Model Rocket Launcher WiFi ESP8266 Part 5

Introduction 
This short blog update continues from Part 4 of the ‘Wi-Fi-controlled rocket launcher’. A prototype design featuring 18650 batteries with the ESP modules was tested.

Rocket Launcher 3D Model

Design Change
As suggested in Part 3 of the blog, the design was updated to integrate two 18650 batteries onto the rocket launcher circuit board (PCB). The battery configuration can be set on the PCB to be either series or parallel using jumper resistors.

Dual Battery Series or Parallel Option

The linear regulator on the Adafruit ESP32 Huzzah board can accept up to an input of 6 V DC, meaning that an additional regulator was not required, depending on the battery configuration.

A fully charged 18650 battery is rated at 4.2 V, so the series battery resistor jumper option should be used with the optional DC-DC regulator. Using the DC-DC regulator should also allow very close to the full capacity of the battery to be utilised.

On the ESP32 Huzzah, the input to the linear is protected by a series diode. This series diode reduces the effective operating voltage of the battery by approximately 0.28 V. Depending on the battery manufacturer and the load current, the cell voltage could be around 3.4 V when the State of Charge (SoC) is between 0% to 10%. The dropout voltage of the ESP32 board may prevent full use of the cell capacity.

A pre-wired Multicomp part MCR13-36A2-11 was selected to replace the previous power switch.

Replacement Switch

Testing
The board was populated with the two output drivers, ESP mounting headers and the dual battery holder.

Bottom Side of Launcher PCB

The jumper resistors were configured for parallel operation. The optional 3.3 V DC-DC converter was not fitted. Since there were only small design changes, the board was powered first with a 3.3 V supply, then with a charged 1650 battery.

Top Side of Launcher PCB

It was noted that during testing with a fully charged battery, the ESP32 did not boot correctly. This issue was caused by GPIO 15. The Status LED is connected to GPIO 15, which is connected to the 3.3 V supply through a current-limiting resistor. A pull-up on GPIO 15 changes the ESP behaviour. To resolve the issue, the LED will be driven to 0 V on the revised design. A pulldown resistor already exists on the ESP board.

Other Changes
To measure the 18650 voltage, the onboard voltage divider values were adjusted to meet the ESP's maximum 1 V limit. The resistor footprint was standardised to 0603. This feature will be tested with the igniter in the next post.

Drill Press Controller Update Part 5

Introduction 
This post details the validation of the remaining hardware item on the drill press controller and Printed Circuit Board (PCB) updates.

Debug Validation
A series resistor was fitted between the microcontroller (PSoC) and the debug (TTL RX, TX) connector to serve as a basic type of input protection. During testing, the serial communications (USB to TTL converter) were intermittent, so the series 100 R resistor was reduced to 10 R to ensure reliable operation.

Updated Serial Input Resistors

PCB Update
As noted in the previous blog, the footprint of the input device (MAX22191) on the PCB required updating.

Updated Footprints on ESTOP PCB

During the PCB update, the top-layer silk screen text near the input and output connectors was increased to 1.5 mm.

Updated Text at Inputs on PCB

 

Updated Text at Outputs on PCB

To allow for easier hand soldering, a selection of component pads was increased.

ESTOP Mains Input Capacitor Pad Sizes

Updates were made to the power supply (AC-DC) and mains input capacitors.

ESTOP Mains Input Capacitor Pad Sizes Updated

The image below shows the updated board.

ESTOP PCB Update

Next, building the updated PCB, reviewing the mounting of the solid-state relay in the original enclosure shown below, and testing with the existing drill press setup.

Existing ESTOP Electrical on Drill Press

TDR Pulse Generator (Microchip) with Housing

Introduction 
This post continues a previous post that used a Microchip microcontroller and a buffer chip to generate a fast-rising edge. In this edition, the design is updated to suit an aluminium housing.

Assembled Pulse Generator Unit

Another Release
To cater to those requesting a USB-powered unit in a ready-made enclosure, the same Microchip microcontroller (ATmega16U2) and driver design from the previous blog were used. Some design changes were made to improve manufacturability, such as using a right-angle PCB-mount SMA connector.

For a small sample run of the new design, the rise time is comparable to the previous design, at 1.1 ns and faster than the original design at 658 ps.

Design Changes
The Microchip microcontroller (ATmega16U2) can be USB-powered (5 V) and operate at a logic voltage of 3.3 V. An external linear regulator was chosen instead of using the UCAP 3.3 V output supply from the ATmega, as the UCAP supply is intended for only low-current loads.

The self-powered connection example is shown in Section 20.3 USB Module Powering Options of the ATmega16U2 datasheet (doc7799.pdf).

ATmega16U2 Bus Powered Example (Courtesy Microchip)

With the power supply reduction to the ATmega I/O, the hex Schmitt-trigger power supply was also reduced to 3.3 V. The output performance was tested with Nexperia parts 74AHC04APW and 74LVC04APW.

To support DFM (Design for Manufacturing), the hand-soldered SMA connector from TE Connectivity (5-1814832-1) was replaced with a PCB-mount right-angle part from Molex (73391-0080).

Updated Pulse Generator PCB (2D)

Updated Pulse Generator PCB (3D)

Lastly, the board shape was resized to suit a metal case. A Hammond aluminium case (1455D602BK) was chosen for the PCB. This case was at the time less than USD 8 in quantities of 10 pieces and available at several online stores. A 3D-printed plastic case would also be an ideal, cost-effective alternative.

The plastic end caps supplied with the Hammond enclosure were replaced with PCBs. This change was aimed at a more turnkey solution, meaning the pulse generator PCB and end caps could come from the same PCB fabricator.

Pulse Generator PCB Assembly Modelled in Fusion 360

Designing the end caps was performed after the pulse generator PCB was completed, with some adjustments.

PCB End Cap (SMA End) in Fusion 360

 

PCB End Cap (USB End) in Fusion 360

After the pulse generator PCB was loaded into Fusion 360 and the end cap dimensioned, the PCB end caps were created in the PCB tool.

PCB End Cap (SMA End)

 

PCB End Cap (USB End)

Modelling in Fusion 360
The pulse generator PCB dimensions were chosen after making physical measurements of a Hammond aluminium case.

Pulse Generator PCB with Hammond Enclosure

Hammond provides a CAD file for the aluminium case; however, a physical measurement for the PCB width was performed because of the powder coating.

Assembly and Tests
The pulse generator PCB Rev 3 and end caps were sent for manufacture. For the 50 Ω controlled impedance on the pulse generator PCB, a predefined layer stackup was selected from the PCB manufacturer's options.

Pulse Generator Blank PCB

After assembling the pulse generator, programming and performing validation checks, the pulse generator PCA was slid into the Hammond case – almost. Checking another case, the PCA slid in very tightly. To make the PCA fit in all the aluminium cases, light sanding was required on the long edge of the PCB – tolerances and case anodising were possibly the culprit.

To fit the PCB end caps, the Hammond-supplied thread rolling steel screws #6 x 3/8" were changed to a set of salvaged pan head Philips screw. Using this type of screw is not ideal, and it should be replaced with a thread-cutting type to suit the C-shaped screw port in the Hammond case.

Pulse Generator SMA End View

Changing the straight SMA to a right-angle SMA connector had a negligible effect on the rising edge.

With a 74AHC04PW fitted, the rise time (20-80 %) was 1.18 ns. Using a 74LVC04PW, the rise time (20-80 %) was 759 ps.

Pulse Generator Output Rise Time (74AHC04)

Pulse Generator Output Rise Time (74LVC04)

Summary and Future Updates
As noted by readers, this design is slower than other designs offered on the market. However, as readers continue to use this design, write their own USB interfaces and highlight the need for faster solutions, new posts on this subject will be made available.

Engineering Files
The same Atmel Studio project applies to this project. Below is the updated revision 4 schematic, PCB Gerbers and PCB BOM for download. The Rev 4 includes only a minor top-layer change near the SMA connector.

In addition to the PCBs, the Hammond case 1455D602BK and suitable screws are required, for example, M3 x 8 mm thread cutting.

Pulse Generator Microchip Studio Project

Pulse Generator Schematic Rev4

Pulse Generator PCB Gerbers Rev4

Pulse Generator End Cap 1 PCB Gerbers Rev4

Pulse Generator End Cap 2 PCB Gerbers Rev4

Pulse Generator Bill of Materials Rev4

Model Rocket Engine Test Stands

 Introduction 
This blog provides ideas and an example for 3D-printed metal rocket test stands designed to accommodate the popular Estes-sized model rocket engines.

Rocket Engine Holders

Why an Engine Test Stand?
To conduct rocket engine tests and measurements, a model rocket engine test stand was required. The stand was designed to serve as a motor anchor and mount on the horizontal axis. Since pricing from 3D metal printing companies has become more cost-effective, two stand designs were tried. Cost-reducing changes were implemented in the first revision test stands with a focus on reducing the printed weight.

Development and Manufacture
Autodesk’s tool, Fusion 360, was used in the development of the rocket engine holders. JLC3DP performed the 3D printing in stainless steel (BJ-316L). 

What Changes?
Since 3D printing can be charged by weight (or volume), and because the thrust axis of the rocket engine is well established, the side walls of the rocket engine stand were partially excluded. The exclusion (cutout) in the stand body, as depicted in the images below, reduced the weight by approximately half. Additional changes could be made to reduce the weight if the focus were on cost-effectiveness, and with that in mind, the CAD files are provided at the end of this post for download.

3D Printed Test Stand
The first test stand was a simple prototype for Estes A-C engines. Some engines did not fit entirely into the 3D-printed metal holder, while others were easily inserted. This was because there are variations in the diameter of rocket engines, and older engines may have expanded with age.

Model of Model Rocket D Engine Holder

Model of Model Rocket C Engine Holder

Some videos below show how the motor was mounted. As a side note, the exhaust temperature was measured at almost 600 C, 100 mm from the engine nozzle.

 

 

 

Downloads
Files are exported from Fusion 360.

Fusion 360 C Engine Holder Model

C Engine STL File

The D engine holder model below has a 0.5 mm larger bore than the model in the above videos.

Fusion 360 D Engine Holder Model

D Engine STL File