Sling stage

[suicidejunkie]

1 minute ago, Buzz light fear said:

How about try sending some thing to minmus using a tether it would be more efficient than using the thrusters on the vehicle does it take less energy to spin some thing than to just excelerate it with the normal engines

In the optimal case, the tether-dropped vehicle will suffer zero gravity and steering losses; you can spin the tether so it matches the surface speed of Minmus when it is pointed straight down, and release or grab the vehicle at 0m/s and 0 altitude.

If you're constantly picking up loads of ore, say, then you'll be net losing energy, but you can re-boost your tether with a maximally efficient burn with the best available engines directly prograde at AP.

On the other hand, *sending* something to Minmus using a tether, results in the station having EXCESS energy afterwards!  Using rocket engines to land is terrible negative efficiency.

 

The main trick would be timing the rotation so your docking port or claw will connect as you go past at ~150m/s.

[Curveball Anders]

20 hours ago, Cunjo Carl said:

I should add that many momentum tether designs aren't just for flinging things. They're for catching incoming ships, too. That would be a terrifying mode of travel. Cheers!

The catching a returning craft and preserving energy is always lovely.

The problem is that most of those solutions are based having a fixed fulcrum.

While mathematically possible, they're not always engineeringly possible.

[Cunjo Carl]

It's even possible to use a momentum tether to fling another momentum tether ('cause why not be Kerbal about it). I'll just repost this from @DoctorDavinci . It's fantastic! If you want to give the video a like, you can find the original posting here: Link. The link is being a bit weird, so just scroll up 2 posts from where it takes you.

For anyone trying their hands at making momentum tethers, I can highly recommend this transfer from Low Ike Orbit to Duna for practice. It's got a gentle velocity requirement, a wide window and a destination you can land on with a skydiving kerbal.

 

Edited August 5, 2016 by Cunjo Carl
link's a bit weird

[moogoob]

13 hours ago, Cunjo Carl said:

It's even possible to use a momentum tether to fling another momentum tether ('cause why not be Kerbal about it). I'll just repost this from @DoctorDavinci . It's fantastic! If you want to give the video a like, you can find the original posting here: Link. The link is being a bit weird, so just scroll up 2 posts from where it takes you.

For anyone trying their hands at making momentum tethers, I can highly recommend this transfer from Low Ike Orbit to Duna for practice. It's got a gentle velocity requirement, a wide window and a destination you can land on with a skydiving kerbal.

 

That is beautiful!!! Great find!

Now, this thread's got me thinking.. for the last few days... even when on the bus. There are a few difficulties in using these. As I see it:

1. I've heard you can shred a compact disc if you spin it fast enough. Same goes for orbital tether systems. Therefore, the total delta-v you can get is limited, compared to a similar weight of fuel and high-efficiency engine. If you spin it too fast, the thing might just disintegrate.

2. Mass of projectile should be very small. I notice that the video quoted essentially just flings lone Kerbals. Increasing that to a half-ton probe would get you less, and a crewed capsule or lander even less. Forget about sending fuel tanks unless your counterweight is ten times heavier.

3. Aim will be difficult. If you're spinning fast enough to impart sufficient Delta-v, timing your release gets more and more difficult. Phrases like "physics tick time" come to mind too.

4. Now this is easy for folks who are good at math, but figuring just how fast to spin up to impart the needed velocity is also difficult. It's kind of like using a Flea SRB to send probes on their way to other planets - you WILL need an engine to fine-tune the burn after.

5. KSP does not have flexible tethers in stock. Unless you want to mod, you're going to need a truss/girder based assembly, like the video above. Those long, thin, floppy things are difficult to launch. *cough*Understatement.*cough*

With all that said, I'm amazed that @DoctorDavinci got it working so well!  Maybe I'll try it myself?

[DoctorDavinci]

I should post my triple momentum tether just for SaG ... and yes, if you spin too fast relative to your mass your craft will go boom

[Sharpy]

Semi-related: Spring-loaded rocketry

 

[DoctorDavinci]

48 minutes ago, Sharpy said:

Semi-related: Spring-loaded rocketry

 

You should enter that thing into my challenge here:

 

[LaytheDragon]

3 hours ago, moogoob said:

. I've heard you can shred a compact disc if you spin it fast enough. Same goes for orbital tether systems. Therefore, the total delta-v you can get is limited, compared to a similar weight of fuel and high-efficiency engine. If you spin it too fast, the thing might just disintegrate.

I have had-unless-I-load-a-certain-quicksave tether in Low Ike Orbit for the the previously-mentioned challenge, and some of the shorter parts (not the main frame) enjoyed shredding off after spinning at a certain speed, including the part (large battery) that the decoupler was stack-attached to that had the command seat attached to it. I still got the kerbal to Duna, though . Later on I did find a trick for making struts as long as you need in KSP, though.

Edited August 5, 2016 by LaytheDragon

[MaxwellsDemon]

1 hour ago, DoctorDavinci said:

You should enter that thing into my challenge here:

 

Coincidentally--- or perhaps not--- trebuchets were precisely what this thread brought to my mind as well!

[DoctorDavinci]

So this ....

 

[Cunjo Carl]

1 hour ago, moogoob said:

That is beautiful!!! Great find!

Now, this thread's got me thinking.. for the last few days... even when on the bus. There are a few difficulties in using these. As I see it:

1. I've heard you can shred a compact disc if you spin it fast enough. Same goes for orbital tether systems. Therefore, the total delta-v you can get is limited, compared to a similar weight of fuel and high-efficiency engine. If you spin it too fast, the thing might just disintegrate.

2. Mass of projectile should be very small. I notice that the video quoted essentially just flings lone Kerbals. Increasing that to a half-ton probe would get you less, and a crewed capsule or lander even less. Forget about sending fuel tanks unless your counterweight is ten times heavier.

3. Aim will be difficult. If you're spinning fast enough to impart sufficient Delta-v, timing your release gets more and more difficult. Phrases like "physics tick time" come to mind too.

4. Now this is easy for folks who are good at math, but figuring just how fast to spin up to impart the needed velocity is also difficult. It's kind of like using a Flea SRB to send probes on their way to other planets - you WILL need an engine to fine-tune the burn after.

5. KSP does not have flexible tethers in stock. Unless you want to mod, you're going to need a truss/girder based assembly, like the video above. Those long, thin, floppy things are difficult to launch. *cough*Understatement.*cough*

With all that said, I'm amazed that @DoctorDavinci got it working so well!  Maybe I'll try it myself?

Give it a try! They're easier than they look.

1. The centrifugal force relative to the your craft's/Kerbal's velocity is m*v^2/r . So the longer you make a momentum tether, the gentler the centrifugal force for a given speed. Unfortunately, the longer it is, the greater the energy you need to spin it up. Fortunately both short and long can be made to work in KSP. "Moar struts!"

2. The good news is you'll never lose speed. If your payload is zipping around the COM at 100m/s, when you release it'll fly away at 100m/s. If you make your payloads heavier and heavier though, the COM will sneak closer and closer to it and that's where the loss is.

3. Aim is difficult. Here's my experience: [alt+F9] nope. [alt+F9] almost [alt+F9] so close! [alt+F9]....

4. Even with math it winds up being trial and error. For starters though, 100-300m/s is pretty average for a tether you just throw together

5. If you're just interested in the tether and not the launch, you can use a mod called Hyperedit to send your tether straight into orbit from the launch pad for your test. Alternatively, there are ways to make launchable momentum tethers using the mk3 parts, which has been pioneered by @LaytheDragon. Once again, pic taken from the Tether challenge page.

 

Come on over! We'd be happy to hear about your trials, troubles and triumphs in flinging Kerbals to their uh... final destination!

[suicidejunkie]

Tether targeting sounds like it might be a good job for kOS scripting.

Even if it has to wait many rotations to get a good timing for launch due to low frame rates, it should eventually catch a near-perfect angle.