BestCoastBrockets
Long Shot
Long Shot is my attempt at a space shot using a liquid bi-propellant rocket that has a low thrust-to-weight at launch in an effort to avoid most aerodynamic heating on ascent. The low thrust-to-weight presents different engineering challenges such as requiring active stability and a very long burn time when compared to other amatuer liquid rockets. For this project I am attempting to complete at least 90% of the design for every system and get feedback from industry professionals in design reviews.
Long Shot Specs:
Dry Mass:
Liftoff Mass:
Thrust:
Burn Time:
Specific Impulse:
Diameter:
Length:
Apogee:
100 kg
250 kg
750 lbf
146 s
> 310 s
12 in
27 ft
> 100 km
Rocket Systems
Structure
My rocket structure is directly based on Robert Watzlavich's Rocket 1 structure which can be found on his project website and is linked in the Rocket Design Resources. He used L-brackets as stringers and thin aluminum bulkheads which was wrapped and strengthened by thin sheet aluminum skin riveted and screwed into place. The number of fasteners adds a lot of time to assembly, but the dry weight of Rocket 1 was right around 45 pounds which is something I've never seen before while using mostly commercially available components. Since mass is one of my main concerns for this project this method is very promising.
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My structure differs slightly because my tanks are integral to the structure to help reduce overall length and provide some more rigidity. I may end up making a custom fiberglass body tube or skin, but currently my design uses 0.032 inch thick aluminum sheet. The target dry mass for this project is 100 kilograms. This seems like a very large mass when compared with other amateur liquid rocket projects, but it gets taken up quickly by the mass of the propellant tanks, other parts of the structure, and the fluid system. After running some simulations of an ideal flight, it looks like I do have some mass margin, but that is something I only want to use in the event of a last minute necessary design change. The current structure of Long Shot without the skin can be seen below.
Fluid System
The fluid system of any rocket has the same responsibility, to contain all the propellant needed for the rocket as well as the pressurant and to deliver propellant to the engine at the designed pressure and mass flow rate to sustain a nominal burn. These systems can get quite complex and uses a type of diagram called a Plumbing and Instrumentation Diagram (P&ID) to show all the flow lines and flow devices in a system in a simplified form to make it clear what components connect together. This is a good method to determine what components and sensors are needed for a system and where they should be installed. I use the first few versions of a P&ID to sketch out the general layout of a system and get a part count for each component. If you are interested in seeing Long Shot's P&ID, it can be seen and downloaded by clicking here.
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Using the design equation resources on the Rocket Design Resources page, the total mass flow rate can be found. Using the mixture ratio (MR), the mass flow rates of the propellants can also be found. Long Shot's engine MR is 1.1 and needs to be supplied with 0.534 kilograms per second of liquid oxygen (LOX) and 0.485 kilograms per second of ethanol at a chamber pressure of 400 pounds per square inch. In the current design, the engine burn time is 146 seconds which we can use with the mass flow rates to calculate the total mass and volume needed for both propellants. The total mass and volume of LOX needed is 77.96 kilograms and 68.33 liters while the total mass and volume of ethanol needed is 70.87 kilograms and 88.99 liters. As a general rule of thumb, 20 percent ullage is used in the propellant tanks to act as a buffer to any pressure transients the regulators cause. This means the physical LOX and ethanol tanks will have a volume of about 82 liters and 107 liters respectively.
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To find the pressurant tank volume and necessary fill pressure we can start with the condition of the vehicle at the moment of burnout where in a perfect world the pressurant tank and propellant tanks all have the same pressure. Currently the designed tank pressures are 550 pounds per square inch in both tanks. Using the ideal gas law, we can find the number of molecules needed in the volume of the tanks to create 550 pounds per square inch, making sure to include the planned volume of the pressurant tank. Taking the number of molecules and the volume of the pressurant tank, the pressure required can be found. I am currently baselining a 30 liter nitrogen tank at a pressure of 3,200 pounds per square inch. When running altitude simulations, it is important to include the mass of the pressurant because even though gas is very light and generally not dense, on larger projects such as this one the mass is not negligible. The nitrogen mass in the pressurant tank is about 7.5 kilograms. Some of you may be wondering why I'm using nitrogen when it has drawbacks such as its molar mass and susceptibility to condense at LOX temperatures (which requires a collapse factor to compensate for) when helium is far lighter and not as willing to increase its density as much. Helium is incredibly more expensive and would easily cost more than the other consumables combined. One plus to nitrogen is that I could potentially run pneumatics off of my pressurant tank far easier since helium leaks through everything.
Propulsion
Rocket propulsion is, in theory, relatively simple. The idea is to create some device that accelerates mass to a very high speed and eject it the direction opposite to where the vehicle is traveling. At this level rocket engines generally do not have moving parts and the lines between "rocket engine" and "rocket motor" are slightly blurred. The debate has raged on for decades, but the only proper name for a rocket propulsion device that uses solid propellant is rocket motor, rocket propulsion devices using liquid propellant can be referred to as rocket engines or thrusters depending on their use. Rocket engines are easy and simple to design and build, with the caveat that building an engine that is stable, reliable, and overall meets the designed parameters at nominal conditions is an incredibly complex endeavor.
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I may create a video at some point explaining how rocket engines and motors work, but there are plenty of resources available for anyone interested in some light research. I'm not going to get into injectors at all, the main points are as follows. The better propellants are mixed in the combustion chamber, the higher the combustion efficiency is, which can help the overall performance of the engine. Injectors can be designed to prevent combustion instabilities and adjust combustion characteristics throughout the engine. All in all, injectors are not overly important and creating something "above average" that is complex and hard to make is generally not worth your time. If you are interested in the different types of injectors and what advantages and disadvantages each type has, I recommend reading NASA SP-8089, the copy quality is trash on some pages so if anyone has or finds a higher quality copy please let me know.
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Long Shot's engine will provide 750 pounds of force and operate at a chamber pressure of 400 pounds per square inch and a MR of 1.1 using LOX and 75% ethanol and be regeneratively cooled with the fuel. The low MR allows for an abundance of cooling to reduce a risk of burn through. I will be using a torch ignitor tapping off of the propellant tanks for ignition. The torch design was taken from Robert Watzlavich's website which is linked in Rocket Design Resources for those curious. The injector design currently uses 8 unlike doublets and 8 Fuel Film Cooling (FFC) elements near the chamber walls. After a recent trip to test fire one of my friend's engines, I think the injector and much of the chamber and jacket is going to be redesigned to simplify ignition and simplify machining time which will hopefully make the cost and lead time drop.
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To optimize the size of Long Burn and the mass and volume of propellant needed to surpass the Karman line I designed Long Shot's engine to have over 310 seconds of specific impulse. To achieve this, Long Shot's engine will be optimized to 10,000 meters above MSL and be over expanded at launch to take advantage of a higher exit velocity. Over expanding the exhaust is far cheaper mass-wise than increasing the chamber pressure which would increase the mass of the propellant tanks and pressurant. There is a risk of flow separation at lower altitudes, but this will be analyzed through testing of the overall system.
Avionics
Avionics is black magic and attempting to wrangle those electron bois should be a felony in my opinion. I'm not aware of a system that is more of a pain in the ass while also being critical to controlling the state of the overall system and data collection that leads to design iteration.
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My avionics abilities are nearly nonexistent. I have taken about a year of casual finagling to understand what components are needed in data collection, but attempting to make a custom system to accommodate my needs is still out of reach. The system on Long Shot will need to be able to communicate roughly a mile, have multiple ways to record data and location in a space environment, handle control and stability throughout the flight, and recover the rocket in as complete a way as possible. I've baselined using a Raspberry Pi Compute Module 4 as the brain and was able to use a development board to breadboard interface with sensors and some communication over WiFi, but I ultimately want to customize a PCB to better fit all of my components.
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I have a long way to go on avionics, but this system will be built out before much of the hardware is ordered. This is the system on Long Shot I could see myself falling short of 90 percent my own design, but I have really talented friends and working together on this system will teach me far more, far faster, than I would be able to learn on my own.
Controls
Active stabilization is at the heart of Long Shot's design due to the low Thrust to Weight Ratio (TWR) at liftoff. I'm waiting for the design of Long Shot to progress slightly farther before committing time to working on the active stability aspect, but it will involve Thrust Vector Control (TVC) of the engine. This involves pointing the engine different directions with respect to the axis of the rocket in order to counteract outside forces, or disturbances, from wind as well as any propellant sloshing that occurs. Depending on how long TVC is involved throughout the flight, I may need to add some form of roll control to avoid a spin rate too high for the TVC to properly work. Long Shot should see a max velocity of 1,300 meters per second at an altitude of about 60,000 meters Above Ground Level (AGL) and a Mach number of 3.5. Because there are different flight characteristics in subsonic, transonic, and supersonic flight, there will be signific research conducted to determine vehicle response characteristics in all flight regimes to ensure stability at all points in powered flight.