

How's this for cross thread pollination?

Larry Niven's The Descent of Anansi. Co-authored with Steven Barnes.

They use the tether trick in that story.

You know - I'd completely forgotten about this one! It's time to keep eyes open for a used copy.

 

[tblaxland]

In this context, the Counter Clockwise rotation won't work. While it's a good idea for the sub-orbital toss to LEO, I don't see it working for LEO to LTO.

You can see for yourself with the attached .scn. The only variable I changed was the rotation direction. And keep in mind that there's still 3.59 g's of force pulling down at the glider.

I'll try and find some time to test this on the weekend. All the tether systems I have seen rotate the same direction as the orbit, ie, for a typical counter-clockwise earth orbit the tether also rotates counter-clockwise and the throw to a higher orbit (or LTO) occurs when the tether is at periapsis with the payload at its highest point.

Going back to Bullethead's post, throwing "overhand" (counter-clockwise) instead of "underhand" (clockwise) makes sense. In an Earth-centric inertial frame, the velocity of the payload is the same at the point of release for both scenarios. However, when throwing overhand the payload's orbital energy will be greater because of the greater height.

 

[n0mad23]

I'll try and find some time to test this on the weekend. All the tether systems I have seen rotate the same direction as the orbit, ie, for a typical counter-clockwise earth orbit the tether also rotates counter-clockwise and the throw to a higher orbit (or LTO) occurs when the tether is at periapsis with the payload at its highest point.

Going back to Bullethead's post, throwing "overhand" (counter-clockwise) instead of "underhand" (clockwise) makes sense. In an Earth-centric inertial frame, the velocity of the payload is the same at the point of release for both scenarios. However, when throwing overhand the payload's orbital energy will be greater because of the greater height.

Agreed,

However, there is another force at work here as well. It's got to do with the spin of the Earth and the spin of the tether-sling in a system that's already spinning. You'd think because there's no atmosphere, there's also no drag. However an object spinning in the same direction as the spin of the orbit (and spin of the object being orbited) is more economical in terms of conservation of energy.

Here's a link to one of the MXER animations that parallels my approach:
http://www.mxertether.com/videos/MXER Tether Animation v2-0.mpg

 

[Bullethead]

Going back to Bullethead's post, throwing "overhand" (counter-clockwise) instead of "underhand" (clockwise) makes sense. In an Earth-centric inertial frame, the velocity of the payload is the same at the point of release for both scenarios. However, when throwing overhand the payload's orbital energy will be greater because of the greater height.

Good point. I'd missed the obvious point that just because the far end of the string is closer to the planet doesn't mean it's going any faster--d'oh! The underhand throw also has the practical disadvantage you mentioned of requiring the projectile to get into a much higher orbit for docking.

 

[n0mad23]

Good point. I'd missed the obvious point that just because the far end of the string is closer to the planet doesn't mean it's going any faster--d'oh! The underhand throw also has the practical disadvantage you mentioned of requiring the projectile to get into a much higher orbit for docking.

And I'm finding my continuing experiments here make this all so much more evident. Sometimes the theoretical makes so much more sense when you get to experience it first hand.

I did some more tests with the counter-clockwise (underhanded) toss last night, and found that it really would be the best tech for sub-orbit to LEO boosts. But the approach has to be very different as well.

For example, the AMCE Test if it was able to be reeled currently, would really work best if the payload rode the tether for a full orbit, thus achieving its maximum velocity.

For the sub-orbital to LEO toss, the catcher would only hold onto the payload for about a 40 degree rotation before tossing it away. The part I'm puzzling over now (back to the HASTOL docs) is the difference in velocity. A capture with more than 1.5 m/s difference in velocities seems impossible.

 

[tblaxland]

I agree that the capture with HASTOL would be the most difficult aspect. I would require extremely precise targeting by the suborbital vehicle and it is also a "one-shot" - if you miss you have no option other than to go back home. And the next launch opportunity won't come for a while.

 

[tblaxland]

Can we agree on a some terminology for this project? I have been getting a little confused with the counter-clockwise's and underhand's and so on, because they are somewhat dependent on the perspective of the observer. Here is what I propose:

A tether rotating in the same direction as its orbit has a prograde rotation. In n0mad23's animation above, the tether is in a prograde earth orbit with and it has a prograde rotation.

A payload release in the same direction as the tether's orbit would be a prograde release.

To summarise, what I was arguing before was that a prograde release from a tether with a prograde rotation would result in a higher orbital energy for the payload than for a prograde release from a tether with a retrograde rotation.

n0mad23, I believe your assertion is that prograde rotating tether is more economical than a retrograde one, particularly if the tether itself is in a prograde orbit. I can understand this with regard to upper atmospheric drag, but I can't see how conservation of angular momentum (energy?) comes into this. Do you have any docs/links on this?

Is the terminology OK? Clear as mud?

 

[n0mad23]

Tblxland -

The timing is perfect. Language issues have been forefront in my conscious mind the last several days, and one of the strands is this project.

I use the term "tether-sling" because of the confusion using "tether" alone creates. Similarly, I'm using "rotovator" specifically because of the "-vator" and implications embedded here.

Prograde toss it is. Perfect. Makes a lot more sense, and it's clear.

It'd be a retrograde rotation to pick up a sub-orbital payload then.

The amount of documentation I've scrounged in the last few years, combined with the volumes I've recently been granted, I'm sure the conservation of angular momentum is addressed very specifically. I'll keep my eyes peeled as I continue to browse and contemplate.

 

[tblaxland]

It'd be a retrograde rotation to pick up a sub-orbital payload then.

Prograde rotation for a pickup. Refer to this diagram from your other thread:

Here the tether-sling rotation is prograde. Let's work it our another way... Determine the direction of the angular velocity vector using the right hand rule for both the tether-sling rotation and the tether-sling orbit. If these both point in the same direction then the rotation is prograde. In the above example, both vectors point into the screen hence the tether has a prograde rotation.

 

[n0mad23]

I agree.

I was referring to the pickup rotation of a HASTOL suborbital to LEO platform only. But in revisiting the documentation, it seems that even there it's a prograde rotation.