Reverse thrust!

[A35K]

Airliners need to be able to take off (I think) at least with 1 of their engines inoperative. This means that twinjets actually have a higher TWR than quads, as a twin jet technically has twice the power it actually needs, (since it has to be able to takeoff on one engine), while a quad only has 1 and a third times the power it needs.

Anyway, in real life thrust reversers are nowhere near as powerful as in KSP, as in most large aircraft they wouldn't even be able to move it backwards when fully loaded. I remember building an An-225 copy in KSP 0.23, using B9 parts, including engines that had reverser, but they were so OP, I would land at say 80m/s and in a second I'd have stopped, and if I didn't turn them off I'd go backwards at takeoff speed!

Edited November 12, 2015 by A35K

[silversliver]

You can make quite old looking planes with the new wheesley.

http://i63.tinypic.com/21awfhw.jpg

http://i68.tinypic.com/300bixg.jpg

[AlexisBV]

Bizjets usually have a TWR around 0.3-0.35 (at max takeoff weight, sea level at 15 degrees C)

As for airliners, yes, quads have lower TWR than twins. That's because they need to be able to climb out with one engine inoperative. With all engines running, they have 4/3= 33% extra thrust. A twin, with all engines running has 2/1 = 100% extra thrust. That's why big whales like the A340 seem to take forever to climb, compared to say an A330.

[RizzoTheRat]

3 minutes ago, AlexisBV said:

That's why big whales like the A340 seem to take forever to climb, compared to say an A330

I was at Farnborough airshow the year a development Typhoon first flew, fitted with RB199's which are about half the thrust of it's intended EJ200 engines.  As an underpowered development aircraft it did a pretty sedate demo, and the next plane up was an A319 (I think) which did a steeper climbout and generally far more impressive display.

 

On ‎12‎/‎11‎/‎2015 at 5:39 PM, A35K said:

most large aircraft they wouldn't even be able to move it backwards when fully loaded

I've seen a C-17 do a 3 point with the loadie hanging out of the side door to direct :D 

[genbrien]

On 11/11/2015 at 5:54 PM, AeroGav said:

Don't think real thrust reversers work like this btw.

3. Only part of the exhaust stream is diverted

4. The diverted exhaust goes in a mostly outward direction with only a small forwards component.

The amount of retrograde thrust is barely able to reverse the jet out of it's parking bay, so why does it give such powerful retardation?

The answer is that this stream of exhaust coming out perpendicular to the direction of motion disrupts the normally smooth airflow over the airframe and creates huge turbulence. The aircraft experiences drag as if it were shaped like a barn door, and it also destroys residual lift making wheel brakes more effective.

Below 80 knots, this "virtual airbrake" effect dies down and the risk of re-ingesting exhaust gas is increasing , so they are usually stowed at this point.

Nothing of that is true for Cascade reversers

[sal_vager]

Yep, reversers give 100% thrust in KSP, this was done for gameplay and makes vtols easier.

Example:

 

[Dispatcher]

Sal, the reverse thrust was not 100% of the forward thrust in tests I performed.  This may be due to design constraints.  See the appropriate link in my sig line.

Edited March 10, 2016 by Dispatcher

[Starwaster]

33 minutes ago, Dispatcher said:

Sal, the reverse thrust was not 100% of the forward thrust in tests I performed.  This may be due to design constraints.  See the appropriate link in my sig line.

The  context menu is an accurate indicator of thrust and it says that engines in reverse put out the exact same thrust as when forwards. There's no change.

So what are you basing your statement on? The only link in your signature that seems relevant...  I'm not sure what to make of it; what there is a direct comparison of thrust?

[Dispatcher]

57 minutes ago, Starwaster said:

The  context menu is an accurate indicator of thrust and it says that engines in reverse put out the exact same thrust as when forwards. There's no change.

So what are you basing your statement on? The only link in your signature that seems relevant...  I'm not sure what to make of it; what there is a direct comparison of thrust?

Hi Starwaster, I had done flight tests of KSP engines, including jet engines; also reverse thrust tests of those with that capability.  But I can see that those tests might be skewed when a similar TWR is sought when launching vertically.  Its also true that when testing the Goliath as radials (for example) that some design differences occurred in order to stabilize it prior to launch, as opposed to the testing of reverse thrust after turning the engines around.  I also note that in order to simulate a similar starting TWR for these jets, I sometimes varied the starting mass, and thus the altitude achieved.

I think its likely that my design constraints allowed differences between otherwise identically engined craft, which would account for my results showing differences in engine mode performance.

I'm satisfied that you and Sal are technically correct in saying that the thrust between modes is the same.  I'm also saying that actual results may vary depending on design constraints of the craft involved.  For my reverse thrust engine tests, I actually turned the engine around and mounted it to the craft backwards, prior to setting it to reverse thrust.  In craft that would normally be built and flown in KSP, the engines would not be turned and moved, only the mode would change.

Looking at my test results, the starting mass figures (when those were different) were relatively small and the altitude differences were similarly small.  I can attribute that to design considerations.  Sal-vager has gotten me interested in VTOLS now.  

[Starwaster]

1 hour ago, Dispatcher said:

 

Looking at my test results, the starting mass figures (when those were different) were relatively small and the altitude differences were similarly small.  I can attribute that to design considerations.  Sal-vager has gotten me interested in VTOLS now.  

You just want that cool flying car

[Dispatcher]

Yes, its 2016; where is that flying car?

I retested the Wheezley in both forward and reverse modes.  These raise a few questions.  You will note in the images that the craft are identical, with the exception of the placement of the shock cone and engine, which I transposed.  I used the pre cooler as the jet fuel tank (which is fair when comparison testing against rocket engines due to the increase in tank mass on the one hand and the fact that the oxidizer is supplied by a virtual column of air, to a point, surrounding the craft on the other hand).  You can see the toggle setting of the engine as well as just the look of the engine for each mode.

I flew each craft using identical settings (full throttle at launch, SAS on) and did no manual flight input.  The starting TWR does not display as 1.2 as with rocket engines I tested, however the pre cooler tank in each case is full of fuel.  The TWR shown may be attributable to the way the game calculates TWR for jets or to the fact that the oxidizer is supplied externally, or both.  My test goals were to maximize the altitude attained and you will see that the craft starting mass is identical for each test.

As you can see, the ships didn't achieve the same altitude.  I suspect that besides possible drag and shock intake variances, the engine outputs do differ slightly.  I say this because when I tested in reverse mode, there were particle effects not only moving retrograde (how we'd want them to move, as these show a downward thrust), but particle effects moving ahead of the ship toward the prograde; indicating that some of the thrust would leak "forward" and thus help slow the ship a little.  So while the engine may operate at the same thrust in each mode, it seems that some of that thrust remains in the "forward" direction of the engine when in reverse mode.  The only improvement to these tests that I can think of is to remove the shock cone and replace it with a simple nose cone.  However, I don't know how significant that change would be.  Check out the images.

Wheezley (default forward thrust):

The crash was so hard that the camera flew through the planet, apparently.  Next, reverse mode:

Since the ship has peaked, the prograde particle effects have shifted to the side.  Altitude results confirmed next:

As my results were different between modes, I wonder what factors actually contributed to that?  My only guesses are differences in "spinning up" time, drag, oxygen intake and possible thrust leakage forward.

Edited March 10, 2016 by Dispatcher

[Kobymaru]

26 minutes ago, Dispatcher said:

Yes, its 2016; where is that flying car?

[EnderKid2]

If you look at the thrust vectors in rcs build aid, there are actually 2, and they rotate from parallel to 45 degrees from forward.  ---- >     to     \

                                              /

[Val]

47 minutes ago, Dispatcher said:

I retested the Wheezley in both forward and reverse modes.

You're not really testing thrust using that method. Mostly the difference is down to aerodynamics.

If you want to do that test without variation in aerodynamics, have a Wheesley at both ends, with the top one in reverse. Then do the test, but only activate one engine at a time.

Another way to test could be to make the vehicle have 0.99 TWR. Run a test once with each engine and record at what mass it lifts itself off the pad.

Edited March 10, 2016 by Val
Typing is hard

[Dispatcher]

3 hours ago, Val said:

You're not really testing thrust using that method. Mostly the difference is down to aerodynamics.

If you want to do that test without variation in aerodynamics, have a Wheesley at both ends, with the top one in reverse. Then do the test, but only activate one engine at a time.

Another way to test could be to make the vehicle have 0.99 TWR. Run a test once with each engine and record at what mass it lifts itself off the pad.

What you say is quite interesting.  I may well do something like that.  However, my tests so far have indeed been about not only performance but also of practical design.  See my rough launch chart/ graph here:

http://forum.kerbalspaceprogram.com/index.php?/topic/133878-engine-launch-graph-for-ksp-105/

Edit:  OK, I performed the first (and easiest) implementation you listed.  The first image is of the craft as you described it.  What follows are the flight test altitude results.

I set the default (lower) engine as the first to activate via the space bar.  Notice the altitude reached in the forward (normal) thrust test:

Next I used the upper (reverse thrust) engine; activated using action group 1 (or I could have just right clicked on the engine and activated it there).  Notice the lower altitude reached:

There is no aerodynamic difference between the two flights, but maybe the reverse thrust from the engine above is adding drag to the parts below it.  Just for fun, I next launched with both engines firing at the same time (using action group 2 in this case):

While the altitude wasn't doubled, the engines did flame out at about 23,000 meters.

Here's what I think.  If you want to design a ship with no real differences between the forward and reverse modes, you will need to attach these engines radially and with enough space between them and the fuselage/ hull to negate the effects of reverse thrust drag on other craft parts.  Otherwise, for practical purposes, there IS a difference between using forward and reverse thrust modes; at least when limiting a flight to only one mode.  As for VTOLs, I suspect that there will be no problems.

Edited March 10, 2016 by Dispatcher