using only the more efficient engine while carrying others

[cephalo]

I have a question. I'm making a small spaceplane with 2 rapiers and a dart. The idea is that I would use all 3 getting to orbit, but just the dart once in space. Do I gain anything by this? The dart is more efficient, but then I'm lugging around 2 dead engines. Would it be better to just continue to use all 3, or perhaps just get rid of the dart?

I have another spaceplane that uses a nuke as the third engine, and in that case I do gain a lot from carrying the two dead engines, as I only need oxidizer on the ascent. Because of the nuke efficiency, I can go all over the Kerbin system for tourist contracts. This plane is a bit sluggish though.

This new plane is meant for simply going up to a space station in LKO. Rather than max efficiency, I'm looking for a fast, comfortable ride. I love spaceplanes that leap into orbit.

[Streetwind]

Using only the Dart is definitely better than using the Rapiers and the Dart in combination. If you did the latter, then the combined Isp of the three engines would follow the formula: Sum(Thrusts) / Sum(Thrusts/Isp's). So: (180 + 180 + 180) / ((180/305) + (180/305) + (180/340)) = 315.84. Compared to using only the Dart, you are losing 24.16 seconds.

If you were to get rid of the Dart, you would save 1 ton of mass, but your Isp would drop further to 305s, 35 less than the Dart. Whether or not that results in better or worse dV depends on your craft, and you need to solve the rocket equation for each case to know for sure. (But if I were to go out on a limb, your dV would drop if your craft is large enough to seriously require two rapiers, as that makes the saved mass very small in comparison to the total dry mass of the plane.)

Edited April 21, 2017 by Streetwind

[GoSlash27]

Cephalo,

 You'd be better off without the Dart. Ditching it saves you a tonne and costs you 10% of your fuel efficiency on orbit, which is a very small DV budget.  The Dart is actually adding weight to your ship overall, which means more fuel, more wing, etc.

 Moreover, using 3 engines instead of 2 means you need an additional parallel node to mount it, which means more drag.

Best,
-Slashy

[Spricigo]

4 hours ago, cephalo said:

The dart is more efficient, but then I'm lugging around 2 dead engines. 

Notice you will carry the RAPIERs around either way.  The argument that a engine turned off is a waste is only valid when not bringing it is an option. 

 

 

[GoSlash27]

25 minutes ago, Spricigo said:

Notice you will carry the RAPIERs around either way.  The argument that a engine turned off is a waste is only valid when not bringing it is an option. 

 

 

^ Agreed. The real question is whether turning an engine *on* is a waste or not. Or more importantly, whether it was a waste to bring it in the first place.

Best,
-Slashy

[Red Iron Crown]

6 hours ago, cephalo said:

This new plane is meant for simply going up to a space station in LKO. Rather than max efficiency, I'm looking for a fast, comfortable ride. I love spaceplanes that leap into orbit.

For this role it is not worth carrying a separate engine for orbital work. You're just not using enough propellant in space to make up for the extra engine mass. 

Specialized vacuum engines only start being worthwhile once you're headed to the Mun or further. 

[Wanderfound]

7 hours ago, cephalo said:

I have a question. I'm making a small spaceplane with 2 rapiers and a dart. The idea is that I would use all 3 getting to orbit, but just the dart once in space. Do I gain anything by this? The dart is more efficient, but then I'm lugging around 2 dead engines. Would it be better to just continue to use all 3, or perhaps just get rid of the dart?

I have another spaceplane that uses a nuke as the third engine, and in that case I do gain a lot from carrying the two dead engines, as I only need oxidizer on the ascent. Because of the nuke efficiency, I can go all over the Kerbin system for tourist contracts. This plane is a bit sluggish though.

This new plane is meant for simply going up to a space station in LKO. Rather than max efficiency, I'm looking for a fast, comfortable ride. I love spaceplanes that leap into orbit.

How much are you planning to lift?

If it's just passengers, a single RAPIER is adequate and two should make a very quick hotrod. For cargo, you'd be after a minimum of roughly one RAPIER per ten tons of payload.

For LKO, stick to pure RAPIER power. Add nukes if you're going interplanetary; darts only really become useful on Eve ascents or if you've got some other reason to want to avoid nukes.

[Wanderfound]

Just to demonstrate that fast ships don't require many engines:

 

[cephalo]

So, I tried two variants of this craft, one with a dart and one with an advanced nose cone on the center rear node. The fuel efficiency at every stage of the flight appeared exactly the same, with the small extra weight exactly countering the small extra efficiency. However, the dart does make the ascent into space quicker and more enjoyable once you switch modes on the rapiers and fire up the dart also.

Edited April 24, 2017 by cephalo

[Wanderfound]

15 hours ago, cephalo said:

However, the dart does make the ascent into space quicker and more enjoyable once you switch modes on the rapiers and fire up the dart also.

If fast is as important as efficiency: tried a three-RAPIER version?

[ajburges]

On 4/24/2017 at 7:28 AM, cephalo said:

So, I tried two variants of this craft, one with a dart and one with an advanced nose cone on the center rear node. The fuel efficiency at every stage of the flight appeared exactly the same, with the small extra weight exactly countering the small extra efficiency. However, the dart does make the ascent into space quicker and more enjoyable once you switch modes on the rapiers and fire up the dart also.

While that is the quick comparison, it's not a very fair one.

Three engine nodes on a space plane is a special number because it commonly requires additional fuselage stacks. This creates a huge drag cost. The performance between a three engine plane without an engine and a two engine plane is significant. It can allow for higher aerobic velocity (and/or more efficient speed run) or allow you to drop some jet engines and thus mass and additional nodes!

While more thrust on accent is fun, the fuel efficency of a shorter burn is outweighed by the losses in mass efficency. For Kerbin accent, the RAPIER anerobic thrust is much higher than needed!

On RAPIER power, dV to minimum stable orbit is around 800-1200 m/s and mostly dependent on final aerobic velocity. Drag, thrust, and direction are  minor complicating factors. RAPIERs need to burn up to 49% of the mass you need in orbit to produce that. Your engine cluster has a combined ISP of 316 so it needs up to 47% of it's mass in orbit in immediate fuel (340 Isp needs 43% as a lower bound for a more complicated staging). For low orbital dV budgets, that 2-6% of mass in orbit is quickly nullified by the extra tonne of engine you take with you and the savings in OMS fuel for low budget orbiters is similarly small.

[cephalo]

Speaking of two engine nodes, I have found this problematic for a couple of reasons. The first reason is that the Mk2 adapter that splits in two seems to create a huge amount of drag according the aerodynamic forces overlay, probably a lot more than it should. The second reason is that without that, it's not that easy to have an even amount of engine nodes. The cockpit requires one fuselage stack, then what do you do? If you don't put an engine on that stack, then it's still an extra stack. Maybe you could put the two stacks vertically to keep horizontal symmetry? I haven't tried that.

[Spricigo]

1 hour ago, cephalo said:

The first reason is that the Mk2 adapter that splits in two seems to create a huge amount of drag according the aerodynamic forces overlay, probably a lot more than it should.

 

An image helps. A disproportional drag may be caused by an mismatched joint, however MK2 parts are know to be considerably draggy for their  performance.

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The second reason is that without that, it's not that easy to have an even amount of engine nodes.The cockpit requires one fuselage stack, then what do you do? If you don't put an engine on that stack, then it's still an extra stack

You may just 'close' the stack with a nosecone. Or even design a twin fuselage plane.

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Maybe you could put the two stacks vertically to keep horizontal symmetry? I haven't tried that.

Yes, however vertical symmetry can be a concern.