Space X Falcon 9 Crew Dragon BT-60

[Switch]

I've started working on a new build at 1/88th scale of the Space X Crew Dragon that flew Bob Behnken and Doug Hurley to the International Space Station (ISS) on May 30, 2020. You can view the launch here.

The goals of this build are ambitious:

1: 1/88th Scale so BT-60 tubes can be used for the body
2: A working two stage rocket with the Crew Dragon as the second stage
3: Hidden fins for the second sage based on Tim's design at Apogee Rockets
4: Working landing fins based on Antsinafrica's design on Thiniverse.
5: No glue, so parts can be replace as needed.

I will also leverage Apogee Rockets' Falcon 9 Crew Dragon since it was scaled for BT-60 tubes.

The two stage ignition will work like my previous Falcon Heavy where a BT-50 tube carries the ejection charge to ignite the second stage motor in the Crew Dragon.

I plan to use the same rear ejection technique as I have used before. But I am planning to move the upper motor centering ring down to the level of the top of the landing legs where small magnets will hold them in place until the ejection charge ejects the motor tube assembly. Then the landing legs will be deployed into landing position using the tension of those tiny rubber bands used for dental braces.

Lastly, I plan to attach the recovery parachute line of the first stage to the top of the body (rather than out of the bottom) so the F9 lands "legs first" (before it tips over and explodes).

If this new design works out, I'll carry this forward to a Falcon Heavy II that integrates the new landing legs and hidden fins for the second stage.

The 3D printing will be done on a Prusa i3 MK3s 3D printer and all of the designs will be created using FreeCAD software.

Here is the first print of the landing legs section. I forgot to change the filament to white...

 

[Switch]

I've started working on a new build at 1/88th scale of the Space X Crew Dragon that flew Bob Behnken and Doug Hurley to the International Space Station (ISS) on May 30, 2020. You can view the launch here.

The goals of this build are ambitious:

1: 1/88th Scale so BT-60 tubes can be used for the body
2: A working two stage rocket with the Crew Dragon as the second stage
3: Hidden fins for the second sage based on Tim's design at Apogee Rockets
4: Working landing fins based on Antsinafrica's design on Thiniverse.
5: No glue, so parts can be replace as needed.

I will also leverage Apogee Rockets' Falcon 9 Crew Dragon since it was scaled for BT-60 tubes.

The two stage ignition will work like my previous Falcon Heavy where a BT-50 tube carries the ejection charge to ignite the second stage motor in the Crew Dragon.

I plan to use the same rear ejection technique as I have used before. But I am planning to move the upper motor centering ring down to the level of the top of the landing legs where small magnets will hold them in place until the ejection charge ejects the motor tube assembly. Then the landing legs will be deployed into landing position using the tension of those tiny rubber bands used for dental braces.

Lastly, I plan to attach the recovery parachute line of the first stage to the top of the body (rather than out of the bottom) so the F9 lands "legs first" (before it tips over and explodes).

If this new design works out, I'll carry this forward to a Falcon Heavy II that integrates the new landing legs and hidden fins for the second stage.

The 3D printing will be done on a Prusa i3 MK3s 3D printer and all of the designs will be created using FreeCAD software.

Here is the first print of the landing legs section. I forgot to change the filament to white...

Here's the prototype of the landing leg:


I suspect that I'm not going to need the rubber band in flight. I think once the landing leg is released, air resistance will pull it out. There are two landing leg struts. Limits of 3D printing at this scale prevent doing four sections like in the actual landing legs. The upper strut has an internal collar and the lower strut has an external collar so they lock when fully extended:

Here's a test of the landing leg deployment. I'm moving a magnet on the inside of the tube with my finger.

 

[Switch]

Here's the prototype of the landing leg:
View attachment 432918

I suspect that I'm not going to need the rubber band in flight. I think once the landing leg is released, air resistance will pull it out. There are two landing leg struts. Limits of 3D printing at this scale prevent doing four sections like in the actual landing legs. The upper strut has an internal collar and the lower strut has an external collar so they lock when fully extended:
View attachment 432931
Here's a test of the landing leg deployment. I'm moving a magnet on the inside of the tube with my finger.

Here is the FreeCAD rendering of the second stage flip out fins. The Prusa 3D printer failed with a "Bed preheat" error. I've tested as much as I can from their excellent support web resources, but I'm stuck until I get that fixed. In the pic below, the interstage tube is made transparent so you can see what is inside. The peg on the lower arms and slot in the fin are for small rubber bands. Once I get this printed I'll verify geometry and no doubt have to do the whole thing over again.

 

[Nytrunner]

second stage flip out fins

Very nice! Do you have sufficient fin airea in the airstream with the fin pivots that close to the motor tube?

 

[Switch]

The transparent inter stage extends longer than the fins in the first drawing I posted.

The fin pivots are just inside the BT-60 tube. Here's a view from the bottom:

 

[Nytrunner]

Wow that's a lesson in parallax for me, thanks!

 

[Switch]

I've started working on a new build at 1/88th scale of the Space X Crew Dragon that flew Bob Behnken and Doug Hurley to the International Space Station (ISS) on May 30, 2020. You can view the launch here.

The goals of this build are ambitious:

1: 1/88th Scale so BT-60 tubes can be used for the body
2: A working two stage rocket with the Crew Dragon as the second stage
3: Hidden fins for the second sage based on Tim's design at Apogee Rockets
4: Working landing fins based on Antsinafrica's design on Thiniverse.
5: No glue, so parts can be replace as needed.

I will also leverage Apogee Rockets' Falcon 9 Crew Dragon since it was scaled for BT-60 tubes.

The two stage ignition will work like my previous Falcon Heavy where a BT-50 tube carries the ejection charge to ignite the second stage motor in the Crew Dragon.

I plan to use the same rear ejection technique as I have used before. But I am planning to move the upper motor centering ring down to the level of the top of the landing legs where small magnets will hold them in place until the ejection charge ejects the motor tube assembly. Then the landing legs will be deployed into landing position using the tension of those tiny rubber bands used for dental braces.

Lastly, I plan to attach the recovery parachute line of the first stage to the top of the body (rather than out of the bottom) so the F9 lands "legs first" (before it tips over and explodes).

If this new design works out, I'll carry this forward to a Falcon Heavy II that integrates the new landing legs and hidden fins for the second stage.

The 3D printing will be done on a Prusa i3 MK3s 3D printer and all of the designs will be created using FreeCAD software.

Here is the first print of the landing legs section. I forgot to change the filament to white...

Here's the motor mount for reverse recovery. The magnet mounts to secure the landing legs are in green. The magnet mounts for attaching to a Falcon Heavy core are in blue and red at the bottom left.

 

[Switch]

I've started working on a new build at 1/88th scale of the Space X Crew Dragon that flew Bob Behnken and Doug Hurley to the International Space Station (ISS) on May 30, 2020. You can view the launch here.

The goals of this build are ambitious:

1: 1/88th Scale so BT-60 tubes can be used for the body
2: A working two stage rocket with the Crew Dragon as the second stage
3: Hidden fins for the second sage based on Tim's design at Apogee Rockets
4: Working landing fins based on Antsinafrica's design on Thiniverse.
5: No glue, so parts can be replace as needed.

I will also leverage Apogee Rockets' Falcon 9 Crew Dragon since it was scaled for BT-60 tubes.

The two stage ignition will work like my previous Falcon Heavy where a BT-50 tube carries the ejection charge to ignite the second stage motor in the Crew Dragon.

I plan to use the same rear ejection technique as I have used before. But I am planning to move the upper motor centering ring down to the level of the top of the landing legs where small magnets will hold them in place until the ejection charge ejects the motor tube assembly. Then the landing legs will be deployed into landing position using the tension of those tiny rubber bands used for dental braces.

Lastly, I plan to attach the recovery parachute line of the first stage to the top of the body (rather than out of the bottom) so the F9 lands "legs first" (before it tips over and explodes).

If this new design works out, I'll carry this forward to a Falcon Heavy II that integrates the new landing legs and hidden fins for the second stage.

The 3D printing will be done on a Prusa i3 MK3s 3D printer and all of the designs will be created using FreeCAD software.

Here is the first print of the landing legs section. I forgot to change the filament to white...

With the Prusa 3D printer awaiting parts, I have not been able test fit the design of anything. So I've moved forward with other design work that needed to be done.

Outstanding were the fins for the F9. I thought about mounting the fins at 45° to the landing legs as others have done. But that won't work for the Falcon Heavy core because the fins would project into the boosters. This is one of the many reasons my previous Falcon Heavy doesn't have landing legs. So I have been looking at integrating the fins into the landing legs themselves.

It looks like there is a lot of room to work with:



The figure above shows how much surface area the landing leg mounted fin can have in both the retracted and extended position. I'm going to need some time with RockSim to figure out the minimum fin size. And that will differ with the F9 for the Crew Dragon and the Falcon Heavy core, which are completely different core rocket payloads.

Here is what it would look like with all four fins in the retracted and extend landing leg configuration:


Here's what a minimal landing leg fin would look like:




promising...

 

[Switch]

Here is the FreeCAD rendering of the second stage flip out fins. The Prusa 3D printer failed with a "Bed preheat" error. I've tested as much as I can from their excellent support web resources, but I'm stuck until I get that fixed. In the pic below, the interstage tube is made transparent so you can see what is inside. The peg on the lower arms and slot in the fin are for small rubber bands. Once I get this printed I'll verify geometry and no doubt have to do the whole thing over again.

View attachment 433046

The Prusa 3D printer is fixed! Here's the first prototype of the flip out fin. I'm going to move the rubber band mount to the middle of the motor tube.

Flip Fin test video



The motor mount is printed in two parts. The upper section has a hole to receive the motor tube with fin arms. Here is an older version with the parts separated. I'm adding a notch on the next version so the two parts align perfectly.



This is based on this Apogee Rockets design:
https://www.apogeerockets.com/education/downloads/Newsletter313.pdf

 

[Switch]

The Prusa 3D printer is fixed! Here's the first prototype of the flip out fin. I'm going to move the rubber band mount to the middle of the motor tube.

Flip Fin test video

View attachment 433695

The motor mount is printed in two parts. The upper section has a hole to receive the motor tube with fin arms. Here is an older version with the parts separated. I'm adding a notch on the next version so the two parts align perfectly.

View attachment 433696View attachment 433697

This is based on this Apogee Rockets design:
https://www.apogeerockets.com/education/downloads/Newsletter313.pdf

Fortunately Tim created a RockSim file for his Apogee Rockets' Falcon 9 Crew Dragon. So it was easy work to modify that for a 2 stage version.

RockSim can't do flip out fins but I just wanted to see that the second stage was stable.


Here is a video of the completed prototype of the Crew Dragon second stage flip out fins.

Here's the FreeCAD rendering of the completed flip out fins motor mount. The fins work as the coupler to the bottom BT60 tube. So the top ring can be completely inserted into the 2nd stage tube.

I've also attached the Rocksim files to this post.

 

[Switch]

So here is my update on this build.


I'll start with a summary of the past two weeks:



Basically, a lot of trial and error (mostly error). I figured out that it is easier and faster to print smaller sections to focus on specific issues. The weakest part of the landing legs are the holes at the ends where they attach to the rocket. I had a lot of trouble with the position of the upper landing leg strut mount. Too low and the leg cannot fold completely in the upright position. Too high and there isn't enough room for the landing leg struts (that's a pile of failed attempts at landing leg struts on the lower right in the photo above).

From this work, I concluded that the landing leg struts could not be printed and would have to be made from telescoping aluminum tubing. Then two options were developed. The first one uses a dental rubber band to extend the landing leg:



The second option uses a small spring made from guitar string:



Here you can see the coil of the spring around the screw that holds the smaller landing leg strut in place and a small magnet used to secure the landing leg in place during launch:


The spring extends out of the smaller tube and is bent so as to protrude through a hole in the larger tube to lock the extended landing leg tubes into place:


The force of this spring and the friction of the end against the larger tube is a delicate balancing that is not completely debugged. I may end up using the rubber band for the "pull" and the smaller strut spring just to lock the extended landing leg into place. Note that it is not clear that any force is needed to extend the landing legs. The actual Falcon 9 uses gravity for this purpose. But as Galileo Galilei observed in his Dialogues Concerning Two Sciences: the effects of falling differ greatly between larger and smaller animals...

But this guitar string spring mechanism does look much better than the rubber band, so I'm going to keep working on it:


I also finished the lower motor centering ring (shown here with the magnets that would be used to secure a a booster F9 to the core of a Falcon Heavy).



I'm currently working on the mechanism to hold the landing leg securely during launch. The small magnet you see at the end of the landing leg is not enough to withstand the tension of the spring (although the magnet works fine for the rubber band version because the rubber band isn't applying much lateral force when fully extended).

Once that is done, I can work on the 1st stage recovery and finish the work started on the second stage.

 

[Switch]

So here's my update. I want to land the Crew Dragon like it landed on March 8, 2020. This means recovering the Crew Dragon separately from the second stage (and "Trunk" service module). So I need an attachment/detachment mechanism so that the Crew Dragon can separate from the 2nd stage (and trunk) so that recovery of the Crew Dragon and second stage are separate. I don't know how many parachutes can realistically be done at this scale but the Crew Dragon used 4 parachutes. So that is the goal. For the Crew Dragon Capsule attachment mechanism, my solution was to embed a ring of magnets in the Trunk and Crew Dragon Heat Shield like this:


You will noticed 16 holes but I determined that only 8 magnets were needed. You'd be surprised how strong the magnetic attachment is.

Then for fun, I setup the Capsule so that it had its own magnetic connection to the heat shield. So that I can put people inside (LOL). Basically I needed mass in the nose cone anyway.

This is the bottom of the Crew Dragon Capsule and heat shield showing the attachment magnets.


And the complete Crew Dragon


In the meantime I have a draft of the 4 landing legs ready for testing. Testing is going to be throwing the rocket off a local parking structure...



This whole damned thing is so fragile.

 

[Switch]

Getting closer to completion. I got the grid fins and interstage working and cut the clear polycarbonate fins. I hid the vent holes needed for the second stage ignition behind the grid fins. The idea is that the grid fins will pop out when the first stage ejection charge ignites.

I still need to debug the landing legs, attach launch lugs, complete exterior details, and complete all three recovery systems.

 

[hermanjc]

This is a great build! Will you be willing to share your 3D Print files when completed?

Keep it up!

 

[Switch]

This is a great build! Will you be willing to share your 3D Print files when completed?

Keep it up!

I'll post the stl files when I have a working version.

I have a lunch window this weekend! The build is not as far along as I had hoped. Lots or details missing but all of the functional parts are ready to do a test flight...

 

[Switch]

I'll post the stl files when I have a working version.

I have a lunch window this weekend! The build is not as far along as I had hoped. Lots or details missing but all of the functional parts are ready to do a test flight...

View attachment 439729

Well, I finally got a chance to launch this F9 & it wasn't pretty. Unfortunately I don't have a video of the launch itself. I used a A8-0 in the first stage and A8-3 in the second stage. I used the lowest powered motors that I had because I wanted to be able to actually see what happened. What happened after a successful lift off to an altitude of about 10 meters was the second stage ignited from the ejection charge of the first stage and then started to tumble. It landed on the ground before the ejection charge released. The flip out fins never deployed & neither did the recovery chute.
Meanwhile the reverse ejection of the motor in the first stage also failed. The motor tube was only ejected a few centimeters but the first stage landed without damage.

Inspection of the first stage motor tube revealed that it took the full thrust of the second stage motor. This makes sense as the motor did not eject as planned. I think the reason the motor tube did not exject was the lack of a centering ring at the top of the motor tube. This was due to the design of the landing leg release ring. The result was that the surface area of the motor tube that was exposed to the pressure from the ejection charge was the area of the BT-20 tube (~254 square mm) instead of the BT-60 tube (~1256 square mm) or about 20% of what I had expected. To fix this, I'm going to have to redesign the landing leg release mechanism and move the top centering ring closer to the top of the core motor tube. This will also give me a much larger space for the recovery chute which had already forced me to use a BT-20 rather than a BT-50 motor tube.



The second stage:

Upon ignition or perhaps as a result of the first stage ejection charge, the second stage motor slipped upwards into the second stage body tube. This prevented the fins from flipping out. It also compressed the recovery wadding just enough to prevent it from ejecting. The solution to the upper motor mount sliding into the second stage body tube is to have a small ring at the mid point which is the diameter of the BT-60 tube (see CAD drawing below).



Another design flaw was the use of arms for the lower fin mounts rather than a disk of the appropriate diameter. The gap between the arms allowed hot gases to partially melt the lower fin mounts and fins. The next version will have a disk at the bottom to support the flip out fins.



The section that enclosed the second stage motor and flip out fins also shows evidence of extreme heat.


This shows the revisions to correct the problems with the second stage noted above. You can see the lower fin mount arm is replaced by a disk and a ring has been added to the upper motor mount to prevent it from sliding into the second stage BT-60 body tube.



I still need to redesign the landing leg release mechanism. Hopefully I can have everything read for my next launch window at the end of March.

 

[Switch]

Well, I finally got a chance to launch this F9 & it wasn't pretty. Unfortunately I don't have a video of the launch itself. I used a A8-0 in the first stage and A8-3 in the second stage. I used the lowest powered motors that I had because I wanted to be able to actually see what happened. What happened after a successful lift off to an altitude of about 10 meters was the second stage ignited from the ejection charge of the first stage and then started to tumble. It landed on the ground before the ejection charge released. The flip out fins never deployed & neither did the recovery chute.
Meanwhile the reverse ejection of the motor in the first stage also failed. The motor tube was only ejected a few centimeters but the first stage landed without damage.

Inspection of the first stage motor tube revealed that it took the full thrust of the second stage motor. This makes sense as the motor did not eject as planned. I think the reason the motor tube did not exject was the lack of a centering ring at the top of the motor tube. This was due to the design of the landing leg release ring. The result was that the surface area of the motor tube that was exposed to the pressure from the ejection charge was the area of the BT-20 tube (~254 square mm) instead of the BT-60 tube (~1256 square mm) or about 20% of what I had expected. To fix this, I'm going to have to redesign the landing leg release mechanism and move the top centering ring closer to the top of the core motor tube. This will also give me a much larger space for the recovery chute which had already forced me to use a BT-20 rather than a BT-50 motor tube.

View attachment 454161

The second stage:

Upon ignition or perhaps as a result of the first stage ejection charge, the second stage motor slipped upwards into the second stage body tube. This prevented the fins from flipping out. It also compressed the recovery wadding just enough to prevent it from ejecting. The solution to the upper motor mount sliding into the second stage body tube is to have a small ring at the mid point which is the diameter of the BT-60 tube (see CAD drawing below).
View attachment 454164


Another design flaw was the use of arms for the lower fin mounts rather than a disk of the appropriate diameter. The gap between the arms allowed hot gases to partially melt the lower fin mounts and fins. The next version will have a disk at the bottom to support the flip out fins.
View attachment 454163


The section that enclosed the second stage motor and flip out fins also shows evidence of extreme heat.
View attachment 454162

This shows the revisions to correct the problems with the second stage noted above. You can see the lower fin mount arm is replaced by a disk and a ring has been added to the upper motor mount to prevent it from sliding into the second stage BT-60 body tube.
View attachment 454168


I still need to redesign the landing leg release mechanism. Hopefully I can have everything read for my next launch window at the end of March.

So from the wreckage of the last failure comes version 1.1:
1: The 2nd stage motor now has a disk at the bottom of the flip out fins.
2: The flip out fins now have two rubber band mounts.
3: A ring was added to prevent the 2nd stage motor from sliding into the 2nd stage body tube.
4: The magnets that secure the landing legs are now on a ring that ejects with the 1st stage motor.

Also new are graphics courtesy of AMX Paper Scale Models. These were printed on sticky label paper.




Here is the back



And here is a close up of the recovery string. My long term plan is to paint the string black and hide it in the LOX line. The point of this string is for the recovery parachute to attach to the top of the 1st stage so that it lands bottom first with the landing legs extended. Recall that I'm using rear ejection so the motor tube ejects out the bottom of the 1st stage with the recovery parachute.


I still need to apply water slide decals to the Crew Dragon. I tried to do that today and forgot to spray the front of the decals with clear spray paint. The ink is water soluble...

 

[hermanjc]

Any chance you can share your flip out fin design .STL files for this and/or the ULA Delta IV? Love the work you are doing!

 

[Switch]

I'm still trying to build rockets that work, so it seems wrong to publish parts that fail. My latest test flight of the Crew Dragon suffered failure of 2nd stage ignition and failure of reverse recovery of the 1st stage.

Analysis of the wreckage:

1: I think the problem with reverse recovery is the multi-part design with multiple sections with coupling rings. Unlike my original Falcon Heavy, the top centering ring of the engine is traversing multiple segments instead of one straight BT-60 tube. Things are just not loose enough.

To address this: I've reduced the diameter of the top centering ring for the 1st stage motor to make reverse recovery more likely.

2: The new flip out fin bottom ring was below the exhaust holes in the 1st stage. So the Champaign cork effect took over. The 2nd stage popped out before the engine ignited.

To address this I've moved the 2nd stage motor upward so that the bottom flip out fin ring is above the exhaust ports in the 1st stage.

I don't have a place to run test flights. This makes progress very slow...

 

[Switch]

Last week was not my week to shine.



The second stage ignited but recovery failed. First stage recovery was a disaster as you can see in this video.

but it was fun to get this bird in the air, and there's lots of lessons learned from the wreckage. Mostly around reverse recovery and securing the parachute...

I'm glad I've been using B6-0 and c6-0 motors on these launches so I can see what is going wrong....