The Dual Spaceplane Heavy Lifter: How to make a Spaceplane Sandwich.

[Brainlord Mesomorph]

The Dual Spaceplane Heavy Lifter Part 1: How to make a Spaceplane Sandwich.

This one of several posts I have about the idea of a “Spaceplane Sandwich.” If you haven’t, please read the General Discussion thread first.

Good candidates for a Spaceplane Sandwich are any SSTO planes with extra thrust, extra lift, extra fuel and some space on the top of the fuselage for the payload. (i.e. no vertical stabilizers in the way)  It does not need any cargo capacity of its own.

One simple way to make one is to take your largest cargo plane, remove the cargo capacity (payload bays, passenger cabins, fuel tanks etc), move the vertical stabilizers out to sides (somehow) and add really heavy landing gear (it will be carrying more than twice its weight).

Once you have selected your plane, you’re ready to make the sandwich. (I’m using the YB101 that I built for the DSHL project, you can get it here.)

By default the SPH wants to put the top plane on upside down or sideways. So you have to go through these steps to build a top plane to go with your existing bottom plane (takes about 3 minutes) .

1.       First make whatever part that contains the CoM the Root Part. In this case it’s a Mk2 long LFO tank. (from this point on I’ll be calling that the “bottom CoM part”)

2.       Add a Decoupler over the CoM.

3.       Add a Demo payload to the Decoupler at the CoM,  in this case we’ll use a 40 ton fuel tank. (it’s good to use a fuel tank as a demo payload to figure out the maximum lifting capacity of the sandwich, later you’ll replace it with a real payload.)

4.       Add  another Decoupler over the new CoM.

5.       Now go to the parts sidebar, and get fresh copy of whatever your bottom CoM part is. In this case it’s a Mk2 long LFO tank.  It will usually attach sideways. That’s fine.

6.       Use the Rotate and Offset tools to rotate the part right side up and center it over the decoupler. Now look from the side and move it forward or rearward so it is directly above the bottom CoM part.

7.       Use Alt-Click to take a copy of the forward fuselage from the bottom CoM part and attach it to the top CoM Part.

 

8.       Repeat with rear fuselage.

9.       And the wings. (Note: if the wings of your plane are not attached directly to the CoM part, you may want to rethink that.)

10.   Add any other parts from the bottom CoM part that are needed (RCS, chutes, etc)

11.   Finish out the demo payload with nosecones, tailcones and fins. (For more on payloads see: The Dual Spaceplane Heavy Lifter Part 2: Payloads: Putting the meat on a Spaceplane Sandwich.)

12.   Retract the gear on the top plane.

13.   Moar Struts (actually just these struts) Running struts from around the landing gear of the top plane to above the landing gear of the bottom plane seems to distribute the weight nicely. If you run struts from top to bottom, they will re-attach when you change payloads.(Struts are usually not necessary on the payload if it properly packed.)

14.   And now you have a Spaceplane Sandwich.

 

Don’t be surprised if the first time you put it on the runway if falls backwards, you may need to go back to the SPH and use the offset tool to move the payload (and/or the top plane) forwards or rearward. I don’t know anything about the physics of hypersonic biplanes, but I do tend to end up with the top plane a little forward of the bottom plane. The result is sometimes a little silly looking. (This is KSP.) It looks basically like two planes carrying a bomb bigger than either of them. (To me it looks like a last-ditch attempt to win a war in a cartoon.)

But it looks a lot less silly when you release the payload in orbit.

 

The Dual Spaceplane Heavy Lifter Part 2: Payloads: Putting the meat on a Spaceplane Sandwich. 

This part took me a while. For the longest time I could get fuel tanks to orbit on a Sandwich Lifter, but I couldn’t reliably do it with spacecraft of similar mass. The problem was drag. If your payload has too much drag you will burn too much fuel on the way up and not make orbit, and even if you have a reasonable amount drag but it’s in the wrong place the vehicle still will become highly unstable at transonic speeds and may just tumble out of control.

The problem was that I was designing the fairing the way I did for a rocket; primarily minimizing mass (which means hugging the payload shape tightly) and secondarily minimizing drag (which meant adding some slanty areas). That was wrong. 

First, l learned what “Center of Drag” was. It goes along with CoM, CoL, and CoT but is not shown in the VAB/SPH. If the CoD is forward of the CoM, the vehicle will tend to flip around. If you have ever put too wide a fairing on the nose of a rocket, you know all about that.  Think of a dart; heavy, pointy nose in front, thick body a little bit forward of the middle, and light draggy fins in back.

You do also want to minimize mass, of course, but that’s secondary.  These parameters lead to one simple shape; the fairing forms a gentle slope from the nosecone out to widest point (the CoM should be just at or slightly forward of the widest point) and then another, slightly longer, gentle slope from the widest point to the tail, and fins on the tail. Putting the fins at 45 degree angles keeps them out of the jet wash. You’ll recognize this shape. It’s the shape of a bomb.

Form follows function; bombs are designed to be carried on high-speed aircraft. It just works. Making the payload consistently bomb-shaped is what makes the entire Spaceplane Sandwich concept work.

There are a few other considerations, order of assembly, getting the girder segments to poke through the fairing, etc.  So I’ve made the following step by step.

Interestingly, as I went through the process to get the screenshots for this tutorial, I made a mistake. The payload doesn’t get to orbit at first. I had to go back and fix it. I thought about redoing the screenshots over. But I realized it made for an excellent “teachable moment.” Can you catch where I go wrong?

This is the Icarus, a hybrid interplanetary tug/lander I just loaded with a bunch of fuel.  Because the ship started in the VAB we’ll start there. We’ll move to the SPH shortly.

1.       First we find the CoM and mount decouplers on what will be the top and bottom of the payload.

2.       We add a spacer, here I’m using an empty fuel tank, on the will become the rear of the payload.

3.       Mount a fairing to that, and build the fairing in the bomb shape we discussed earlier. Note: “clamshell” fairings seem to get all explodey with girders sticking through them; “potato chip” fairings appear to be safer.

4.       (optional) Temporarily remove the spacer. The fairing will move with it and stay intact. (This is just to see better, I can never work with all those fairing its flying around.)

5.       Attach long girder segments to the decouplers.

6.       Run struts from the girder segments to the payload, supporting it evenly.

7.       Reattach the spacer/fairing assembly. Hopefully the girder segments will be long enough to poke through the fairing, if not add more.

8.       Attach a tailcone and fins.

9.       Use the reroot tool to make the bottom (ventral) girder segment the root part.

10.       Save the ship.

11.   Go to the SPH, load your Spaceplane Sandwich.

12.   Remove the top plane WITH its decoupler. (It may flip sideways, that’s fine.)

13.   Remove and delete the demo payload.

14.   Click the Open button and Merge with the craft file you saved in step 10.

15.   Mount the payload on the bottom plane. Note:I have had a problem w/ the Long Girder Segment not wanting to attach to the decoupler properly. The green ball attachment point appears on the wrong end of the girder (the end already attached to the payload) ,then it wants to attach there, with the girder segment clipping through the plane. If that happens, putting a small girder segment on the decoupler first puts a green ball there. Then the payload will attach correctly.

16.   Mount the top plane on the payload. (Struts should reconnect by themselves.) Note:  At this point you have just combined three very complex vehicles, usually with conflicting forms of symmetry. Depending on partcount, computer memory and/or whatever KSP bugs exist, we appear to be pushing the limits of the SPH at this point. This is when symmetry goes wonky, struts and fuel lines may get disconnected. Adding more symmetrical parts is iffy at best.  None of this, however, has anything to do with this tutorial. All I can tell you is to watch out for that kind of thing.

I’ll say again: Don’t be surprised if the first time you put it on the runway if falls backwards, you may need to go back to the SPH and use the offset tool to move the payload (by the bottom plane’s  decoupler) and/or the top plane, forwards or rearward. I don’t know anything about the physics of hypersonic biplanes, but I do tend to end up with the top plane a little forward of the bottom plane.

Now, as I said in the beginning; that did not get into orbit. Can you tell me why not?

The shape.  The back half of the fairing isn’t long enough. Back to the SPH where I replaced that short spacer with two size one hollow tubes. Then I rebuilt the fairing, this time making it a mid-fairing and adding an actual nosecone.

That one went into orbit nicely.

One last note: If your payload is atmospherically streamlined, you might not need a fairing, (i.e. a sleek Mk2 ship or a fuel tank that just requires nose and tail cones). But the fins do not appear to be optional. Just a girder with four fins will do. But you will need some fins back there.

The Dual Spaceplane Heavy Lifter Part 3: Orbit:  Properly serving a Spaceplane Sandwich. 

(OK, enough sandwich puns  I've been told they're metaphors)

This one of several posts I have about the idea of a “spaceplane sandwich.”  This article is about packing the payload in a fairing. If you haven’t, please read the General Discussion post first.

So you’ve got your Spaceplane Sandwich and you’re about to make orbit. There are still a couple of things to consider.

1. What about Debris?
Once you clear 55km, and before the orbital insertion burn, you can jettison nosecones, tailcones, and the fairing. Note: “clamshell” fairings seem to get all explodey with girders sticking through them; “potato chip” fairings appear to be safer.

But that still leaves the girders and decouplers holding the planes to the payload. Do you want the planes to carry the payload (and that debris) all the way to orbit?

I like to aim for an Ap of 72 km and a Pe of about 52 km. Just a little short of orbit. Then I separate everything. The payload and one plane make a short burn at Ap to orbit. Then I fly the other plane home, and all the debris deorbits with it.

2.       Are you really making orbit?
Normally when you design a spaceplane or rocket, once you make orbit, you can usually make orbit again, the same way, every time. That’s not true with a Spaceplane Sandwich. Every time you fly it, you have a different payload; different weight, different shape and size of fairing. In short it’s a different plane every time. And if you’re like me, you’re probably pushing max payload capacity (and drag).

So you might not quite be making orbit every time. Fortunately, the Spaceplane Sandwich concept gives you a lot of options here. Does the payload have engines? After jettisoning the fairing it might help carry everything up. “Borrowing” fuel from the payload is another option. Or you could go back to the SPH and add fuel to the payload for the purpose of transferring it to the planes on the way up. (Did you use empty fuel tanks as a fairing spacer?)

My point here is that even though the planes should be able to carry the payload (fairing, girders, and all) to orbit. It doesn’t have to. And debris-wise you probably don’t want to.

So, that’s putting Spaceplane Sandwich in Orbit,
Comments about the Spaceplane Sandwich concept in general, belong in the General Discussion thread.

 

Edited September 10, 2016 by Brainlord Mesomorph