Radical Rocketry X-15 Build with Fuel Tanks & Ablative Scheme

[KenECoyote]

Hi Fellow TRF'ers!

I love the X-15 and one of my favorite rockets is a 4" Hangar 11 kit that I built in 2015 and still fly from time to time. I got it back when there were almost no X-15 offerings for rocketeers (even Hangar 11 was mostly out of rocketry when I got their kit).

Well things have definitely changed in the past few years! There was the big release of the Apogee kit as well as am offering from SNG Aero.

When @BigMacDaddy announced that he was making a BT80 version, I jumped on it! In addition to his kit imo appearing to be a quicker and easier build than the others, it also seemed more durable, more detailed, more flexible with motor choices and I really appreciate the amount of support he offers (based on my experience with his F104 Starfighter kit).

So this ended up being quite a "Right Stuff" story...

 

[KenECoyote]

Note: I rushed this build to make a club launch, so I may be missing some pictures and skipping some finishing steps. I'll also be going back often to edit things in as I proceed since this was a crazy blur and I'll often recall more things later. Also I had purchased this kit on my own.

 

[KenECoyote]

So in conversation with @BigMacDaddy about the X-15, I mentioned that it would be really cool if he made the external fuel tanks, noting that I hadn't seen anyone else do it before (probably because scratch building the odd fronts would be difficult), 3D printing them would be easy, and that it would also set his kit apart from others.

Rob being the super-accomodating guy that he is*, said he'd try making them and if I wanted to test them...

*Read through his kit threads and you'll see him often taking up suggestions and incorporating them.

 

[KenECoyote]

So that conversation was in early April, but due to having to make the fuel tank parts and BMD going on a trip, I got the kit on Saturday 4/29 (when I got home late at night). By Sunday night, I had realized that the next club launch was the upcoming weekend!

I initially told BMD that I wouldn't be able to build the kit in time due to having a demanding full time job (which really blew up that week...of course ).

However, the kit kind of had some pull on me and I found myself starting it on Monday.

I've always been one to try to do what it takes on short timing, so what the heck.

 

[KenECoyote]

First off, I was originally going to do the ubiquitous black X-15 like everyone else. Why not? It's GORGEOUS!

Well, I already had one in 4" and I kept reading comments about nobody ever doing the White Ablative one, so I thought it'd be cool to be different!

Furthermore, while researching the white one, I found out that the one I chose to model with the fuel tanks was actually the one with the speed record!

https://www.whiteeagleaerospace.com/the-fastest-x-15-flight/



Edit Add: I've built this as a "Sport Scale-ish" rocket. This is because I was thinking of possibly changing it to the black version later if I didn't like the white. Plus this is a very on-the-fly rushed build!

 

[KenECoyote]

For those who don't know or didn't read in the article, the ablative was a coating applied to the X-15 for high speed tests since at the higher speeds, the heat would damage the aircraft, so the coating was applied to absorb the heat. It was actually pink (like an eraser) with a final coating of white.

Also in case you're not aware, Radical Rocketry kits are builder kits where you have to provide the tubes and a few other parts (shock cord, chute, etc.). Parts provided are 3D printed & cut plywood and he has instructions on his website. I also consider his kits for advanced builders. You can read through my F104 build (linked in post #1) for a better sense of the parts provided.

 

[KenECoyote]

So, where do I begin the build?

Tubes! First off, with the RR kits you need to check the list of supplies needed and the right tubes are the most important.

Given that this kit is offered with different motor options (2 X 24mm, 29mm or both), the motor tubes needed may depend on what motor configuration you want. I'd recommend heavy 29mm motor tubes if you're going to use 29mm motors.

I went with both 2x24mm and single 29mm options and that works because it's modular:


So cool, right?

 

[KenECoyote]

Now just a heads up that the 29mm heavy tubes I had were just a bit short, but I made do and adjusted as needed given that I didn't have time to get another.

The motor configuration is the toughest part of this (especially if you're doing the swappable mounts), so take your time reading and laying out the parts until you can figure it out. Also you can reach out to BMD (or me) with any questions.

 

[KenECoyote]

The next important step is determining the spacing of where the CRs are glued. They need to be clear of the wing and rear fin roots.

Note how I've marked up the body tube and was checking where the CRs are (my tube is a bit short, but BMD suggested I cut the wing tab a bit shorter).

Pic of fairing, wing and fins to show placement:

(CRs need to be in the space between the wing and rear fin and one in front of the wing tab.)

Note: Don't forget to attach the shock cord before gluing in the motor tube!

 

[Cape Byron]

It was actually pink (like an eraser) with a final coating of white.

Any idea what that looked like after testing? My mind has gone to pink undercoat and weathering…

Sorry for the thread detail.

 

[KenECoyote]

Another tricky build step is the side fairings. They are in three pieces for each side and the instructions suggest assembling them first, gluing onto the bt and then gluing in the wings and rear fins.

I found it easier to assemble the rear two pieces, then gluing in the wing and rear fin into the fairing (inserting temporarily onto & into the bt to get correct alignment), then gluing that assembly to the body, then gluing on the forward fairings.

I felt this order allowed a more solid connection since I am gluing the items to the fairings first and can add extra glue on the wing and fin tabs inside of the fairing, then after the assembly is checked and dried, add glue to the slots, then glue on the entire assembly (which felt very solid).

EDIT ADD: Make sure that the fin slots are big enough to fit the wing & fin tabs without resistance. In my rush, I made the mistake of trying to push in a tab when the slot was tight and the fairing split. I then widened all slots and had no other issues.

 

[KenECoyote]

Any idea what that looked like after testing? My mind has gone to pink undercoat and weathering…

Sorry for the thread detail.

Sorry, but it's top secret!

Haha, just kidding! I have no idea. I'd imagine it's streaked with some metal or bluish black showing.

Anyone else knows and can post here?

Edit: I found this Googling.

 

[BigMacDaddy]

Another tricky build step is the side fairings. They are in three pieces for each side and the instructions suggest assembling them first, gluing onto the bt and then gluing in the wings and rear fins.

I found it easier to assemble the rear two pieces, then gluing in the wing and rear fin into the fairing (inserting temporarily onto & into the bt to get correct alignment), then gluing that assembly to the body, then gluing on the forward fairings.
View attachment 579567
I felt this order allowed a more solid connection since I am gluing the items to the fairings first and can add extra glue on the joints inside of the fairing, then after the assembly is checked and dried, also add glue to the slots, then glue on the entire assembly (which felt very solid).

Great build thread Ken - thanks for sharing.

I think gluing the fins to the fairings first is a very good tip (just be sure that the rear fin is angled at 15 degrees - it gets this angle automatically when the tabs are inserted into slots at middle line of tube).

As a reminder you should also sand out the slots in the fairing to make sure that tabs insert easily. It is somewhat easy to crack the fairings until they are glued to the body tube. You could also paints a bit of ABS slurry onto the inside around the slots to further reinforce them (again making sure that the wings/fin tabs insert through slots easily.

One benefit of gluing the fairings to body before putting in fins is that it is easier to sand them, fillet them, sand again, etc... before the wings are attached. Downside is that you might wind up with a slightly crooked fin or wing if everything is not straight.

 

[BigMacDaddy]

Sorry, but it's top secret!

Haha, just kidding! I have no idea. I'd imagine it's streaked with some metal or bluish black showing.

Anyone else knows and can post here?

Edit: I found this Googling.

That is very interesting!

 

[BigMacDaddy]

I shared a few other interesting documents in the build support folder. For example:

 

[KenECoyote]

Here she is with the basic assembly done.
(Excuse the mess... chaos follows rushed builds lol!)

At this point I wanted to plan out where and how to attach the fuel tanks. This was something new, so we really didn't have much to go on regarding how it would affect flight.

• The fronts curve up, so it would add drag to the bottom, but would the curve counter that?
• Would moving the tanks back more lessen the amount the CP moves forward?
• Would there be added base drag behind the tanks treating the rear almost like a tailcone?
• Would shorter or longer tanks be better?
• How to mount them? (BMD didn't have time to figure out and print the mounts and I told him no worries at all).
• Where would the launch lugs/rail buttons go?

Placement testing...

Length testing...

In the end, I went with 7" tube lengths and placed it at the middle of the fairing curve. This looked close to the actual and of course they knew what they were doing.

I initially thought of velcro to attach the tanks (so I could remove for a "first flight" baseline test of this model before adding tanks for a second flight.

However, I decided to use screws and a "keyhole" mount figuring it looked better and was secure enough given when I have a wall hanging with a pair of them, it's quite a pain to remove it.

 

[KenECoyote]

Great build thread Ken - thanks for sharing.

I think gluing the fins to the fairings first is a very good tip (just be sure that the rear fin is angled at 15 degrees - it gets this angle automatically when the tabs are inserted into slots at middle line of tube).

As a reminder you should also sand out the slots in the fairing to make sure that tabs insert easily. It is somewhat easy to crack the fairings until they are glued to the body tube. You could also paints a bit of ABS slurry onto the inside around the slots to further reinforce them (again making sure that the wings/fin tabs insert through slots easily.

One benefit of gluing the fairings to body before putting in fins is that it is easier to sand them, fillet them, sand again, etc... before the wings are attached. Downside is that you might wind up with a slightly crooked fin or wing if everything is not straight.

Ooh, thanks for the reminder about the slots! I'll add the update.

 

[KenECoyote]

Next up, a nose job!

Since I planned to fly this with and without the tanks (as well as on different motors), I decided to apply my usual HP nose anchor build where it's an adjustable nose weight system.

I use BBs and epoxy for the base weight, but also add in all-thread (or a long bolt in this case), a coupler, an eyebolt and a quick link. This alone I measured out to weigh 4oz with the estimated epoxy.


To add an extra 2oz, I have a lead coin weight with a hole punched and drilled through.

I used a wood chopstick to temporarily position and center the anchor while the epoxy cures.

Viola! (Coin weight added to check fit.)

So 6oz nose weight in total for flying with tanks and 4oz without tanks seemed right for my build and the motors I planned to launch on first.

 

[KenECoyote]

At this point it was Wednesday or Thursday and I decided to go all out with the Ablative Record flight scheme...

This scheme isn't for those who can't deal with asymmetry since both sides are different and it gets a bit wacky with one "mummy" tank, diagonal stripes and only one apparent window... a black oval on the right side.

The biggest issue with this is that BMD had printed rectangular windows on both sides. I brainstormed a removable* workaround, but I wasn't sure how well it would work.

(*I was considering having the ablative scheme just temporarily and maybe switching to black after.)

Here's my plan, which worked out very well!

1) Make a paper template of the front cockpit windshield section.



2) Trace that paper template onto white vinyl, cut out and apply.


3) Then use an oval pattern for the one black window and do the same with black vinyl.



Arrr, matey!

 

[BigMacDaddy]

The biggest issue with this is that BMD had printed rectangular windows on both sides. I brainstormed a removable* workaround, but I wasn't sure how well it would work.

I think the oval windows were only used with the external tanks which were not part of the original plan

 

[KenECoyote]

I think the oval windows were only used with the external tanks which were not part of the original plan

@BigMacDaddy is so accommodating that he wants to explain why he didn't account for the odd version that I've never seen done before!

In fact, when I had noted that I was planning the single oval window version to him, he said that he could perhaps print covers, but I said it wasn't necessary (for me) at that point.

 

[techrat]

SO I'm realizing I met you at the METRA launch on Saturday and didn't even realize it. You were wearing a blue shirt. I was that dorky guy with glasses with a brown hat and a green shirt and had "Der Rust Max" that survived a flight with half the ignitor still stuck to it. And we're all X-15 fanboys.

 

[KenECoyote]

Some small deets...

CA'd the body tube edge and added an ear plug to the kevlar line to help prevent zippers given that the appropriate delay would be a bit if guesswork with the tanks.



I painted up the tanks and added black vinyl strips. Of note is that I ended up painting the "mummy" pattern on the wrong end (was done quickly outside around 2AM in the dark... don't do that lol), so I had to add a new set of key holes to that tank.



A day or so earlier (Tuesday?) I had painted the body white with Rusto 2X, however, it was still tacky on Saturday, which was flight day.

I didn't have matte white paint, so I used the one I had which I think was semi-gloss. I'll likely repaint in the future.

 

[KenECoyote]

SO I'm realizing I met you at the METRA launch on Saturday and didn't even realize it. You were wearing a blue shirt. I was that dorky guy with glasses with a brown hat and a green shirt and had "Der Rust Max" that survived a flight with half the ignitor still stuck to it. And we're all X-15 fanboys.

Hey there good buddy! I remember that rocket, it was great looking!

Were you joking that you shouldn't get the rocket weighed with an ignitor attached because it's heavier? Then I replied thasometimes the rocket takes the ignitor along and then later you said it just happened to you?

Always great to meet another TRFer in the field!

 

[KenECoyote]

Swingers baby!

So the tanks posed somewhat of a dilemma with regards to simming since it was difficult to correctly model them. This meant that swing testing was called for.

Initial swing tests inside the house Thursday night were very good and the rocket appeared very stable with tanks and 6oz of nose weight. This included starting backwards and sideways.

It was a bit stressful doing this indoors given I wasn't sure if the heavy rocket may break loose and then break hitting something (or fly out a window).

Friday morning I went outside to swing test with a longer string. Similar results showing the rocket was stable. I tried to take a lic of it mid-swing while spinning and it wasn't easy! (And was very dizzying )


Results were better than I had been hoping for!

 

[BigMacDaddy]

Ken - Great build thread with lots of good advice.

One thing to note for others that got the model, Ken opted for the tall ventral wedge fin. Taller vs. shorter ventral wedge fin makes some difference in simulations so keep that in mind when planning nose weight.

Also simulations do not accurately account for increased base drag of these wedge fins so you may be able to experiment with less weight depending on motor configuration, recovery weight, etc...

Swing test is good practice if varying substantially from previously tested models.

 

[KenECoyote]

After the swing testing, I went about copying some of the red markings on the plane. Luckily there were only a few and they were simple to duplicate (rectangles and triangles are all that is needed).

So given that she was pretty much done, I took a few more glamor pics before first launch the next day.

With the tall bottom tail, I had to use something to prop up the front and I found a white rock (not to scale ).

 

[KenECoyote]

I later found a replacement for the X-15 rock (probably the first time that's been said here lol) and took a few more shots.


Next up... launch time!

 

[burkefj]

Ken - Great build thread with lots of good advice.

One thing to note for others that got the model, Ken opted for the tall ventral wedge fin. Taller vs. shorter ventral wedge fin makes some difference in simulations so keep that in mind when planning nose weight.

Also simulations do not accurately account for increased base drag of these wedge fins so you may be able to experiment with less weight depending on motor configuration, recovery weight, etc...

Swing test is good practice if varying substantially from previously tested models.

Afaik the tall ventral was not used with the tanks and white scheme in particular. Most flights did not use it after a certain time as it wasn't needed. The oval windows were added in the a-2 version to prevent cracking at high temp/speed when it was re built. Tall ventral helps stability though and most people won't know better.

There is one test flight in black with the tanks that shows an orange painted extended ventral.

 

[BigMacDaddy]

Afaik the tall ventral was not used with the tanks and white scheme in particular. Most flights did not use it after a certain time as it wasn't needed. The oval windows were added in the a-2 version to prevent cracking at high temp/speed when it was re built. Tall ventral helps stability though and most people won't know better.

Thanks Frank. I thought as much -- Ken got both the 2x24mm cluster as well as the 1x29mm engine options so will fly lots of experimental variants. The full ventral was used with both engines but the white ablative scheme had a pretty narrow operating window.

Now Ken has started talking about the Ramjet (which I think only flew as a dummy)...

I guess only option is to build 3 or 4 models with the correct options and paint schemes