DRAKE TEST VEHICLE

BACKGROUND
First conceived in the early days of Tripoli Pittsburgh rack-rocket activity (late 1960’s), “internal motor-feed staging” was an alternative staging concept explored in design sketches by the author. Rack-rocket staging, which in essence facilitates multi-staging in a one-piece, open hulled airframe, was (and is) simpler to construct, and was consistent with the technology available at the time, thus internal motor-feed staging remained a concept only for many years. From 1969 through the present, the author and several other Tripoli members (AJ Reed, Tom Blazanin, Korey Kline, Mike Dennett, and Joe Peklicz) pursued “conventional” rack rockets (if such an adjective applies to a relatively unusual staging technique) with a great amount of success.

In the late 1990’s the internal-motor feed staging concept was examined anew, since it was clear that onboard electronics could be utilized to trigger the flight events necessary to deploy the design. An initial design specification for a two-stage ultra-high performance rocket, using O-motors, was initiated in 1996, and was assigned a design code KG-24 and named “TERRA NOVA.” This rocket promised a potential of altitudes in excess of 80K feet, if constructed of lightweight materials. Several variants were simulated, including one incorporating a conventional clustered lower stage, boosting a minimum diameter two-stage motor-feed upper flight vehicle. In these enhanced forms, simulations took the flight vehicle to over 100K feet. To pursue the concept logically, it was determined that a smaller version of the TERRA NOVA should be built as a proof-of-concept vehicle. Thus was born the design of the KG-25 “DRAKE” – a scaled down version of the basic TERRA NOVA original design, with K motor power as the intended impulse range. By 2003, fairly detailed drawings of DRAKE were taking shape, and in 2004, Eric Haberman, of Dynacom/AirX fame, agreed to be the principle engineer and component fabricator for a finalized design. Ultimately, the project grew to involve several members of the Tripoli Pittsburgh prefecture as part of the project team, with an initial test flight targeted for Black Rock, NV, in September 2006

MOTOR-FEED STAGING • BASIC CONCEPT
The basic principle of the two-stage motor-feed staging approach involves an internal motor delivery method that ejects one motor that is encased in a recoverable sabot (containing it's own parachute) while it slides the upper stage downward, locking it into firing position, then igniting that stage. The whole eject/feed/firing process should take place as fast as one would cycle a bolt-action rifle, the only real delay being stage two coming up to power.

The downward movement is triggered by a staging board that activates a Rouse CO² system (or systems), pressurizing a motor tube actuating piston, forcing everything downward. The upper stage motor/sabot locks into place when two spring-tensioned bolts reach openings in the motor tube, springing outward and locking the sabot into the lower, firing position. Any excess gas pressurization will be vented out through vent holes in the airframe once the upper stage motor is in the full downward position. The locking of the motor into firing position triggers a switch that ignites the stage two motor. After it is pushed from the airframe, the first stage free-falls for 1 - 2 seconds, after which a recovery parachute is deployed

.
Draft design views of “Drake” motor sabots in mother tube, illustrating the motor-feed staging scheme.

DRAKE TEST VEHICLE
The test vehicle, which actually has been in design stage since 1997, is named "DRAKE" with the KG-25 numeric design designation. Following some early design concepts focused on a lower power range, the motor size eventually selected for this test vehicle is 54mm, using 54/1706 NS casings. It was deemed that 3" airframe tubing would be used, ensuring that the fairly complex framework of the test vehicle could be fully and comfortably enclosed in this first design application, while keeping overall diameter as minimal as possible. The resulting airframe structure would be based upon the proven and successful “AirX LUCERNE” production rocket.

RATIONALE FOR MOTOR-FEED DESIGN
To understand why motor-feed staging was deemed worthy of investigation, one must first answer the question, “Why stage, and not just create a single stage rocket with one motor equivalent to the total impulse of two staged motors?” The answer to this is clear – one single motor performs in one way, with a single thrust profile. Some of these profiles may be more complex than others, but with any one motor, you get just one profile, period. A staged flight, even with just two stages, can combine individual motor profiles of very different natures into one flight plan. For example, a high average impulse motor of short duration may be used for the first stage, maximizing the power-to-weight ratio of the rocket at take-off and ensuring safe, stable initial flight, while a second stage motor of longer duration and lower average impulse can take advantage of the initial velocity gained, powering the rocket for an extended high-altitude climb. Such a blended flight profile cannot be achieved with a single-motor rocket. An additional advantage over a single motor flight is that approximately 50% of the total motor casing mass is ejected as part of the staging event, thus increasing the sustainer stage’s potential velocity. Further examining the point about losing mass during the staging event, one may question why motor-feed staging, which loses only the mass of the motor and the recovery sabot, is a better solution than using a conventionally staged airframe, where a significant portion of the airframe is separated and a larger amount of mass reduction takes place. The answer to this is that in most cases of conventional staging, the actual mass reduction should be larger, because the airframe probably weighed more than an internal motor-feed-staged airframe in the first place. Consider that a conventional lower stage would not only have motor tubing, it would also have airframe tubing, a full set of fins, the required reinforcing/bracing for these fins, a larger recovery system, and (especially in a HP rocket) a fairly well-reinforced stage coupler system to resist bending at the 1-2 stage joint under thrust. All this material adds weight that is not present in a single-piece motor-feed rocket. Thus if the conventionally staged rocket weighed more in the first place, the loss of greater weight during staging is not a net advantage, and in fact, the heavier initial weight of the rocket would negatively impact such a rocket’s flight performance from the start.

Then consider the aerodynamic problems of a conventionally staged rocket. A set of fins for each lower stage is required for such rockets, and these are not only a weight penalty, they increase drag and aggravate weathercocking. Thus, a conventionally staged rocket normally has far greater fin area than required for the airframe of the rocket, if it was a normal single-piece airframe, and it inherits all the negatives of vastly “overfinning” a rocket.

Finally, the question arises as to why this relatively complex approach to multi-staging a one-piece rocket is better than a rack-rocket, or for that matter, just a conventional clustered rocket using airstarts. In the case of a rack-rocket, it has always been a challenge to create an airframe that is open enough to permit upper stage motors to fire in-situ, after the lower stages have been ejected. Such airframes have tended to be constructed of a dowel-rod array, or use a slotted-tube hull. In either case, the materials used must be made resistant to hot motor exhaust, a consideration that becomes more challenging as the motor size increases. Also, slotted hulls and/or open motor racks cannot be said to be especially aerodynamically clean, thus a certain amount of performance is lost to drag and turbulence. In the case of a conventional cluster-rocket using airstarts, the problem is simple – excessive airframe diameter. Much of the benefit of airstarting motors at later points in the rocket’s flight is lost, since these motors, arrayed as a side-by-side cluster, require the diameter of the rocket (a direct enemy to high-altitude potential) to increase in direct proportion to the number of motors clustered. The motor-feed-staging method permits the rocket diameter to be as narrow as the motor tube, plus whatever internal space may be needed for the feed-staging mechanisms to function, plus the thickness of the outer airframe. The designs of the DRAKE and TERRA NOVA both show that the resulting diameter could be far less than even that of a three-motor clustered rocket.

Complete “DRAKE” motor-feed sabot frame, with retaining hardware and CO² generator/actuating piston visible at the forward end.

CONCLUSION
It is believed that motor-feed staging can and will be made to function reliably, and with a minimum of weight penalty. The main sources of potential problems would seem to focus more on the mechanical features of the design, since the electronics required to trigger the flight events are reliable, off-the-shelf components, and are no more complex than what is required for any conventional airstart or multi-stage rocket. Should the DRAKE prove to be a success, and measured performance confirms the advantages of the design, immediate plans will be set in motion to design and build the TERRA NOVA, potentially with O-motors compatible with TRA-Research rules, for test flight at Black Rock in 2007