Reusable Slingshot-Tug Concept

[Neici]

Hi dear rocket-sience colleagues

Today i thought of a intersting concept for accelerating sience crafts into interplanetary trajectories.

(Yea i relate to KSP in this "Off-topic" forum, but it is a very physical concept)

Concept principle:

You have a SST(SlingShotTug) in your LKO.

After you have launched, rendez-voused and docked your craft with the tug you will burn prograde into a direct trajectory to your target. Duna for example. So you have a trajectory that leads direct into dunas SOI and you are still around 100km over kerbin.

Undock now your craft from the tug and let it fly. Change to the SST immediately, turn it around and burn retrograde. Reaccomplish LKO.

The SST is ready for further use.

To reduce fuel waste with a tradeoff to use more time, you could burn to hold the SST trajectory in the SOI, lower ti periapsis into atmosphere and do an aerobrake with recirculation afterwards.

Pros:

+ Reusable interplanetary tug that stays at Kerbin.(Less part costs for economics)

+ You don't have to Launch new Tugs befor every launch.

+ (In KSP)You may save some gametime(gaming time not ingame) if you launch several crafts to other planets

+ No interplanetary debris, or at least less if you count your crashed craft

Kons:

- It wastes some fuel because of spending the delta v for the trajectory one time and for returning the tug into LKO a second time.

- You have to set up a accurate and direct trajectory.

- Not recommendable for a journey further than dres.

My thoughts:

- Pro 1 speaks for it self

- Pro 2 is realated to 1. You also don't have to build big launchers to bring a interplanetary stage into orbit.

- The game time you save is: Pro 2 and you can designe a SST that can launch more than one craft without refueling. You only have to rendez-vous a new craft with the tug. So minus refueling launches. But plus the time you need to refuel it later and bringing it back to LKO after use.

So you need to balance the the SST between Partcost and fuel waste.

- Pro 4 speaks for it self

- Yes you waste fuel, but it is much less than used for launching your craft. Because you have less fuel and no payload to slow down again.

- The last two Kons are related. Accurate trajcetories out of a SOI into an other one can be difficult.(In KSP because of the shakeing orbital lines.)

But it is allowed to bring some sort of low delta v trajectory ajustment device in your payload.

My thought example:

SST = a half Tank (18t) + 4 LV-N

Payload 1 = 25t

Payload 2 = 10t

Duna delta v = 1100 m/s

start SST + Payload 1 delta V = 2820 m/s

slingshot burn time = 54s

remaining delta V without Payload = 3093 m/S (1100m/s will be used to return to LKO)

Delta V left with payload 2 (SST + Payload 2) = 1149.7m/s

So in theorie it would be able to launch a second payload with 10t to duna but not to reaccomplish LKO again.

Well with only 2 LV-N it could have enough delta v to bring 2 payloads (20t and 10t) to duna without leaving Kerbin.

But needs as twice as long to burn => gets on a higher orbit after slowing down => wastes more fuel to lower orbit.

It's all a matter of designing. As more crafts the SST should launch without refuelling as more fuel gets wasted.

You can design a SST for almost every payload and for any Planet it should get.But this i leave to to you as spacecraft engeneer. ^^

Tell me what you think about this concept!

PS: It would also be a cool challenge to land a lander on duna, but you are not allowed to use any thrust generating part at the outside of the SOI of Kerbin.

Neici

[magnemoe]

This will work well, now you do not need to do an full return burn, just burn so SST orbit stay inside of Kerbin SOI, do an small adjustment burn at Ap to do an aerobrake for LKO return.

Option 2, modify your high orbit for an Minmus intercept. Refuel at the orbital depot or launch an miner to refuel it.

This plan pretty much require mining to make sense so going to Minmus would be smart as your Ap would be outside it, raise Pe until you get an suitable intercept.

Works best to Duna and Eve as you can aerobrake at target and the dV after leaving kerbin SOI is low.

Now for Jool and the other planets an option is to use the SST as an first stage, burn until just outs

Less suitable for Jool because the 800 m/s dV more than to leave Kerbin SOI, other planets also requires an braking burn. Here you can simply burn the SST until your Ap reach edge of Kerbin SOI, undock and continue with the other ship.

Edited October 4, 2013 by magnemoe

[Kommitz]

I was just thinking about something like this the other day, might be useful. Thinking that refueling it by shipping fuel from Minmus could work well plus you can aerobrake it back down to a low orbit once it's done pushing pushing things.

[Neici]

yea nice thougth about minmus refueling.

But don't you need much luck to have a minmus encounter after slowing down from a interstellar trajectory that actually hit duna for example?

Well actually a SST is e interplanetary stage. But you return it to LKO fur further use. If you let it burn out it would be just a simple interplanetary stage.

It would be possible to designe a SST for Jool, but it would be very fuel wasting. and hard to hit Jool System with the payload.

Cool to see some interesst

[Stochasty]

There are a couple of problems with this idea. The big one is the first one: you return your tug o LKO; now what? It needs refueling before the next mission, but it's easier to launch a second tug than it is to refuel the first one. This leads directly into problem two: if launching new tugs is easier than refueling, then tugs are essentially disposable. But, if tugs are disposable, why did we bother to return the tug to LKO in the first place? It's a waste of fuel, and worse yet is means we carried unnecessary fuel (ie, dead weight) during the transfer burn itself.

The only way this makes sense (in KSP or in real life) is if there is an on-orbit refueling infrastructure waiting in LKO/LEO (and one that doesn't rely on constant launches from Kerbin/Earth). Remember: once you're in orbit, you're half way to anywhere. The idea behind a reusable tug is to minimize trips to the bottom of Kerbin's/Earth's gravity well, but most of your mass for any voyage is fuel, so unless you have a fuel source that's not Earth/Kerbin it's pointless.

[magnemoe]

There are a couple of problems with this idea. The big one is the first one: you return your tug o LKO; now what? It needs refueling before the next mission, but it's easier to launch a second tug than it is to refuel the first one. This leads directly into problem two: if launching new tugs is easier than refueling, then tugs are essentially disposable. But, if tugs are disposable, why did we bother to return the tug to LKO in the first place? It's a waste of fuel, and worse yet is means we carried unnecessary fuel (ie, dead weight) during the transfer burn itself.

The only way this makes sense (in KSP or in real life) is if there is an on-orbit refueling infrastructure waiting in LKO/LEO (and one that doesn't rely on constant launches from Kerbin/Earth). Remember: once you're in orbit, you're half way to anywhere. The idea behind a reusable tug is to minimize trips to the bottom of Kerbin's/Earth's gravity well, but most of your mass for any voyage is fuel, so unless you have a fuel source that's not Earth/Kerbin it's pointless.

The return will cost almost no fuel as you only has to get Ap back into Kerbin SOI, cost of return to LKO is trivial, and in dV you are almost in Minmus orbit who is more important as you would do mining.

An large reusable could also bring up fuel who are cost effective. In KSP this is an SSTO.

One downside of the plan is that its easy to optimize the transfer stage so it takes you from circulate into LKO to low Duna orbit from anything from 0.5 ton probes up to 50 ton bases while the SST would have to fuel up to have correct dV for the job.

Note it has been thought of using an reusable tug from getting satellites from LEO to GEO. Would make some sense if it used ion engines or some other sort of high isp ones, or could mine fuel in space, probably one of the most obvious marked for this.

[DaveofDefeat]

Or I could just build my craft with the neccesary equipment to make the interplanatary burn by itself

[Seret]

it's easier to launch a second tug than it is to refuel the first one.

Depends how much fuel you have available in LKO. If you've got a well-stocked refuelling depot you can fire off several interplanetary shots with a minimum of launches from Kerbin. Bring up an occasional heavy launcher with loads of fuel.

At this stage the cost of parts and fuel isn't an issue, but in the non-sandbox game it will be. Non-atmospheric tugs will be really useful, as the longer you keep an LV-N in space and the more times you refuel it the more it's higher efficiency pays off.

[magnemoe]

Depends how much fuel you have available in LKO. If you've got a well-stocked refuelling depot you can fire off several interplanetary shots with a minimum of launches from Kerbin. Bring up an occasional heavy launcher with loads of fuel.

At this stage the cost of parts and fuel isn't an issue, but in the non-sandbox game it will be. Non-atmospheric tugs will be really useful, as the longer you keep an LV-N in space and the more times you refuel it the more it's higher efficiency pays off.

Lets take the most cost effective version of this, Launch to Duna or Eve, you do an 950 m/s burn and let the engine on the cargo do the rest, it has to do adjustment and circulate after aerobrake anyway.

You do a finalize and adjustment burn for cargo, an adjustment at the SST Ap for aerobrake, circulate and meet depot in LKO.

So far the extra cost of the SST over an disposable transfer stage is perhaps 200 m/s so it makes sense.

However this require that you did not bring a lot of fuel with you back so you have to be very careful calculating fuel uses.

Now you do not want to use an four engine SST to launch an one ton probe while it will be to small for an 100 ton orbital launchpad facility.

You will also have to have multiple SST if you want to launch multiple cargoes during one transfer window unless you increase your dV cost a lot by doing an long braking burn.

Again mining changes this totally, as fuel becomes free.

[Seret]

You will also have to have multiple SST if you want to launch multiple cargoes during one transfer window unless you increase your dV cost a lot by doing an long braking burn.

Personally I always have at least two tugs (aka Space Tractors) in LKO anyway, so I'd be able to use hardware I've already got and that has paid for itself many times over. They only have two LV-Ns normally, but it'd be trivial to slap more on the rear docking port. I have used them for taking landers out to the Mun, not actually used one for proper interplanetary yet, but for reasonable sized payloads I don't see what would stop me. They'd be too big and heavy to make it worth it with little stuff.

[architeuthis]

I did something similar in my Integrated Program mission report:)

[Neici]

Interessting Report architeuthis!

Especially the real life mission remake.

Well but keep it in mind. If you launch a SST to orbit with enough fuel to fire 2 or 3 Payloads of 10 t for example without refueling, you would already compensate the fuel waste with less launcher fuel and part cost plus less Tug part cost. Then you could refuel it and do the same again.

In case of very low weight payloads, like little probes you could take a ionengine SST with enough batteries and enough engines to do the burn and then just recharge the batteries.