Spaceplane Difficulty

[BlockTrooper]

Hello everyone! I'm new to the forums (I probably should have checked these forums out ages ago though as I have been playing before 1.0 4 years ago) so I would first of all like to say how much of a good job people are doing to keep these forums a nice place!  But I would like to ask what are some tips to get me going on spaceplanes as I have had much difficulty throughout these years getting them to go a few inches above the ground before they blow to pieces. I get the general idea of center of mass in front of center of weight but my spaceplanes still don't seem to work that well, and also I have found jet engines can't produce enough thrust to lift any plane I create off the ground even though I have 2 air intakes on the plane! It would be nice to know what I could possibly be doing wrong! It would be very appreciated if someone could give me some tips on ways to make my planes more stable and why mine could be simply flipping out of control!  Any advice is appreciated!

[EliteGuy3]

Here's some tips from me:

Being a special person in spaceplanes myself (I'm terrible), you don't want your centre of mass to be far away from the centre of lift. If so, than your plane won't take off. Instead, it will crash in front of the runway. You must also add the appropriate amount of wing surface to your plane to make it lift. Too little, and your plane won't fly. Too much, and your plane will be slow.

Jet engines may have low TWR (thrust to weight ratio), but that means your plane is just too heavy. Get rid of those extra fuel tanks.

air intakes do not affect the thrust of an engine, but for some engines, like the RAPIERs or turbojet engines, increase the maximum height of which the engines will operate. You only need 1 intake (non-radial) per engine.

tell me if you need more help.

Edited December 4, 2016 by EliteGuy3

[Ten Key]

47 minutes ago, BlockTrooper said:

I get the general idea of center of mass in front of center of weight but my spaceplanes still don't seem to work that well, and also I have found jet engines can't produce enough thrust to lift any plane I create off the ground even though I have 2 air intakes on the plane!

Just one random thought here that may or may not apply. Jet engines take time to spool up to full power-- when taking off, leave the wheel brakes on your landing gear locked until the engines have had time to build up thrust. 

[BlockTrooper]

35 minutes ago, EliteGuy3 said:

Here's some tips from me:

Being a special person in spaceplanes myself (I'm terrible), you don't want your centre of mass to be far away from the centre of lift. If so, than your plane won't take off. Instead, it will crash in front of the runway. You must also add the appropriate amount of wing surface to your plane to make it lift. Too little, and your plane won't fly. Too much, and your plane will be slow.

Jet engines may have low TWR (thrust to weight ratio), but that means your plane is just too heavy. Get rid of those extra fuel tanks.

air intakes do not affect the thrust of an engine, but for some engines, like the RAPIERs or turbojet engines, increase the maximum height of which the engines will operate. You only need 1 intake (non-radial) per engine.

tell me if you need more help.

 

2 minutes ago, Ten Key said:

Just one random thought here that may or may not apply. Jet engines take time to spool up to full power-- when taking off, leave the wheel brakes on your landing gear locked until the engines have had time to build up thrust. 

Thanks for the tips!

[MaxxQ]

Here's a couple other tips:

1. Make sure your Center of Mass/Center of Lift relationship doesn't change much when the tanks are dry.  This will help immensely in case you have a gliding return/landing as opposed to powered.  You MAY need to transfer fuel from fore to aft (or vice-versa) to balance the craft for your return flight.  Learn to do that, although the new fuel flow priorities feature introduced in 1.2 may make that unnecessary.

2. Place your main landing gear fairly close to your CoM, but behind it.  This will help you when it's time to rotate the craft for liftoff.

3. If possible, either have the nose of the spaceplane a little higher than the tail, or angle the leading edges of the wings upwards slightly.  This will give you a built-in positive Angle of Attack that should help you get off the runway a little sooner.

4. If you have a long tail, make sure there's enough clearance for you to take off and land without banging it into the ground.  This can be done with longer landing gear, placing the landing gear on the lowest part of the plane, or using a tailpiece that has an upward sloping underside.

5. Make sure your Center of Thrust is pointing through the CoM, or as close as possible.  If the main thrust of the engines is above or below the CoM, you will either pitch down or up, respectively.  Forex, if you have your main engines on top of the wing, and your wings are even with the CoM, your thrustline will be too high and the craft will want to constantly pitch down, requiring a lot of pitch up on your controls.

[LordKael]

1 hour ago, MaxxQ said:

Make sure your Center of Thrust is pointing through the CoM, or as close as possible.  If the main thrust of the engines is above or below the CoM, you will either pitch down or up, respectively.  Forex, if you have your main engines on top of the wing, and your wings are even with the CoM, your thrustline will be too high and the craft will want to constantly pitch down, requiring a lot of pitch up on your controls.

This gets to be especially important in space. In atmo, your wings and control surfaces will handle the correction, often without you needing to even notice, but once you get to space, the craft will either need a crazy overpowered RCS set up, or enough reaction wheels to stop Kerbin from rotating. In both scenarios, you lose a lot of efficiency by having offset thrust, so building your spaceplane on a single plane will make life much easier. 

On the subject of building in a single plane: Don't radially attach things other than wings, lights, comms, etc if you can help it. The way the game handles aero occlusion is overly simplified, and node attached parts will create very little drag, especially if you can manage to use only matching node sizes. It doesn't take into account the offset tool (hotkey 2) when calculating the drag, so you can create some really bizarrely shaped planes that would never fly in real life, but fly like a champ in Kerbal 

Edited December 5, 2016 by LordKael

[katateochi]

This thread is worth a read for aircraft construction:

Try to balance the plane so that the CoM doesn't shift much as fuel is consumed (although that's more important later, sounds like your issues are at the start). Either manually drain the tanks in the editor, or (much less hastel) use the RCS build aid mod (it adds a dry mass marker as well as the default CoM marker).

Drag...I can't stress enough how important it is to minimize drag on spaceplanes.  

 

6 hours ago, BlockTrooper said:

I have had much difficulty throughout these years getting them to go a few inches above the ground before they blow to pieces

If they're exploding after take off, you're possibly pulling up too hard? I have had some craft which had a lot of lift be prone to losing their wings on take off.

6 hours ago, BlockTrooper said:

and also I have found jet engines can't produce enough thrust to lift any plane I create off the ground even though I have 2 air intakes on the plane!

Intakes are much less of a factor than they used to be. 1 intake for two engines is sufficient, especially at sea level. That sounds like your plane is simply too heavy for the number of engines you're using. If you posted a pic of the plane we could probably be more helpful. 
And on that note, start small. You don't need a lot to just reach LKO with a single pilot and no cargo (as an example, this will reach 100km orbit and return: https://kerbalx.com/katateochi/Knat-Delta) (also have a look and maybe pick apart some craft by other players, heres some 1.2.x spaceplanes)

 

[Leafbaron]

I usually design my Space planes to have a CoL behind the CoM when dry about the distance the CoM indicator ball is in size. When designing the plane i try to achieve a minimum of .5 TWR on the air breathing engines when on the runway. This should be over 1 when reaching the limits of there operating altitude. Most ram jets and turbo ram jets get increased thrust when over mach 1. So to save fuel my flight profile is pretty steep, the quicker you can get to high altitude the less fuel you will burn to get there. so you can keep you craft lighter. around 20-30 degrees until reaching about 8000m, velocity usually starts to decrease at this point so to keep building speed pitch down slowly, start pitching down achieving 5-10 degrees of pitch around 10-12km. this is when the drag from the atmosphere really starts to decrease and velocity really starts increasing. Keep a low angle of attack here and build as much speed as possible before reaching 20-22km. at this point your engines will have almost no thrust and will barely keep you going 1100m/s to 1300m/s. at this point i switch to a closed cycle engine preferably with a TWR of .7 as a minimum, as you burn fuel it will get higher and thanks to the lift of the wings is sufficient. but the faster you can accelerate the less gravity losses you will experience. When i turn to closed cycle engines i pitch up gradually so that I can get to the thinner atmosphere quicker. reduced drag lossess on d'/v when my plane is about 35-40 km i pitch to almost 0 and start building speed until in orbit. Recently i've been launching with space planes exclusively and through lots of testing this is the most efficient way to get to orbit that i have found. Only bring enough jet fuel to carry you to 20-22km, this will save weight and increase the d'v of your LFO engines. any other questions id be happy to help. Space planes are infinitely fun to play with. 

[RentedTritium]

Wait I thought additional intakes helped you keep going as you got higher? Two engines on one intake is a lot less than I'm using. I thought I needed extra to raise my ceiling. 

[suicidejunkie]

My spaceplane has got a precooler for both of my turbojets, but that is simply for symmetry and is overkill for intake air.  I replaced the frontal intakes with nosecones.

I have tried using the supersonic ramp intakes, but that requires idling down the runway at 10% throttle to pick up speed and slowly, slowly throttling up as the wind starts blowing through.  Not worth the hassle since the oxygen cuts off at the same altitude regardless.

 

[Leafbaron]

13 minutes ago, suicidejunkie said:

...I replaced the frontal intakes with nosecones.

 

Use circular intakes instead of nose cones assuming you also have an intake designed for supersonic flight. As per this video someone posted a while back explains. They have the least amount of drag even compared to nose cones, additionally you won't have to throttle back to build up speed on the runway.

 

Edited December 5, 2016 by Leafbaron

[suicidejunkie]

It was quite a while ago, but I believe I dropped the front mounted intakes due to them overheating and exploding during the LV-N stage of the ascent somewhere in the 30-40km altitude range.

[BlockTrooper]

Thanks for the tips everyone! I made a nice little career mode plane to fly!
 

[Leafbaron]

post pictures!

[BlockTrooper]

4 minutes ago, Leafbaron said:

post pictures!

Where is the button to download pic? Cant find it!

[Leafbaron]

f1 to screen shot, then you have to post the images to imgur, no account required. then copy the image link and paste it in comments here!

screenshots will be in the screenshot fold of you ksp folder

Edited December 5, 2016 by Leafbaron

[BlockTrooper]

http://steamcommunity.com/sharedfiles/filedetails/?id=813138282http://steamcommunity.com/sharedfiles/filedetails/?id=813138601 Used Steam to get the pic! It's engines can run efficiently until about 10,000 m (which isn't bad saying it's for long distance travel around kerbin) And it's average speed is 220 m/s to 150 m/s and it's engines can run for (drumroll) 3.174 (Earth Hours)!!!

Edited December 6, 2016 by BlockTrooper

[EliteGuy3]

I'm glad you were able to get you plane running. Well done, and welcome to the forums!

[Snark]

Moving to Gameplay Questions.