Year In Review

The 2015-2016 school year saw BURPG kicking off development of a new engine, Lotus. Replacing the hybrid Mk. V on the Starscraper rocket, Lotus is BURPG’s first crack at liquid engine development, and has proven to be an incredible learning experience and significant step in the journey to space. As the group takes a semi-hiatus from rocket development for the summer, it’s time to look back on the progress with Lotus, Starscraper, and where things are going next.

Lotus liquid engine on the test stand

Lotus liquid engine on the test stand

One of the major reasons for BURPG’s transition to liquid engines is the faster testing pace they offer. Lotus testing began in December 2015, after a marathon build and prep process during the fall semester. In just the spring semester alone, BURPG executed more engine tests than ever before accomplished in an entire year. Not only that, but Lotus testing requirements birthed a totally new ground support system, software, and upgraded electronics that were refined and matured through the test campaign. BURPG now has a very capable, flexible and reliable test infrastructure.

 

ENGINE AND SUPPORT SYSTEMS DEVELOPMENT:

LOTUS LIQUID ENGINE:

BURPG executed 26 tests of Lotus during the spring semester. Six of these tests were aborted before or shortly after engine ignition due to the detection of faults in either the ground system or the engine itself. While these tests may not have provided data about engine performance, they still yielded data used to improve the engine or ground support system. There were 10 cold flows of Lotus.

Cold flow test 

Cold flow test 

Hot fire test

Hot fire test

There were 10 hot fire attempts of Lotus, 8 ignited, with all tests being 2 second duration. These tests focused on ignition and establishing stable combustion. A brief breakdown of the tests are as follows:

Testing Summary:

Date

Tests

Equipment Notes

December 5, 2015

4 oxidizer cold flows

3 fuel cold flows

-Rev 1 fluids

February 20, 2016

1 fuel cold flow

3 hot fires

-Rev 1.1 fluids: reduced propellant line lengths

 

April 3, 2016

1 hot fire

3 aborted tests

-Rev 2 fluids: panel mounted pressurization system, further optimized propellant lines

-Corrected injector fuel port sizing

April 17, 2016

1 fuel cold flow

1 oxidizer cold flow

2 hot fires

3 aborted tests

-Rev 2.1 fluids: panel mounted propellant feed, introduction of flow meters, panel mounted propellant valves and purge system

-Rev 2 ground electronics and software

-New concrete pad for tank stand

April 30, 2016

4 hot fires

-Rev 2.2 fluids: increased propellant line diameters, optimized flex lines on run tanks

-New blast shield

-New water cooled flame diverter and firex system

-New APCP igniters

The last test of lotus resulted in the copper chamber liner being expelled from the aluminum chamber sleeve. This was from investigations into the length of ox lead needed to get the engine to ignite compounded by inconsistent valve timing. With nitrous oxide as an oxidizer, a precisely timed oxidizer lead is needed to allow the oxidizer to decompose in the chamber due to contact with the burning igniter. With too much fuel being present at the same time, the oxidizer is unable to decompose well enough to allow combustion. The film cooling ports at the throat of the engine make proper propellant timing difficult, which led to combustion outside the chamber during most of the earlier testing. Combustion in the chamber on the first two tests may have been achieved due to the high hydraulic resistance of the original fluids system leading to lower flow rates. Less propellant entering the chamber means the igniter can more easily decompose the oxidizer, even with fuel present, preventing ignition from being smothered.

"Flamethrower" behavior from combustion occurring outside chamber

"Flamethrower" behavior from combustion occurring outside chamber

With each test, the oxidizer timing was being incrementally advanced until proper ignition could be achieved. However, due to a slow valve from trapped pressure and oxidizer bleed-in issues, the timing was incorrect going into the last test and the oxidizer was too far advanced. This led to detonation in the chamber upon fuel entry, creating over-pressurization and failure of the retaining ring holding in the copper chamber liner. This is the designed failure mode for the chamber, since all components that are ejected are comparably low energy and are deflected in a safe direction.

To address the problems that led to the chamber failure, an ox bleed valve is going to be added so the oxidizer will be starting at a consistent place in the feed line, right behind the main valve, with each test. The main oxidizer valve is going to be swapped for a vented valve to prevent pressure buildup leading to inconsistent valve opening times.

Other potential changes are to the engine itself, focusing on making the engine easier to ignite. One is increasing the L* of the engine. L* relates the volume of the chamber to the throat area, with a larger L* increasing the dwell time of the propellants in the chamber. A longer dwell time in the chamber means the propellants will have more time to react. More planning and redesign is going to occur over the summer to allow manufacture and testing to begin as early as possible in the fall semester.

GROUND SUPPORT EQUIPMENT:

To test Lotus, the existing hybrid test stand infrastructure was modified to better suit the liquid engine, and a new ground fluids system built. As mentioned in the test breakdown, there were two major revisions of the ground fluids system.

The first had many of the components and plumbing removable from the stand.  This was found to be subject to high line loss, so the line lengths were shortened where possible.  The setup was still cumbersome, so the pressurization system was built onto a panel, followed by the feed system with the addition of flowmeters in the second revision of the fluids system.

Even after this reorganization, the line losses were still higher than were acceptable, forcing the run tank pressures very high. This led to the upgrade of the feed lines again to a larger diameter. The end result is that the fluid system performs to the requirements of the engine and is fast to set up and break down in the field.

Along with the test stand and fluids system, a fire suppression system was added to the ground equipment this year. The initial setup relied on a pressurized water tank, and a pyrotechnic valve turning on the supply to the nozzles. This was then changed with an upgrade to the blast deflector. A larger tank supplies water to an electric pump, which can be controlled by the ground support electronics. A water spray cools the blast deflector during normal operation, but in the case of a fire needing to be extinguished the flow is switched on to nozzles mounted on the test stand.

SOFTWARE AND ELECTRONICS

With the new requirements presented by Lotus, the ground electronics and software had to be upgraded for testing as well.

The software was substantially rewritten focusing on both increasing its capabilities and reliability. The autosequence and abort handlers were changed to allow them to be more flexible and easily modified, including configuration files that could be changed with the software running, without having to reset it. This increases testing pace substantially as the ground support system does not have to be safed and shut down for the reset like it was before. The abort handler was made more reliable and predictable, and not in need of being reset after being triggered. This allows faster recovery of manual control after an automatically triggered abort.

Usability was also upgraded, including an improved UI with window configuration saving. The UI also changes to reflect abort states for easier recognition of fault scenarios and the safe recovery from them. The software is also capable of running reliably across multiple OSs.

Example screenshot of BURPG's ground control software

Example screenshot of BURPG's ground control software

On the hardware side, the Ground Operation Devices (GOD) Box underwent a significant upgrade. Originally designed for Hyperion Rev A, the front panel and wiring harness was not capable of taking advantage of Hyperion Rev B’s full capabilities.  A new front panel was made allowing all channels to be broken out, along with new features like power control for the fire suppression pump. A new wiring harness was made with shielded wire bundles, further lowering the noise floor on measurements. The design of the new front panel and harness also allows easier access and repair to the devices in the GOD Box.

 

VEHICLE AND RELATED DEVELOPMENT:

While Lotus development and testing was indeed the main focus for BURPG this year, there was further work on the Starscraper airframe and avionics. Most of the airframe work from this year is still in the concept stage, and relates to modifications that need to be made to the original hybrid Starscraper airframe for the new liquid iteration. The avionics too were revamped for the requirements of the new liquid engines.

Airframe work focused on 3 main components; the aft structure, the fuel tank, and the intertank bay, which is the area between the fuel and oxidizer tanks.

The aft structure contains the joint that allows a pair of electromechanical linear actuators to gimbal the engine cluster, and is intended to function as a stand-alone assembly to allow lab testing and characterization of the actuator control loops.

There are two different fluids scenarios being investigated for use on the aft structure. One relies on more traditional flexible tubing for the fuel and oxidizer feeds, which while based off of commercial off the-self equipment and is a lower risk design, has higher line losses and lower performance.

Aft structure concept utilizing commercial flex lines

Aft structure concept utilizing commercial flex lines

The other option being investigated is a custom bellows joint for the ox feed, since this is the higher flow propellant and more susceptible to performance issues due to inefficient feed arrangements. The bellows joint would comprise of a rolled metal bellows in the center of a ring joint.

Bellows joint for oxidizer feed system

Bellows joint for oxidizer feed system

Another item of the airframe being worked on is the fuel tank. The fuel tank follows a similar design used originally in the ox tank, with modified bulkheads. The bulkhead design was changed to be lighter, more inexpensive to manufacture, and have the necessary ports for the ox pass-through, instrumentation, and pressurization inlet and tank outlet. The bulkheads on either side of the fuel tank are intended to be identical for faster manufacture, since there will be no change in tooling for each bulkhead. Final volumes and operating pressure will be determined by engine operation parameters verified through ground testing.

Concept model of fuel tank, showing internally mounted pressurization tank

Concept model of fuel tank, showing internally mounted pressurization tank

The final airframe component being designed is the intertank, which is also an important structural area of the rocket since it transfers load from the fuel tank to oxidizer tank. Housed in the intertank are critical avionics components including the inertial measurement unit and flight computer, as well as control hardware for the fuel tank pressurization. All of this equipment needs to be accessed easily. The structural members of the intertank are struts placed radially around the bay, and are removable for servicing the equipment installed in the bay. There is also a central, non-structural mount for the avionics.

For the avionics on the liquid version of Starscraper, the decision was made to create a new avionics framework that would offer increased flexibility and performance through the use of modular components networked over Ethernet. These components are:

·         Linear Actuator Controller (LAC), which drives the brushless motors used in the linear actuators that gimbal the engine cluster

·         Data Acquisition and Control boards (DQCs) several of which collect data and control valves across the rocket

·         Flight Computer and Switch (FCS) which handles the major navigation calculations and serves as the central communications node of the avionics network

·         IMU board (IMU) which interfaces the internial measurement unit to the avionics network

·         Telemetry Interface Board (TIB) which allows the telemetry system comprised of COTS components to interface with the avionics network

While a modular, Ethernet networked avionics framework is new for BURPG, the avionics components rely on mature technologies developed by BURPG over several years of avionics improvements. These boards are in various stages of development. The LAC has already undergone debugging and preliminary linear control loops have been tested. The DQC is undergoing assembly, with the other boards having completed the layout stage.

Work also began this year on a rebuilt ASTRo, used as a testbed for inertial control system implementation. The ASTRo airframe was rebuilt with minor modifications to allow more reliable recovery and more secure camera mounting. The most drastic change was to the flight computer, which was redesigned to utilize a higher performance IMU; the same one intended to be used on Starscraper. The ASTRo flight campaign will happen during the 2016-2017 school year following the completion of a first revision control loop.

New ASTRo flight computer with IMU

New ASTRo flight computer with IMU

 

WHAT’S NEXT:

Over the summer, BURPG will be working on several things to prepare ourselves for Lotus testing as early as possible in the fall semester. This work will largely focus on the changes to Lotus and test infrastructure as highlighted earlier. Concept development for the vehicle will continue, as will work on ASTRo's control system. The plan is to begin vehicle integration with the start of the spring 2017 semester. Next stop is space!