1/47th Semi-Scale SpaceX Starhopper, aka "Deskhopper" Scratchbuild

[eugenefl]

Ahead of the attempt by SpaceX to test their Raptor engine on the Starhopper test article on 8/26/19, I released an image on various platforms of my scratch build of the Starhopper that I am very proud of and excited to share with you here.

Brief background: I've be a TRF member since late 2001, or early 2002, hard to recall, but I've been an avid supporter of the hobby and basically lived out most of my "online" existence on TRF. I was a child rocketeer in the early 90's, became a BAR in 2001, NAR L1, left for a while, and am now going through a Re-BAR cycle post-40 years of age. Fun fact: I had a brief partial ownership of the forum during some challenging times while in transition into v2.0. Most of my contributions can be found in the old TRF 1.0 archive. I hope not much has changed as I've been away from the hobby for a few years. Having navigated a few social media platforms with rocketry themed groups recently, I still think TRF might be the best long threaded format to document a build, share information, and collaborate with other enthusiasts. I am glad to see this forum is still alive.

For starters, attached is a photo of the near-completed model which I've interchangeably dubbed "Deskhopper" and "DesktopHopper" although I think "Deskhopper" sounds more synonymous with the real thing.

Quick design specs:

- 1/47th Semi-Scale "Eyeball Engineered"; most measurements were taken from visual imagery of the actual test article. I worked on this many late nights after the family was in bed. Being tired played a role in saying "Ah, that's good enough."

- Initially designed for use with a 24mm SU AeroTech F21-4W, I installed a core 24mm midpower MMT tube. That may have been a design error as this model is nearly at neutral stability, with a slightly positive CG/CP relationship and will require more nose weight likely making a 24mm MMT inadequate based on motors I've researched. I may at some point either add 2 side core MMTs and cluster it or remove the 24mm center mount and install a 29mm tube.

- Diameter = 7.7", air frame is made of internal 1/4" foam skeleton discs wrapped with poster board.

- Leg/Fins = this was a buildup of dowels wrapped in body tubes to simulate the thickness of the legs used on the real test article.

- Nose cone = Floral foam half-sphere, shaved down to approximate height and curvature. This will require a little more trimming, but being that it's "sport scale" I can live with it. If I increase any variables in thrust, I may shave it down again, and coat it with a lightweight fiberglass. At the moment it's "static display ready" with wood filler, primer, and silver paint.

- "Raptor" engine (removable, display only) = Estes Nose Cone (#72057), cut to size. The remainder of the nose cone will make a nice boat tail for some other build.

- "Stainless Steel" = Oy...80% of my time was spent painstakingly hand cutting and placing each piece of self-adhesive chrome reflective vinyl. Originally intended to have panels cut by a Cricut machine, my wife was on Elon time so I had to move forward and use a ruler and hobby blades. I may have to learn the interface for that thing so I can cut my own stuff in the future. But, it was worth it. The panel counts are accurate to the exterior of the test article. Mis-alignment was intentional, and very much a relief for an OCD person like me. I'm glad SpaceX was in a hurry too.

- Build time = About 1 month start to finish.

Main Goals/Drivers:

- Reconnect with the hobby and it's people via social media and in-person launches. I've been away for a while. I figure a project that could gain visibility and interest wouldn't hurt. There are lots of nice folks in rocketry.
- Feel like I'm a part of the commercial space race, at hobby scale.
- Commemorate a test article that is now retired after only 1 tethered hop and 2 test hops.
- Build a flyable model that likely won't get much replay in the hobby enthusiast market, but will be recognized by space fanatics.

Stretch Goals:

- Merit a "like" from Elon or Gwynne Shotwell, or both, but for very different reasons. That'd be instant printable souvenir material. I'm afraid I might never meet Vern or Gleda in person for an autograph , so I'd be thrilled to garner the acknowledgement of the man at the forefront of the SpaceX company himself or the lady that drives the operations and growth.

- Accomplish a "scale like" hop. I've run enough calculations to determine that it is entirely feasible on an Estes E9-P.

- Consider a conversion to use a BPS Signal R2. The real test article was intended to hover. I think the Signal R2 is the perfect application for this scale model. Of course, the model would have to achieve a high enough altitude to survive a parachute deployment, but the low thrust and low speed climb would look amazing.

Design Inspiration: A few names come to mind.... Tim Dodd (Everyday Astronaut), @JoeBarnard , Oli Braun (Buzz Space Models), Boca Chica Maria, Scott Manley, Martin Molin (of Wintergaten/Marble Machine), countless spaceflight photogs, and the many nameless/faceless SpaceX engineers that do this stuff for real. We just get to be the fun cheerleaders building flyable trophies in our spare time.

I'll be adding some build photos in the days to come. Please let me know if you have any questions, comments, suggestions, etc.! Thanks for checking out this really long initial build intro thread.

 

[Cape Byron]

Mmmm. Shiny. Looking forward to the thread!

 

[OverTheTop]

Nice work .

 

[eugenefl]

Like many scratchbuild projects, this one started last month while roaming the aisles at Michael's Arts/Crafts. With the upcoming SpaceX test looming on the horizon, inspiration struck. (All model rocketeers have this in common. LOL...junk finds, craft stores, thrift stores, yard sales, toy stores, dollar stores, etc.) I've incorporated foam into rocketry projects before, but the half-sphere looked like something that could easily be worked into the profile of the Starhopper nose cone. I did a quick lookup online, and if you picture it, there I stood in the aisle holding my phone sideways and the 1/2 sphere visually comparing. My 13 year old daughter, always astute in her observations, sarcastically proclaimed "Dad, what are you doing?" I settled on the 7.7" figuring the finished weight of the model would be in mid power territory. During that same visit I picked up some dowels, 1/4" foam board, and poster board - components that later were the legs and airframe/bt.

Quick scaling math: Starhopper air frame diameter = 30ft, or 360" / 12 = 46.753 or roughly 1/47th scale.

 

[neil_w]

This is cool.

 

[boatgeek]

This is a super cool project. To get around the liftoff weight issue, could you use an F39 Blue instead of the F24W? The flame would look more "right" though it wouldn't burn as long.

 

[eugenefl]

I definitely have not looked at the rest of the available 24mm engines nor their performance characteristics. My biggest concern on high thrust and short duration is that this isn’t the most aerodynamic design. I’ll run more sims later. I also don’t know at what point the internal 1/4” foam ring skeleton that also doubles as centering rings would collapse under high thrust. I’ll post more build photos this evening. Perhaps collaboratively we can figure out some better options. I’m also not opposed to making a different model altogether better suited for high thrust flight, aka overbuild. Thank you for your comment/suggestion!

 

[boatgeek]

I definitely have not looked at the rest of the available 24mm engines nor their performance characteristics. My biggest concern on high thrust and short duration is that this isn’t the most aerodynamic design. I’ll run more sims later. I also don’t know at what point the internal 1/4” foam ring skeleton that also doubles as centering rings would collapse under high thrust. I’ll post more build photos this evening. Perhaps collaboratively we can figure out some better options. I’m also not opposed to making a different model altogether better suited for high thrust flight, aka overbuild. Thank you for your comment/suggestion!

I'll grant that this is eyeball engineering, but the center core should be fine for this amount of thrust as long as it's glued up well to the body. Foam core board is surprisingly strong. You need it to hold about 10 pounds, which isn't too terrible. My first thought was a CTI F51BS, but those motors start getting ridiculous (and fun!) fast and Aerotech has a SU in the right size range.

 

[neil_w]

Honestly, a low-thrust F sounds best to me; this thing is gonna drag like a saucer, so long slow burn is likely better. If you had a 29mm mount in there I'd say the Estes F15 would be great.

Dunno about stability, but I wouldn't be surprised if base drag made it stable even with marginal CP/CG relationship. Could try entering into OR and adding the base drag correction and see what it looks like.

This would indeed be a fantastic candidate for the BPS system.

 

[eugenefl]

The next main task in developing the model was figuring out what materials would stand up to hard landings. Being that the fins were also a part of the legs, I figured I'd frame the fin-legs with dowels, and instead of balsa, more 1/4" foam core. Of course, I needed some rudimentary angles to design off of so a bit of image scaling to size, trace, then transfer to a drawing. I also had to account at this phase the approximate thickness of the body tube that would "wrap" the dowels. I didn't want to risk a hard landing on Estes tubing causing a kink in the tube. This was one of those weight trade-off decisions that I think I can live with.

 

[eugenefl]

@neil_w - I don't fully trust my OR skills, but this sim was intended to try and get a ballpark figure. The "fin" design for this model is awkward as is being that the top of the legs is effectively a BT20 tube. I guess that would make the fin thickness .74-ish with a rounded leading & trailing edges since the "skins" cover from leading edge to trailing edge. I could use a lot of help with the program, but honestly, I wanted to get a guesstimate and use as many realistic values for the components as possible. The CG, according to the OR sim, is almost spot on for having weighed and balanced it by hand. I still haven't installed a recovery system and the max lift-off weight recommendation for an F21W is 18oz.

Per @boatgeek suggestions, I ran some other sims with these results: The F21W (max lift-off weight exceeded) has an apogee of 462ft, the F24W at 372ft, F39T at 406ft, and the CTI F51BS at 401ft. I think my only option at this point, if I want the model to stay true to the single Raptor design, is to rip and replace on MMT size from 24 to 29mm. EDIT: Else, I can bore out 2 holes directly adjacent to the center MMT and add 2 more 24mm parallel tubes for an inline cluster. That would actually be easier. Also, I'd omit a black power ejection from those.

 

[neil_w]

If you'll attach the OR file I'll have a look-see. I do think the base-drag correction is going to change things up quite a bit on this one.

 

[mbeels]

This looks like a really cool project! Well executed build.

Did you consider any of the 3 grain CTI 24mm motors? They'll have more impulse than the F21/F24/F39 options you were looking at. The F30 long burn may be a good candidate.

 

[eugenefl]

This looks like a really cool project! Well executed build.

Did you consider any of the 3 grain CTI 24mm motors? They'll have more impulse than the F21/F24/F39 options you were looking at. The F30 long burn may be a good candidate.

Sounds like a good reason to get into CTI hardware. I've honestly not launched a rocket in about 3 years and haven't looked at the availability of commercial hardware or motors. The F30 you referenced does appear to fit the bill. If I can at all solve for the thrust/weight issue with this model I'm ok "justifying" some hardware purchases.

 

[eugenefl]

If you'll attach the OR file I'll have a look-see. I do think the base-drag correction is going to change things up quite a bit on this one.

See attached. Thanks for offering to check it out!

 

[neil_w]

Here it is with base drag correction:


If you trust this model, than you have plenty of stability. I'm not sure if the base drag correction should be included in flight sims, or just for CP calculation. I briefly re-read the Apogee newsletters that describe it, but don't recall them clearly explaining that aspect.

My general take is that this rocket should be stable, but I don't consider myself expert in this area. What do others think?

[aside: the motor looks so small and lonely in there ]

 

[eugenefl]

In an earlier post I showed a photo of the internal foam disc skeletal structure. In this series of images, I've already wrapped the skeletal frame with poster board and glued the discs in place. I printed out a fin marking template to align the 3-leg-finned rocket, made marks, and inserted the first leg into the base of some corresponding foam discs. I had to hand drill some pilot holes so I could bore out and fit the dowel. The main attachment points are the top and bottom dowels for rigidity, and of course, the root edge of the 1/4" foam panel "insert" to the leg frame structure. The thought is - dowels glued together, dowels supported by inserts, and entire unit glued to root edge and internal discs. Once satisfied with the assembly, all 3 leg-fin units are installed and for the first time I propped up the model and put its goofy overly rounded half-sphere foam nose cone on for sizing.

To get the legs to near scale size, I decided to "cover" the dowels with BT20 tubing. Of course, I only had enough to cover the lower legs, so I decided to "roll my own" for the upper parts. I used a much thinner cardstock to roll tubes using a slightly diluted white glue so it'd smear. The finished product was way more rigid than I could have hoped for. All tubes had to be sliced down their mid sections so they could be "wrapped" over the dowels. Top and bottom tube angles were measured for fitment. This took a bit of trial and error. Fitting 2 cylindrical tubes together at an angle was a fun challenge, and likely one of the more difficult parts of this build.

 

[boatgeek]

Sounds like a good reason to get into CTI hardware. I've honestly not launched a rocket in about 3 years and haven't looked at the availability of commercial hardware or motors. The F30 you referenced does appear to fit the bill. If I can at all solve for the thrust/weight issue with this model I'm ok "justifying" some hardware purchases.

The CTI 24mm motors are an awful lot of fun. They have a lot of variety and tend toward higher thrust than you see from AT (F85! G150!). That said, they do ship hazmat, so they're kind of expensive unless you're already buying a hazmat shipment or you have a dealer nearby you can go to.

 

[eugenefl]

I'll add a personal anecdote to this build thread, that hopefully you can relate to or have yourself experienced. At this step of the build, as I was admiring the model sitting awkwardly on my build desk with its twiggy dowels, I had a nostalgic flashback to 1990 - the first year I got into rocketry as a kid - and was reminded of a kit I once considered buying. At 2.6" in diameter, the Estes Alien Space Probe (ASP) already looked like an old retro sci-fi classic. The rocket sat a bit goofy and had a faux cockpit essentially represented by a decal. I was really attracted to this kit because if it's beefy size, but because my thoughts of the future were tainted with more futuristic space vehicle designs, and because the rocket seemed a bit under powered for its stout size, I never did buy the rocket. What stands out to me as an adult, and the recollection I have that is as clear as day, is that I remember standing there in the hobby aisle of a now defunct KB Toy store at the local mall where my dad worked. After quietly deliberating, multiple times in the day, I still remember making that final decision to slide it back on the display peg. While I wanted the bigger sized rocket, instead of the ASP, I ended up buying a Voyager II with the limited funds I had from helping my dad with yard work, washing cars, cleaning the garage, etc. Ironically, I had considered modifying the ASP to make it look more futuristic because even in 1990 the ASP looked dated. The Voyager II ended up winning out on my decision that day. The kit was shoe-in since I had recently visited Kennedy Space Center on a field trip, and clearly, there was NASA bias on my mind. Despite being a much smaller rocket at 1.325", both kits used 18mm engines and roughly had the same performance, so I took home with me the Voyager II. Eventually, the rocket got dirty ('cause it's white) after multiple flights, some of the decals peeled off, and I remember the disappointment in the rocket now not quite resembling the cover artwork. So, I did what any kid would do and sanded it down. I repaint the bare kit glossy black - I always loved the shine of a black rocket. I hand painted the canopy and mock light pods silver and added chrome vinyl accents on the tail fins, body tube, and other places to make it look even more sci-fi futuristic. Ironically, here I am near 30 years later quietly reliving the memory of that choice, and reliving the chrome vinyl decal experience - all of it - with the "Deskhopper" build. This, my friends, is what rocketry is all about for me - reliving the excitement of my youth and the unfulfilled promises of space exploration or a career in engineering. Stay young. Stay imaginative. Build. Fly. Create.

 

[mbeels]

I'll add a personal anecdote to this build thread, that hopefully you can relate to or have yourself experienced.

Oh my, yes. I remember the 1991 Estes catalog very vividly, each page.

I mail ordered the Estes Alien Space Probe and built it as a young teenager. Other things happened, and then last year I found it. I had survived years sitting in my parent's basement and various places of storage. I finished it 25 years later, painted it red, and bought replacement vinyl from Stickershock23 because the original decals were long gone.

I also remember picking out rockets and weighing the price difference between a $11.95 rocket and a $13.95 rocket. Those $2 were a big deal on newspaper route money.

As a youth, I did imagine a career in aerospace, and while that didn't happen for me either, I did go down a path into the sciences. But yes, rockets are still fun!

 

[eugenefl]

Here it is with base drag correction:
View attachment 391872

If you trust this model, than you have plenty of stability. I'm not sure if the base drag correction should be included in flight sims, or just for CP calculation. I briefly re-read the Apogee newsletters that describe it, but don't recall them clearly explaining that aspect.

My general take is that this rocket should be stable, but I don't consider myself expert in this area. What do others think?

[aside: the motor looks so small and lonely in there ]

On a much smaller rocket, I might consider a swing test to validate stability, but I could see my attempt to get it moving go badly and damaging the model from testing. At this point, if/when I decide to make a flight attempt, I'm going all in. I'll snap many photos before and then "send it." OR, I might actually make a smaller scale model based on a 4" dia tube and call it a day. I've been having conversations with my wife (who is a 4th grade teacher) and have been asking her if her school does an STEM or "Great American Teach-in" days at her school. I think this model as a static display could capture some imaginations!

 

[eugenefl]

The leg-fins were a bit of a challenge overall. Between rolling my own tubing, then later figuring out what geometry makes them sit correctly and what cuts to make, I finally got everything situation. Happy with the appearance, I glued them all into place. You can see in one of the photos where somehow I made a measurement error and cut the tube length too short. So, of course, I cut a piece off to make up for the short tube and glued it on to make a completed length. I later sanded the seam, added filler, and called it good enough.

The next sequence in the build process was to cover the leg fins with "skins" as well as the "fillet" areas that run the length of the air frame to the top of the body. More tricky geometry was involved here trying to get flat paper to bend and shape to 3 dimensional areas. You'll also notice in one of the images where the leg length was too long. I marked my cut-off lines and shortened those before adding the round foot pads.

 

[eugenefl]

The next task I tackled was adding a launch lug and designing a "removable" Raptor engine bell housing.

Of all the methods to custom make a bell housing, I settled on chopping the tip off of a BT60 from an Estes US ARMY Patriot. (I think I noted the part # in the initial build thread message.) Anyhow, I tried a few other ways to make a cone shape including stacking 1/4" foam mini-discs, printing out a paper cone, and even cutting slits into a tubing to try and form a conical shape, but was quickly dissatisfied with the results. While I probably could have turned a cone lathe-style on a drill press, I still wanted the hallowed cone so it looked a little bit more like an engine bell housing. Satisfied with the semi-scale shape, I glued the Raptor to a spent D12 casing. The remaining shoulder of the BT60 nose cone now makes for a nice boat tail on a future scratch build, so all is not lost. Of all the features on this model, I think I like the Raptor engine the most.

The launch lug is brass tubing for a 1/4" rod. I mounted it to a thin strip of plywood, plywood to body tube, then a generous coating of epoxy to make the unit one assembly. Not too different from all of my other mid-power scratch builds.

 

[gwh]

@eugenefl this is an awesome build! Nicely done.

@neil_w I'm wondering how the base drag will affect my Starhopper model, hoping you can add some input? See the post here: https://www.rocketryforum.com/threads/spacex-starship.151484/#post-1915729

 

[eugenefl]

Fun with foam - not. This was probably my least favorite part of the build, so much so that I probably needed to shave the thing down a bit more to bring it closer to scale. Foam of this type is probably just messy and not very satisfying to work with since I know it needs a lot of help. Quite honestly, when I got close enough, I called it a day, but in reality the dome height is probably about 1/2" to tall. This being said, I will likely need to repair dents later on as the foam was filled with white glue, wood filler, primed, then painted (with sanding steps in between). So I'll call it good for now and have some margin for repair later, possibly by covering it in a lightweight fiberglass. Ironically, fiberglass was the preference when I started, but weight was becoming an issue far too early in the build.

I derived a profile from images found on the internet and traced it in Photoshop (all in scale). I then printed and transferred the template to 1/4" foam board so I could test fit after several passes of shaving down the foam.

 

[eugenefl]

I knew nose weight would play a factor in this design. My approach here was to create a base for the foam to sit on, and on top of the "base plate", I would epoxy the weight. Weight of choice was BB's in 3 small BT50 "cups." The cups each hold 2oz of weight, are glued to each other, then glued to the foam dome. The entire assembly was glued together, and for good measure, I added to of those really thick and spiraly plastic wall anchors. They really had some bite and pulled the base plate and foam cone/dome together.

Not pictured is the finished underside of the nose cone. The assembly "sits" on the base of the model with the edges of the base plate of the cone resting just inside of the upper air frame. Think of it as a reverse shoulder. The very center of the nose cone has about 1.5" of tubing protruding downward - this part inserts into the center tubing which is also the motor mount. This design decision in retrospect leaves very little room for a recovery system. As I built the model I later worked out a possible solution to this problem and that would involve modifying the top disc that holds the structure together, or bore out the base of the nose cone. At this point I have to make space.

 

[eugenefl]

The joys of finishing. You sand, you prime, you repeat. On top of the task of traditional priming and painting was the effort to give the model a scale appearance based on its polished stainless steel panels. I actually counted the panels on the real test article, scaled them down to size, then began the process of cutting each one for placement on the model. The Starhopper exposes areas of the legs that are not covered by stainless steel so painting the whole model silver before applying the chrome vinyl was probably overkill.

 

[eugenefl]

My last post brings the build process current to the finished model. The next posts, which are likely to be spaced due to personal time constraints, will largely focus on an upcoming "hop" attempt. More aimed at the novelty of controlling a severely under powered model, and probably outside the boundaries of what is considered "model rocketry", but in a controlled and as-safe-as-possible manner I hop to achieve a simulated and scale-like hop, albeit to a short altitude. I actually ran a sim that accounts for a peak altitude of about 4 feet. In the absence of active motor gimbal assistance, we either rely on airspeed for fin guidance, or, in this case, a long launch rod to keep the model from veering off. Here are some more of my thoughts:

The goal is to get the model off the ground in a controlled fashion, but not have enough velocity to clear the launch rod, and have it slowly creep back down the rod and touch back down while under power - a simulated hop! The simulation data looks very promising and actually aligned closely with the manual calculations I did. To achieve this feat I had to examine the thrust curve data of the Estes E9 to understand its performance characteristics during the burn. To match the motor characteristics for the flight and achieve the desired "hop", I have artificially weighted down the model to 36.2oz fully loaded. According to the thrust curve data of the engine, the peak thrust output is 19.47N for only .25 (1/4) sec of the initial burn at which point the engine takes a rapid .25 (1/4) second to drop down in power and stabilizes at 8.75N for 2.5 seconds for the remainder of the burn before abruptly extinguishing. Here's the kicker - in order to continue gaining positive altitude at 36.2oz, the engine would have to produce more than 9.45N throughout the burn, but we've established that it settles in at 8.75N for the duration of the burn (or 2.5 seconds) after the initial .5 (1/2) second thrust.

So, according to calculations and sim data, with that .25 (1/4) second initial thrust, the model should travel no higher than 4.12ft off the ground in total, and since the thrust is incapable of positive vertical altitude, the model will actually "weigh" down on the exhaust and achieve a controlled negative altitude! The usable launch rod length is 3'11", but the sim has it exceeding this height, so to account for this I will do 2 things: 1) attach a fishing line to the rocket and anchor the other side to heavy elastic which when combined will prevent the rocket from accelerating past the top of the rod and, 2) attach zip ties and duct tape at the tip-top so the launch lug physically can't slide off of the end of the rod. Both of these should prevent the model from leaving the launch rod and going off vector should the calculations be off. What's really amazing is that the simulation shows negative altitude or negative vertical velocity (aka descent) and motor burnout about 3-4" off the pad! Attached is the simulation data plotted out. This all of course is largely dependent on the consistency of the motors performance. It'll actually work really well, or not at all.

Also attached is a photo of me test fitting an 808 camera to achieve "Raptor" view. SpaceX has an area of at least 15' to work with while I have about 3.5". While the math accounts for scaling of a model to a much smaller size, I'd probably need a 4mm fisheye lens with a macro lens of some kind to gain a much wider and in-focus view. For the novelty of internet, I'll go with what I have for now.

 

[neil_w]

The finished model is gorgeous. The proposed flight profile is... non-traditional. Good luck!

 

[mbeels]

Wow, fantastic results, the rocket looks great! And a scale-like flight would be something to behold, I'll be watching!