Why is my spaceplane's drag so weird?

[jebe]

My spaceplane somehow has more drag acting on the nose during re-entry than on the delta wing and tail, causing it to be unstable and flip to retrograde no matter what I do. Why is the drag so mismatched?  The center of lift is well behind the center of mass.

[Daveroski]

Before re-entry Move all of your fuel as far forward as possible and try again.

 

Oh.. and welcome to the forums

 

[ForScience6686]

I am assuming that your com must be near the back.  The nose is acting like a big lever, since it is further from the com it has a greater effect than the lifting surfaces which are next to the com.

Give me a lever big enough, and I'll move the world.

[jebe]

1 hour ago, Daveroski said:

Before re-entry Move all of your fuel as far forward as possible and try again.

 

Oh.. and welcome to the forums

 

Thanks, when I refueled at my space station i put it all in the rear tank, didn't realize that that would mess up my COM.

[HvP]

Just as ForScience6686 said, the nose is acting like a lever because it's far from the center-of-mass. The wings are probably still creating more drag than the nose, but the force is being applied at a point of low torque, like balancing a ruler in the middle. The force from the nose of the craft may be a lot less, but it's like adding a small push to one end of the balanced ruler; it's still going to tip over.

And as Daveroski said, pumping your fuel from the back to the front will help counter this torque by making the nose heaver, moving the center-of-mass forward.

Edited May 5, 2017 by HvP

[ForScience6686]

One tip that I like to use.  Add an antenna on your fuselage in line with your col.  Since the camera centers on com, you can quickly zoom in on your craft and verify that the com is in front of your antenna.

[Spricigo]

3 hours ago, jebe said:

My spaceplane somehow has more drag acting on the nose during re-entry than on the delta wing and tail, causing it to be unstable and flip to retrograde no matter what I do. Why is the drag so mismatched?  The center of lift is well behind the center of mass.

The image show rockets but its the same principle for planes. For this purpose a plane is just a rocket with lextremely oversized fins.

Also notice that the CoL don't show the whole situation, it only account for aerodynamic forces in one axis when the craft is in the same direction of the airflow. (and, to advertise CorrectCoL, it also only account for part that have a Lift value)

[bewing]

Keep in mind that the nose is exposed directly to the shockwave, and always creates a disproportionate amount of drag. To vastly oversimplify to make a point -- you can pretend that the drag on the nose goes up with the square of the vessel's speed, and the drag on the rest of the vessel goes up linearly. Did you ever wonder why the X15 has such a big tail on it, for such a tiny sleek little spaceplane? It's unstable at top speed with a smaller tail.

 

[GoSlash27]

Drag doesn't act at the same point as lift. If you have more surface area ahead of the CoM than behind, it's probably gonna flip regardless of what you see in the hangar.

For best results, avoid canard delta designs where the CoM is way in the back. Stick with conventional designs with the CoM in the middle.

Best,
-Slashy

[Spricigo]

3 hours ago, GoSlash27 said:

For best results, avoid canard delta designs where the CoM is way in the back. Stick with conventional designs with the CoM in the middle.

Best,
-Slashy

While may be simpler and/or easier to design a aerodynamic stable plane with elevons, the issue of CoM to far back its not exclusive of canard delta. 

Notice , I recognise elevons as the better option for spaceplanes since its activation bring the CoD further back,  contributing to stability. But the assumption that using elevons instead of canards will result in a stable designs is far from reliable. 

4 hours ago, GoSlash27 said:

Drag doesn't act at the same point as lift. If you have more surface area ahead of the CoM than behind, it's probably gonna flip regardless of what you see in the hangar. 

Actually this sum up the whole point.  If there is more drag in front than behind of CoM the vessel will flip.  It's not only probable, it's a matter of time. 

 

[juanml82]

In another subject, are those vectors I'm seeing at the back of a spaceplane? As in, heavy, low isp, excessive thrust, excessive gimbal vectors?

[Ultimate Steve]

1 hour ago, juanml82 said:

In another subject, are those vectors I'm seeing at the back of a spaceplane? As in, heavy, low isp, excessive thrust, excessive gimbal vectors?

It would appear so. Really, just a tip to any spaceplane builders out there, on that type of spaceplane you'd probably be better off with Rapiers than Vectors.

Unless of course you're building it for complete fun. Vectors, IMO, are one of the funnest engines in the game.

*loads up KSP to see how far I can get on a single Vector*

[jebe]

1 hour ago, juanml82 said:

In another subject, are those vectors I'm seeing at the back of a spaceplane? As in, heavy, low isp, excessive thrust, excessive gimbal vectors?

Yes, but im in career mode and dont have rapiers yet.

[Spricigo]

50 minutes ago, jebe said:

Yes, but im in career mode and dont have rapiers yet.

The vector is still a bad option.  Even the Swivel may serve you better in a mixed cycle(jet and rockets) spaceplane. 

I strongly recommend you redesign the whole vessel. Define what the purpose will be and than chose the hardware that allow you accomplish it with less. Specifically less mass,  less drag and less cost. 

Also,  use the collective wisdom of this community.  I can demonstrate how effective this is with a link :

 

[bewing]

Well, what juanml82 was pointing out was that your spaceplane could get a lot higher and further if you used something smaller. Such as terriers, which have significantly more performance for their mass.

 

[AeroGav]

On 2017-5-5 at 7:13 PM, jebe said:

My spaceplane somehow has more drag acting on the nose during re-entry than on the delta wing and tail, causing it to be unstable and flip to retrograde no matter what I do. Why is the drag so mismatched?  The center of lift is well behind the center of mass.

Because the CoL indicator is broken.

It only takes account of wing/control surface parts.    Case in point, you don't even get a blue indicator if you build a rocket with no fins.

In reality, all parts generate lift and drag.    Also, "lift" and "drag" are basically the same thing - aerodynamic forces acting on your craft.    We just call the component that acts perpendicular to the direction of travel "lift", but it's basically the same stuff.

Install the mod called "CorrectCoL"

This will fix the blue indicator and probably show it to be further forward than you thought it was.

Also, get RCS build aid 

The red indicator shows where your CoM will be after the fuel burns off.   I suspect it moves way, way aft in your design.

You have two very heavy (4 tons each) vectors at the back, and another four jet engines, a bit over 7 tons worth.    At the front , you only have 2 tons of cockpit to balance it all.     The only thing keeping your plane balanced is fuel.

 

15 hours ago, Spricigo said:

The vector is still a bad option.  Even the Swivel may serve you better in a mixed cycle(jet and rockets) spaceplane. 

I strongly recommend you redesign the whole vessel. Define what the purpose will be and than chose the hardware that allow you accomplish it with less. Specifically less mass,  less drag and less cost. 

Also,  use the collective wisdom of this community.  I can demonstrate how effective this is with a link :

You will be above 10km and probably above 15km when you fire up your rocket engines , so the atmosphere ISP rating of your rocket engines isn't important.    You want the best possible vacuum isp instead.   TWR is less of an issue than it is on a rocket, because a spaceplane has wings to support it, and because the mass of the rocket engines is a lower percentage of total vehicle weight than it is on a rocket, which chucks away its empty fuel tank and doesn't have wings, intakes and jet engines to drag into space.

Two aerospike "Dart" engines would serve you better i think.    Ten percent more fuel efficient and saving 6 tons of weight...

[Urses]

On 5.5.2017 at 9:30 PM, jebe said:

Thanks, when I refueled at my space station i put it all in the rear tank, didn't realize that that would mess up my COM.

A good wayaround is to empty all tanks in the SPH and build the plane, to bring the drain COM in front of COL.  Than fuel up and correct for fuel COM. And as last step correct fuel flow this way that the COM and COL dont wander to far around. My way around is to build one front tank for reentry and close it till than. And build the rest of the craft around. 

One stable way is to begin with a central tank and add all other structures around to hold the COM and COL together.

Hope this helps.

Funny Kabooms 

Urses 

[qzgy]

17 minutes ago, AeroGav said:

Two aerospike "Dart" engines would serve you better i think.

Actually, looking at the design, why not go for RAPIERs? Seems like it would serve him better than dedicated rocket/jet engines.

[AeroGav]

45 minutes ago, qzgy said:

Actually, looking at the design, why not go for RAPIERs? Seems like it would serve him better than dedicated rocket/jet engines.

he doesn't have that tech yet