A reusable transfer stage: a good idea?

[Laie]

I have yet to leave Kerbin SOI.

While planning my first mission to Duna, I designed a transfer stage, then thought I could make it reusable. As I have no idea what further missions to (say) Jool might look like, I also made it modular.

I ended up with lots of radial docking ports where I could attach any amount of additional engines and tanks, when and if that ever becomes necessary. Even in it's most minimal state, the contraption has 8 nuclear engines and weighs about 30 tons, sans fuel.

Then I wondered if it's even worthwhile to bring that whole weight back to Kerbin.

I guess a lot of you have found the answer already: is it even a good idea to have a re-usable transfer stage? Or should I rather use dedicated going-there and getting-back stages, each tailored to it's particular mission?

Edited June 17, 2014 by Laie

[Dkmdlb]

Until we start paying for parts, dedicated transfer stages are better. When we start paying for parts, it's possible that a reusable transfer stage will be cheaper.

[Kasuha]

This was my trusty modular interplanetary ship, I visited all planets (except Eve) and moons with it. Each part was reusable and I could always decide if I want to keep it in orbit for next mission, crash it into the atmosphere, or (in case of the engine unit) land it safely on Kerbin's surface.

Regarding your space program, do whatever you consider the most fun. If you don't enjoy designing separate ship for each mission, go for reusable. If you enjoy designing ships or making optimal designs, keep designing tailored ships.

[WWEdeadman]

Yeah, in 0.24 careere mode you will probably benefit from reusing as much stuff as possible, but right now it's just unnecessary work, IMO.

[Cirocco]

re-usable transfer stages don't really give you a benefit right now because economics aren't implemented yet. They aren't really a bad idea though since they do save you the hassle of putting big stages up in orbit every time. You do have to refuel them, but that can be done with re-usable refuelers as well.

Modular transfer stages on the other hand I am not too sure of. Swapping out large tanks and heavy engines on docking ports means they won't have any struts to connect them to each other. That means low structural integrity and a TON of wobble when burning. You might not notice it on Duna transfer stages (because those are still pretty light) but trust me, you'll notice on the heavy duty ones when you want to push large cargo further out.

Personally I think it's best to (if you have the skill an patience for it) make one big really powerful transfer stage with a variety of docking ports and re-use that all the time. Sure it will be overpowered for the lower delta-V transfer burns, but you'll be able to use it for (almost) everything. And too much delta-V is always better than too little.

I personally haven't gone past Duna and Ike yet so I don't have a big interplanetary transfer stage (yet), I still custom build each one (or go full on single stage ). I plan to build one when I finally get around to flying my Eve lander/ascent vehicle over to that purple hellhole but until then: custom built one-time only transfer stages.

[cantab]

There's no significant fuel penalty to parking your return transfer stage, which may or may not be the same as your outbound stage, back in LKO on your return. It will however be more complex and more real-world time consuming, since you'll need to hit your aerocapture right and deal with rendezvous and docking to use it again. Of course whether you feel like spending that time is up to you.

Modular ships can be useful if you need to reconfigure things. For example I had an asteroid tug that had a single LV-N in its engine section, which was docked to the command (+ claw) section. I was later able to swap this for a much bigger engine section with 6 LV-N's, without needing to send a new clawing bit, and I put the original engine to use bringing the old crew back to Kerbin. Then again, I could have done the job just as well by sending a whole new tug.

[Skorpychan]

I just use space tugs. Missions and fuel tanks are docked to the back end of a fuel tank with nuclear engines on outriggers.

It works great, because it's good for the return mission too. And then it's parked in LKO for the next trip out. Saves launches!

[Pecan]

Another vote for 'yes' - mine looks very much like Kasuha's so it must be good :-) Personally I favour a reusable infrastructure of crew/fuel launches to LKO spacestation, interplanetary tugs/transfer vehicles and destination spacestations with dedicated landers (see link in signature). Lunar stations within a system probably aren't worth it but having one around each planet can be handy, if only as an emergency fuel reserve.

[Red Iron Crown]

Nukes are expensive, it will be costly to throw them away when costs are a thing.

[Laie]

This was my trusty modular interplanetary ship, [...]

You've got like, what, one LVT-N per 20tons? Is that fully adequate or merely sufficient?

I accounted for twice as much thrust per mass. Would that be way overpowered?

[Taki117]

You've got like, what, one LVT-N per 20tons? Is that fully adequate or merely sufficient?

I accounted for twice as much thrust per mass. Would that be way overpowered?

Depends on how long you want your burns to be. If you don't care about time less is more (at a lower TWR), if however you are impatient more is better.

[LethalDose]

I ended up with lots of radial docking ports where I could attach any amount of additional engines and tanks, when and if that ever becomes necessary.

I remember having a similar idea for multiple radial docking ports that could dock dedicated engine pods to increase thrust. The conceptual problem I was never able to solve (and therefore never even attempted it's construction) was how to line the engines lined up along the axis of the ship.

Specifically, let's say your ship will thrust along the x axis (the axis around which an aircraft would roll), and your radial docking ports are along the y axis (the axis around which an aircraft would pitch). Now, when you dock your engine pods to the radial ports, if the axis of the engine isn't perfectly in line with the x axis, you're going to get thrust oblique to direction of your intended travel, causing rotation in one or more axes. You need a way to lock the docking ports in a specific and precise rotation around that y axis, and I'm not sure if that can be done. At least without mods.

Or maybe this just isn't a problem in reality and just doing a really good job eyeballing the rotation on the docking is all you need. Anyone else run into this?

[Claw]

I remember having a similar idea for multiple radial docking ports that could dock dedicated engine pods to increase thrust. The conceptual problem I was never able to solve (and therefore never even attempted it's construction) was how to line the engines lined up along the axis of the ship.

It is difficult to get things to line up straight on a docking port. I'm mostly a stock player so I don't have any alignment mods either.

I've been experimenting with using mounted rails to help with alignment, although that is on pause at the moment. It's hard to build an alignment system that is tight, but also leaves enough space for coarse alignment during docking.

I haven't done much with multi docking, but that might also be a way to ensure good engine alignment.

[Red Iron Crown]

If the engines in the docked module have gimbals, it might be enough to compensate for a bit of misalignment.

[Kasuha]

You've got like, what, one LVT-N per 20tons? Is that fully adequate or merely sufficient?

I accounted for twice as much thrust per mass. Would that be way overpowered?

I'd call it sufficient. Some transfer burns were almost 20 minutes long. Could be improved with physics warp but it still needed corrections after or even during the burn since the maneuver indicator was no longer reliable near the end of the burn.

[LethalDose]

I've been experimenting with using mounted rails to help with alignment, although that is on pause at the moment. It's hard to build an alignment system that is tight, but also leaves enough space for coarse alignment during docking.

I haven't done much with multi docking, but that might also be a way to ensure good engine alignment.

If the engines in the docked module have gimbals, it might be enough to compensate for a bit of misalignment.

These are three good points and should be useful for the OP.

RE: multi-docking, I have gotten that to work for engine pods, but the entire ship I was using it for had to be designed around the attachment pylons. I used two station hubs separated by stacks of equal lengths, which was a little tricky. I haven't gone back to that design method, though, if that tells you anything.

You could also launch several identical engine pods with multiple radial ports and assemble them around another identical pod, so the multiple docking ports would all be aligned.

So yeah, all options. This is why you crowd source design ideas.

[Pecan]

...Anyone else run into this?

When I was working on the idea of docking engine modules for my long-duration tractor I simply adapted my standard tug (4 out-rigger nukes around a Mk2 lander can). Extra fuel on the outriggers and large docking ports for the engine, fuel and payload modules. Meant I could add any combination of engines depending on mission.

Edited June 22, 2014 by Pecan

[LethalDose]

That's similar to the solution I came up with, too. The "problem" for me is that I try to send up everything beneath a fairing if it's not nice and aerodynamic. I use KW's fairings (I remove all the other parts), and there ain't no way that would fit in even the largest one. However, this:

does work. at least for me.

I'm waiting until I've unlocked the 3.75 fairings in my career, though, before I start sending these up. I want to have a broader central tank so I can effectively strut the pylons. With just a girder or I-beams, I'm worried about getting too much bowing from the thrust from the engines's thrust.

I would have presented this earlier, but I haven't "road-tested" it yet. I was curious what others had come up with.

[asb3pe]

Until we start paying for parts, dedicated transfer stages are better. When we start paying for parts, it's possible that a reusable transfer stage will be cheaper.

Which is why this really needs to be first and foremost, in the next game update - an "economy" of some sort. I just started playing KSP but that's my opinion after about 2 weeks of play. I fully understand I purchased a "work in progress", not a fully fleshed-out game yet. I hope they keep improving the "game" part, the astrophysics is already very well done (harmless atmospheric friction and lack of re-entry worries notwithstanding).

[Claw]

...(4 out-rigger nukes around a Mk2 lander can).

I like this idea, modular engine/fuel pods. I could see using the large-medium multi adapters to mount 2, 3, or 4 engines to the sides. Anywhere from 2 to 16 total engines.

I would have presented this earlier, but I haven't "road-tested" it yet. I was curious what others had come up with.

LeathalDose, I think this has some real potential. If you were to launch two of these things (or more), you could dock the middle ports together into an X type configuration. It would probably end up looking similar to what Pecan posted. (I don't know if that's what you planned.)

With this, even if the docking is slightly misaligned in roll, it would still be symmetric. And the engines would be on straight.

[Pecan]

I like this idea, modular engine/fuel pods. I could see using the large-medium multi adapters to mount 2, 3, or 4 engines to the sides. Anywhere from 2 to 16 total engines.

LeathalDose, I think this has some real potential. If you were to launch two of these things (or more), you could dock the middle ports together into an X type configuration. It would probably end up looking similar to what Pecan posted. (I don't know if that's what you planned.)

With this, even if the docking is slightly misaligned in roll, it would still be symmetric. And the engines would be on straight.

Oh yes - someone even gave me an array of 7 LV-Ns in a large adapter ^^. Don't forget you can attach engines on the core as well, behind the payload - so every number of engines, from 1 to 35 (5x7) is possible without clipping (needs the arms a bit longer than pocket I-beams).

KAS/IR are handy with designs like this too, by the way. Mine has fuel-lines from the 'core' to the out-riggers but that makes it tricky to use with drop-tanks. You can put as many as you like in the centre, of course, but that makes for some very long payloads. Using 2-riggers with lines IN to the core and the other 2 OUT to the riggers means you can have 2 engine stacks and 2 fuel stacks, without disturbing the payload.

IR is great for unfolding things from inside fairings, of course.

ETA: My fuel and engine modules tend to be 'smart' too - here is the last of a set of twin-nukes docking with the tractor pictured above:

Edited June 17, 2014 by Pecan

[Red Iron Crown]

Oh yes - someone even gave me an array of 7 LV-Ns in a large adapter ^^.

Be careful, I have it on fairly good authority that the array in question has a nasty exhaust leak and a tendency to explode. Besides, that someone is totally stealing being inspired by your modular ship core.

I wonder if such a design is going to work out economically though. It might be cheaper to launch it in a less modular configuration, to save on probe cores, docking ports and RCS systems. I've been working on a Jool-5 entry that has a single stage, reusable mothership that carries its payload near the center of mass, but I'm not sure whether it will be cheaper to save some fuel by dropping tanks and docking new ones upon return to Kerbin. After all, the new tanks will have to be launched anyway to get the propellant up there in the first place...

Here's the mothership with the Jool-5 mission package. The main docking port is just aft of the 2.5m RCS tank and battery.

[LethalDose]

LeathalDose, I think this has some real potential. If you were to launch two of these things (or more), you could dock the middle ports together into an X type configuration. It would probably end up looking similar to what Pecan posted. (I don't know if that's what you planned.)

With this, even if the docking is slightly misaligned in roll, it would still be symmetric. And the engines would be on straight.

Thanks. I'm looking forward the actually getting a final design up into space. A lot of modular components are about to get tested on an E asteroid capture and a planned Eve mission. If those are successful, then we'll move onto using a refined version "cross spar" section to mount larger engine pods. Overall I think that kind of design is better than using radial ports.

And yeah, you could cross-dock the sections to increase the placement for pods. Overall, though, I'm not sure if I'll need to use that configuration since my missions tend to be substantially leaner than a lot of designs I see here, so I try to keep mass down and launch smaller rockets.

[Red Iron Crown]

Thinking about the modular designs that Pecan and LethalDose have shared, it might be a good idea to have a "parking" station for storing modules awaiting retasking in LKO. Maybe a simplified (i.e. cheaper) version of the ship core.

[cantab]

Be careful with parking lots on larger stations though. I have one on mine, and several modules hanging off it, all heavy and with a fair bit of torque, the SAS can try to destroy the station.