A question about engines and efficiency

[Macko939]

So far I've figured out that the ISP is what matters the most in the interplanetary space.

Soo, lets say I'd launch a big payload to the orbit with a standard rocket and then dock it to a tugboat with them engines with 390 ISP and transfer all the fuel to it, would it be much more efficient compared to a standard launch with big engines?

Because docking takes some time and fuel as well, so I am not sure if it is actually worth it

[MarvinKitFox]

It... could be better.

Depending on so many factors...

How heavy is your Tug?

How heavy is your primary spaceship?

How heavy would it have been, if you had built those engines into it?

Where are you going? A tug to Mun is a waste of time. A Tug to MoHo or Eeloo is a brilliant plan.

In general, you will want to build your launch vehicle to have just enough fuel for its "big launch" engines to do just that. Launch. On orbit, or very shortly before orbit, you want to dump your inefficient but strong launch engines and change over to using smaller + weaker but more fuel-efficient engines.

Whether these better-ISP engines are built in to your rocket's upper stages, or whether you use engines on a tug is up to you, and is virtually an matter of Aesthetics rather than efficiency.

P.S.

If you *are* going to use a dedicated tug, at least run it using Nuclear engines. The difference in fuel efficiency between ISP 390 and ISP 800 is ***enormous***

For that matter, with their recent buffing, you might even consider running your tug on ION engines, at ISP 4200 they are unbeatable and at thrust 2 they are quite usable, if you don't mind coffee breaks during maneuvers.

[Taki117]

If you are going interplanetary you want the nukes, 800 ISP. For interplanetary travel, you are worried more about efficency more than TWR, with most interplaetary vessels you are going to have to build it in sapce if you want to carry enough fuel to get anywhere.

[technion]

I *was* primarily using nuclear engines with the "obvious" fact that the ISP is better. But sometimes it just isn't.

My moho unit, which had five orange tanks (one center, 2 asparagus stages) to deal with its transport needs (because moho is a ... something) was of course fitted with LV-N's. Then the ejection manoeuvre was calculated at.... something over half an hour. It's not just about the time you spend idling waiting for it. It's about the fact you would start that burn when you're on the wrong side of Kerbin and spend half of it firing in the wrong direction.

I ended up fitting 2 x T30's on each tank on the first asparagus stage, which then emptied themselves near the end of the burn and I dropped them. The remainders all ran LV-N's and didn't mind the slow burn to get captured when they could, so much more accurately, get themselves on an intercept.

[Streetwind]

Yeah, thrust/weight ratio is important. You can get by with just one or two LV-Ns on large interplanetary vessels, but you often can't do the ejection burn in one orbit. It all depends on how comfortable you are with long burns (same issue that always comes up when discussing ion engines).

But, maybe the OP isn't concerned about TWR, but simply doesn't have the tech for nuclear rockets yet.

In general, if you launch your interplanetary spacecraft in one go or if you assemble it in orbit doesn't make that much difference. Ultimately if you do the former, you'll have to dock with it to refuel it, because it will expend some of its fuel to get up there; and in the latter case you'll have to dock with it in order to attach all the parts. So both approaches are comparable. Do what you feel more comfortable with!

[Dispatcher]

The above posts are informative. As indicated above, a lot of it is simply based on your design and implementation strategies and preferences. In addition, you might want to consider fine tuning your options by considering such things as an engine's thrust vector range, electric output, whether or not it is bottom-stackable, etc. Show us some pics and let us see what you are up to!

[Nemecle]

Some people made a post on which engine choose, unfortunately, it's in french

http://www.kerbalspaceprogram.fr/forum/viewtopic.php?f=29&t=2675

If you are interested, I could could ask the author if we can translate it

[Taki117]

anote on long ejection burns is to split them up. instead of performing one long 30 min burn, splitit into 3 10 min burns.

[maccollo]

To answer the question I think you are asking:

The difference between launching the tug/transfer stage and payload separately and all in one go is essentially just time.

It takes longer to fly a mission with multiple launches than one with a single launch. Actually, launching bigger payloads is more efficient because it allows you to utilize fuel lines aswell as the new ludicrously efficient size 3 engines at the same time.

[Dust]

In 0.23 I'd say absolutely, for something like Moho and most other planets you should do a multi-launch build in LKO. My usual interplanetary ship involved several 2.5m sections connected by 2.5m docking ports. I'd have a nuke tug module, crew habitation/command module, lander/probe module, and drop tanks at the rear (the last of which having a KW rocket to assist in the ejection burn).

I have yet to do an interplanetary mission in my new 0.23.5 career, but I suspect you can now much more easily get to most planets with a single launch.

[RockoDyne]

The main reason most people end up making a multi-launch craft is because it's too heavy to launch in one go. The actual stats of such a craft don't really change. Unless you are putting docking ports on when there originally weren't any, or adding more RCS tanks and thrusters (basically anything to change the weight), then it's the same craft, just put together differently.

[Streetwind]

Some people made a post on which engine choose, unfortunately, it's in french

http://www.kerbalspaceprogram.fr/forum/viewtopic.php?f=29&t=2675

If you are interested, I could could ask the author if we can translate it

Actually, the google translation is surprisingly readable - still amusingly missing the mark in some places, but I personally have no issues understanding the actual points being made. I've seen posts written in native English that were harder to understand...

[Rokker]

Historically the general consensus is big strong and usually inefficient engines for launch, more efficient lower thrust engines after orbit, with all stages in between usually having ISPs somewhere in between the two.

[Jouni]

For that matter, with their recent buffing, you might even consider running your tug on ION engines, at ISP 4200 they are unbeatable and at thrust 2 they are quite usable, if you don't mind coffee breaks during maneuvers.

I've used the new ion engines in some medium-sized interplanetary ships. When I did my ion-powered Moho mission, I used 12 ion engines in a 41-tonne ship. To get the same amount of delta-v with a nuclear engine, it would have been necessary to increase ship mass to 55 tonnes. Essentially, the ship with ion engines had 75% of the mass and 54% of the TWR of a nuclear ship.

In practice, the ship with a nuclear engine would require less delta-v, as higher TWR allows it to use more efficient maneuvers.

[Streetwind]

Historically the general consensus is big strong and usually inefficient engines for launch, more efficient lower thrust engines after orbit, with all stages in between usually having ISPs somewhere in between the two.

Yeah, just remember the reason why this consensus exists: it's not because people didn't want the inefficient engines to feel left out. It's because efficient engines just play suck at developing actual thrust, and gravity isn't a guy you can just ask nicely about letting you go. If you want real power, it's going to be inefficient. And that's unfortunately the only thing that's able to lift anything useful off the pad.

If it wasn't for that, we'd be slapping things like the space shuttle engines (the single most efficient chemical engine to ever see actual service) on everything we have and call it a day. But no, the shuttle had super inefficient solid boosters for a reason

[Rokker]

Yeah, just remember the reason why this consensus exists: it's not because people didn't want the inefficient engines to feel left out. It's because efficient engines just play suck at developing actual thrust, and gravity isn't a guy you can just ask nicely about letting you go. If you want real power, it's going to be inefficient. And that's unfortunately the only thing that's able to lift anything useful off the pad.

If it wasn't for that, we'd be slapping things like the space shuttle engines (the single most efficient chemical engine to ever see actual service) on everything we have and call it a day. But no, the shuttle had super inefficient solid boosters for a reason

Yup, pretty much... well that and weight. There are certain points at which weight of the rocket is more important than efficiency, because the engine would increase the final mass too much thereby significantly lowering the delta-V.

[Jouni]

Yeah, just remember the reason why this consensus exists: it's not because people didn't want the inefficient engines to feel left out. It's because efficient engines just play suck at developing actual thrust, and gravity isn't a guy you can just ask nicely about letting you go. If you want real power, it's going to be inefficient. And that's unfortunately the only thing that's able to lift anything useful off the pad.

It's more about the fact that kerosene is cheap and easy to handle, while liquid hydrogen isn't. Still, the RS-68 engine used in Delta IV burns hydrogen, and there are not that many liquid fuel engines of any kind that produce more thrust. The F-1 engine of course, and some Russian engines, but that's probably all.

The reason why SpaceX is so successful is that they care more about cost-effectiveness than efficiency.

Edited April 19, 2014 by Jouni

[Rokker]

It's more about the fact that kerosene is cheap and easy to handle, while liquid hydrogen isn't. Still, the RS-68 engine used in Delta IV burns hydrogen, and there are not that many liquid fuel engines of any kind that produce more thrust. The F-1 engine of course, and some Russian engines, but that's probably all.

The reason why SpaceX is so successful is that they care more about cost-effectiveness than efficiency.

Well the higher the efficiency, the lower the mass ratio which means it takes less mass per mass to produce X amount of delta-V. Now lets look at the Russian N-1 and the American Saturn V (which I'm using because the massive amounts of fuel really help show the difference more noticably.) We can see that while they have very similar weights with the N-1 at 2,350,000 kg and the Saturn V at 2,800,000 kg, that the Saturn V's much more efficient LH2/LOX engines helped essentially double the amount of mass it could send to the Moon compared to the N-1's RP-1/LOX engines. The Trans-Lunar Injection masses were 45,000 kg for the Saturn V compared to the N-1's 23,500 kg.

Now honestly, I love SpaceX and I hope to work for them when I graduate, but they have yet to become truly successful (but that's due more to certain politics and business things.) But the use of RP-1 instead of LH2 only plays a relatively small role in the low prices they predict they will be able to achieve and is more because they plan to avoid burning up their rockets every launch. I dont currently have anything I can cite to say that the fuel plays only a minor role, but there is the fact that the Skylon, which uses LH2 is also expected to be extremely cheap due to it being a reusable vehicle, not fuel.

[Jouni]

(Ignore this, I was in hurry and I didn't have time to think.)

Edited April 20, 2014 by Jouni