Thrust Loss on Angled Engine for Asteroid Puller

[Aegolius13]

Hi all,

I've got a bit of an odd question regarding engine thrust.  I'm designing an asteroid "puller" to bring in a Class E.  I have my nuclear engines out on 4 radially-symmetrical nacelles, which were angled at about 45 degrees to try to clear the asteroid for unimpeded thrust. 

On my first attempt, the angle was not sufficient, and I had some of the engines (but not all) obstructed by the asteroid, leading to more asteroid-spinning and less asteroid-moving.  So to address this, I tried to increase the angle of the engines (i.e., make them closer to perpendicular to the main axis of the ship).  

I tested out the new design on the launchpad, with gravity hacked and with just the core ship, not the orbital-launch rig.  Results were not good- I was barely getting any thrust!

In other words, it seems there's a thrust penalty for having your engines angled.  I've looked on the forums for info on this and not seen much, but one person suggested the end-thrust is a function of the cosine of the engine angle.

My question, I guess, is why.  My (probably wrong) understanding was that, if you have two symmetrically balanced engines, as long as there was SOME angle in one direction, they would put full thrust forward regardless of the angle.  Those engines' energy has got to go somewhere, and I presume it's not being lost to deformation or heating of the ship in between.  If not, the only place the energy could go was forward.  The analogy that comes to mind is: squeeze a wet bar of soap between your hands, it will shoot out straight up or in some direction.  Not because you pushed it in that specific, but because it's got nowhere else to go when you apply a squeezing force.  

Can anyone explain what's causing the observed thrust loss?  Is there just extra heat generation?  Is the specific impulse of the engine affected somehow?  Is it just something else?

(By the way, if major engine angling is not the answer, I will probably fix my rig by either extending the "tail" so the ship is further in front of the asteroid, or putting the engines out on longer pylons).  

Thanks for any info you might have!

 

 

 

Edited June 19, 2016 by Aegolius13
Improved soap bar analogy.

[Snark]

33 minutes ago, Aegolius13 said:

I've looked on the forums for info on this and not seen much, but one person suggested the end-thrust is a function of the cosine of the engine angle.

That is correct.  The phenomenon is referred to as "cosine loss" (try searching for that phrase on the forums, you'll find a lot of talk about it). An engine that's thrusting perfectly prograde contributes 100% of its thrust in the prograde direction (since the cosine of zero is 1).  If the engine is pointed any other direction than prograde, then the component of its thrust in the prograde direction is the full thrust times the cosine of the angle.

33 minutes ago, Aegolius13 said:

My question, I guess, is why.

It's basic trigonometry.  The component of a force along an axis is proportional to the cosine of the angle it makes with respect to the axis.

It works with any force, not just rockets.  Ropes, for example.  If you've got a 100-pound weight, and you attach a rope to it, and pull straight up on the rope... what tension do you have to put on the rope to lift the weight off the ground?  100 pounds.  But suppose you have a *horizontal* rope held taut (clothesline-style), with you at one end and a friend at the other, and the weight is hanging from the middle, so that there's only a few degrees of "sag" from the weight.  How hard do you both have to be pulling now in order to lift the weight off the ground?  A whole lot more than 100 pounds!  That's because the vertical component of the rope's force goes with the cosine, which will be small if the rope is mostly horizontal with only a little bit of "sag".

Or, another way of experiencing this:  Try hanging from your hands, with your arms straight overhead, for thirty seconds.  Not so hard, right?  Now try to do the same thing but you're supporting your weight by holding your arms straight out to your sides.  That, my friend, is cosine loss. 

 

33 minutes ago, Aegolius13 said:

My (probably wrong) understanding was that, if you have two symmetrically balanced engines, as long as there was SOME angle in one direction, they would put full thrust forward regardless of the angle.

You are correct, that is wrong understanding.  They only put a slight amount of thrust forward, since most of their thrust is directed sideways (fighting each other) rather than forward.

 

33 minutes ago, Aegolius13 said:

Those engines' energy has got to go somewhere, and I presume it's not being lost to deformation or heating of the ship in between.

Be careful not to conflate "energy" with "force".

A rocket expends its energy the same place no matter what its situation:  out the exhaust nozzle.

Imagine that you have two rockets that are pointing perfectly equal and opposite each other, so they cancel out.  Where does their "energy" go now, hmm?  Simple:  Out the exhaust nozzle.  Their force completely cancels out, which is why they don't go anywhere.

And if you nudge them slightly, so they're not quite perfectly opposed... then there will be a slight drift to one side because they don't perfectly cancel each other out.  In other words, the cosine is non-zero.

The main source of confusion is that you've focused on "energy" as the concept here-- it's not super relevant.  Rockets are poor choices for talking about conservation of energy, because they expend such huge quantities of it and it's very much not a closed system (that's the whole point!) and the vast majority of the energy goes out with the exhaust rather than moving the rocket itself.

Focus on the force instead, and you'll have a much better picture of what's going on.

33 minutes ago, Aegolius13 said:

The analogy that comes to mind is: squeeze a wet bar of soap between your hands, it will shoot out straight up or in some direction.  Not because you pushed it in that specific, but because it's got nowhere else to go when you apply a squeezing force.

Yes, and the force that the soap moves upwards with is much less than the force you're pushing your hands together.  Again, because of cosine loss.  You could squeeze your hands together really hard to try to make the soap squirt upwards... but I could put one little finger on the soap pressing it down, and you wouldn't be able to budge it.  Not because I'm a whole bunch stronger than you, but because you're fighting cosine loss and I'm not.  I've got a better mechanical advantage.

 

[Aegolius13]

Ah, think it's starting to make sense.  I think the part I was missing when the engines are highly inclined, the engines are spending most of their effort just pushing two streams of exhaust in mostly-opposite directions... which of course would not do a lot to move the ship.  Like how a boat with propellers at both ends would push water out in both directions, but would not move.

Back to the drawing board on my asteroid puller, I guess.  Thanks much!

[bewing]

I suspect that one of the major problems that you are seeing with your test rig is that NERV engines barely work at all in the atmosphere. So the fact that you are hardly seeing any thrust at all isn't the tiniest bit surprising. I think you need to do your test in space. Just hacking gravity is not enough.

[Aegolius13]

I thought of that, but I did compare my original and revised versions with the same test launchpad set-up, so the only variable was the engine angle.  They were both pretty slow, but the version with the more-angled engines was much slower -- could barely out-thrust even the hacked gravity.  

The second version was like one angle-snap click away from perpendicular, so given the cosine function for engine loss, I'm now not surprised I got next to no thrust.   

[Jarin]

If you don't care about cheesing the physics a bit, I'd suggest just put a longer fuselage behind your engine arms, so there's more space between the exhaust and the asteroid. The "thrust obscured" problem doesn't actually check backwards very far, so a longer ship with puller engines can work just fine, even without any angle.

[Aegolius13]

That's my tentative plan going forward... stick the Klaw at the end of a couple orange tanks, a long string of Mk 1 liquid fuel fuselages, and/or some girders.  Hopefully the extra rear clearance will allow me reduce ore eliminate the engine angle from the original, which should provide extra thrust to make up for the additional tail weight.  

And of course, even if a little of the thrust is blocked by the asteroid, fuel should still be plentiful as I'll be mining the asteroid while pulling it.  So all I really care about is being able to pull relatively straight.  

 

 

[ajburges]

5 hours ago, Jarin said:

If you don't care about cheesing the physics a bit, I'd suggest just put a longer fuselage behind your engine arms, so there's more space between the exhaust and the asteroid. The "thrust obscured" problem doesn't actually check backwards very far, so a longer ship with puller engines can work just fine, even without any angle.

If you desire less cheese, remember sine functions are non linear. 45° gives you 70% thrust efficiency. An angle of 15° looks plenty realistic on a decently long tug and yields 96.6% thrust efficiency! Push that to 10° and you have 98.5% thrust efficiency.

[Snark]

I've built an asteroid puller that has a claw mounted on the back, and a trio of long, curved arms sticking out (made of girders) that roughly define a hemispherical space big enough to contain a class-C asteroid.  Then engines mounted at the ends of the arms.

It works great, and looks cool... but dang that thing's a bear to launch in the first place. 

[Jarin]

1 hour ago, Snark said:

I've built an asteroid puller that has a claw mounted on the back, and a trio of long, curved arms sticking out (made of girders) that roughly define a hemispherical space big enough to contain a class-C asteroid.  Then engines mounted at the ends of the arms.

It works great, and looks cool... but dang that thing's a bear to launch in the first place. 

I want screenshots