A rather intriguing question was posted in an e-mail discussion forum devoted to sugar propellants. One of the contributor innocently asked “Would it be possible to launch a sugar-propelled rocket into Space?” This query set into motion a flurry of activity, which sought to answer this question, at least from a theoretical perspective. A number of experienced amateur rocketeers pondered the question in-depth, ran computer simulations, and concluded that it might be possible, but barely. Due to the low performance of sugar propellant, which has a “specific impulse” of about one half that of professional rocket propellants, the goal of reaching Space was shown to be very challenging.
A conventional single stage rocket would not be capable, at least not one that would boost a decent sized payload. A two stage rocket would be needed. The key advantage to a two stage, versus single stage vehicle, is that of efficiency. A single stage rocket would propel a vehicle to a very high velocity in the lower, densest part of the atmosphere, losing a lot of energy due to aerodynamic drag. With a two stage approach, a vehicle can coast following the first burn, and soar to an altitude beyond much of the densest air before firing the second stage. An alternative suggestion was then submitted for discussion. Why not a single stage rocket that would behave as a two stage rocket? Deemed a “dual-phase” rocket, two serial propellant charges would be separated by a common bulkhead, and share a common nozzle. Following burnout of the first charge (or phase), the bulkhead would be breeched, allowing the two chambers to act as one. The second charge would then fire with the motor behaving in conventional manner. Simpler than dealing with the complexities of staging, at least in theory. And so out of this innocuous discussion, the Sugar Shot to Space project was born, with a mandate to demonstrate that theory and reality could be merged. Amateur rocketeers with a passion for sugar propellant and a commitment to accomplish something on the edge of feasibility could pull this one off.
That was over five years ago. Over the course of ensuing time, there arose many unexpected challenges, technical as well as organizational, restrained by the limitations of being an all-volunteer, minimum budget project, delving into a little known technology. It turns out that dual-phase rocket operation is simple in theory, but more than a tad challenging to engineer a workable solution. We learned the hard way that we knew even less about sugar propellant than we thought we knew. For example, we were aware that sugar propellant is brittle, but how brittle, and how would that play out in a large-scale rocket motor? Brittleness can be a bad thing, resulting in sudden, unexpected and potentially catastrophic fracture under certain conditions.
Fully understanding those conditions in order to mitigate the risk stipulates a great deal of unglamorous effort. And despite a passion for the goal of reaching Space (a dream nearly every amateur rocketeer shares), many volunteers were simply over-constrained with regard to available spare time, as many have full-time jobs and a life outside rocketry.
Realizing that the approach taken to reaching Space “in one giant leap” was fraught with many hurdles that would likely lead to a disappointing end, the project was eventually reborn as a “program”. Instead of trying to reach Space in a single attempt, the new tactic was to apply an incremental “Apollo” style approach, moving forward cautiously step by step. Three key projects were identified for the program: one-third scale, two-thirds scale and then the full-scale “Space” rocket. Tackled this way we could learn as we progressed, developing scalable hardware and methods.
In hindsight, this appears to have been a wise change of course. As things unfolded, the one-third scale “Mini Sugar Shot”required several static test firings before a successful firing was achieved. The difficulties were mainly a consequence of the very demanding “mass fraction” requirement needed to reach Space on a low performance fuel, and secondly due to the unexpectedly severe thermal loading the rocket chamber experienced during the second phase burn. The first of these, which demands that most of the liftoff mass (at least 80%) must be propellant, asserts that the lightest of materials be used. Gone by the wayside was the inherent comfort of using beefy metal motor casings. Only lightweight composite materials could fit the bill. The second issue, made all the more complicated by the first, was eventually resolved through the development of a lightweight ablative material that lined the motor chamber, and served to effectively insulate it from the torrent of hot, highly pressurized and speedy exhaust gases seeking its escape to greater entropy through the chamber and out the nozzle.
What challenges associated with the use of sugar propellant lie ahead for the Sugar Shot to Space team as we graduate toward the next project, the two-thirds scale “Double Sugar Shot”?
We’ve learned that brittleness of sugar propellant can lead to a catastrophic result. Encouraged by experiments that indicate that storage method, such as deep freezing, can inhibit formation of brittleness; this negative trait can hopefully be tamed. We’ve learned through our Mini Sugar Shot experience that we’ve gotten a pretty good handle on casting sugar propellant. One-third scale propellant “grains” of about a kilogram each (totaling twelve for each motor firing), and of high quality, were consistently produced. Will that same casting technology allow us to cast the much larger grains while maintaining similar and consistent quality? The need for consistency is imperative to scaled-up design approach of a rocket motor, as the burning characteristics and other traits affecting the “internal ballistics” are directly affected. The sheer quantity of propellant needed for Double Sugar Shot, 90 kg for each firing, leads to an unprecedented challenge of safe and efficient mass production. Whatever method we develop should be scalable to the full-sized, appropriately named “Extreme Sugar Shot”, which has a projected motor capacity of 450 kg. That’s a lot of sugar propellant. Other questions come to light when considering mass production of propellant, such as “what happens if sugar propellant is accidentally ignited”? Controlled experiments were performed to gain a better understanding of this critical aspect of sugar propellant usage. Turns out that the risks and consequences associated with such a mishap can likely be mitigated by intelligent design of propellant handling and casting apparatus, and by appropriate response to such an event.
What grain configuration would be most suitable for our requirements? The ubiquitous BATES configuration, as used for most amateur rockets, or some other untried geometry, such as “star” shaped core? Which oxidizer-to-fuel ratio would be best? Stick with the tried and true (65/35) or seek to optimize? These and many other questions remain to be answered. Many challenges undoubtedly lie ahead before we succeed in taming sugar propellant. The only certainty is that success, if that’s to be the fate for the Sugar Shot to Space team, and we truly believe it will be, will demand an unrelenting commitment to achieve an extraordinary goal with a decidedly ordinary rocket propellant.
You can find out more about Richard Nakka and his team on their website SugarShot.org
You can find this article and many more in Issue 01 of Citizen Science Quarterly