Prochron is a booster intended to launch a single pod of CubeSat satellites (3kg) to an orbit of roughly 200km altitude at a recurring cost of about $10,000. The main challenge isn't technical, but whether the market is adequate to support a business producing and operating this booster. It is also available as a high performance amateur rocket, with two stages, or an exoatmospheric sounding rocket with three stages. The orbital version uses four stages.
General Characteristics: Prochron is too small to be provided with an active stabilization system, such as larger rockets. While theoretically possible, it would be extremely expensive, as it is on large boosters, and even more difficult because smaller rockets with higher accellerations react to aerodynamic loads much more rapidly. Prochron is therefore designed to be stable passively, like any other amateur rocket. However, it is not possible to stabilize a rocket this way below a certain dynamic pressure, and an orbital booster must be guided in order to be inserted into orbit. The two situations where the dynamic pressure is insufficient to allow passively stable flight are immediately upon ignition, and after the booster leaves the atmosphere, when there won't be enough air to act on the fins to stabilize the booster. For the first phase, the booster is stabilized by a launch tower. High accelleration is provided by Stage One, allowing the rocket to gain enough speed by the time its aft end exits the 20m tall launch tower.
Stage One: A series of existing amateur strap-on rockets based on 98mm high power rocket motors, each of which produces M class total impulse. (Hypertek hybrids is a leading contender.) It is likely that nine will be used. They surround Stage Two. Stage One is recoverable and reusable in all versions. See versions below for reusability of other stages, which depend on performance level.
Stage Two: An oxyfuel core stage, very likely oxypropane with a linked helium pressurization system in the fuel section to equalize the injection pressure with a vapor pressurized LOX tank. The propellants are pressure-fed into a single motor.
Stage Three: An oxyfuel stage, very likely oxypropane with a linked helium pressurization system in the fuel section to equalize the injection pressure witha vapor pressurized LOX tank. This stage is likely to be pressure stabilized in the orbital version to reduce mass. The propellants are pressure fed into a single motor.
Stage Four: An oxyfuel stage, probably oxypropane with an aforementioned helium pressure system in the fuel section. The tanks are likely to be pressure stabilized, and the stage will probably be equipped with a nitrogen or sulfur hexaflouride cold gas RCS.
Stage Two, the core stage, ignites before Stage One does. Its thrust alone is insufficient to move the rocket. Stage Two is activated by the on board launch timer, while Stage One is activated by the ground launch controller, manually. Stage One burns out after about six seconds, and its motors are are immediately separated...hopefully separation can be actuated by the strap-on's thrust dropping below a certain level. This moment is maximum dynamic pressure. Stage Two, at this point is barely able to overcome gravity and drag and maintain the speed of the booster, but as the air thins and the rocket lightens due to the consumption of propellant, it will again accellerate, but in the thinner air will soon find itself without the ability to keep stable. The rocket is free fin stabilized during the First Stage, and may also be free fin stabilized during the Second Stage, which depletes in less than one minute. If it is free fin stabilized, the aiming of the launch tower to achieve the proper arcing trajectory (or vertical trajectory if it is a sounding rocket version) will be required. It may be fin-stabilized with some positive guidance during the second stage, thus allowing a fixed launch tower. During Stage Three, the booster will be either spin stabilized or positively guided using thrust vector control and the RCS in the fourth stage for active roll stabilization. During the Fourth stage, the vehicle will be controlled by the RCS alone, as the fourth stage motor starts getting rather small for thrust vector control, also may be very close to the center of gravity of what little is left of the booster, and therefore extremely sensitive to thrust vector changes (alignment of a fixed nozzle on the ground might be tricky for the same reason.)
Two Stage Amateur Sounding Rocket: Performance goal is 120km altitude vertical flight. This rocket vehicle will be flyable as an NAR Level 3 or CAR Level 4 amateur rocket. As such, Ascent (i.e.: the as-yet-unformed business which would manufacture and market the Prochron and other Ascent Roadmap boosters) would sell the actual rocket, and the CAR/NAR certified rocketeer would fly it. In this version, Stage Two is recoverable.
Three Stage Professional Sounding Rocket: Performance goal is 600km vertical flight. This version would be operated by Ascent, or an Ascent authorized professional sounding rocket service (to which it would sell the boosters.) In this version, Stage Three will almost certainly be spin stabilized and not recoverable. Stage Two will probably be recoverable. It is an open question whether this vehicle would need positive guidance.
Four Stage Orbital Booster: Performance goal is a 200km circular orbit in a single maneuver, direct ascent terminating within line-of-sight of the launch pad or a tracking station in its immediate vicinity. Stages Two, Three, and Four are probably not recoverable (although Stage Two is likely to hit the surface intact.) Stage Three may be either spin-stabilized or guided via thrust vector control from the Fourth stage guidance platform (if the latter, Prochron will need two versions of Stage Three: one with spin-up rockets and yo-yo despin, and one with thrust vector control gimbal, actuators, and batteries.) Stage Four contains the guidance platform and radar transponder/receiver.
Range Safety of the Amateur and Professional Sounding Rocket versions are provided as they are with existing rockets: make sure they don't leave the range hazard area. Professional and Orbital versions are likely to have a beam-riding flight termination system. This is, the radar tracker is aimed at them at all times. If the radar receiver on board the rocket quits getting radar signals, it shuts the booster down and initiates payload separation. The radar receiver termination system may also have a heartbeat monitor on the guidance system for the Orbital Booster. If the guidance system crashed, the flight termination controller would no longer pick up the guidance heartbeat and shut down, responding to radar pings with a tone that indicates whether the guidance system heartbeat is operating or not. The guidance system may transmit telemetry on a separate (non-radar) channel, and an onboard video system may yet use a third. The radar receiver flight termination system on the Orbital Booster might be used to control normal booster cutoff. It is likely that ground radar will have a better picture of where the Fourth Stage is than the Fourth Stage itself (whose guidance system accuracy and reliability is likely to be compromized for cost considerations...compared to that of a typical orbital booster, that is.) Precise cutoff would require tuning of the radar pulse rate. In this case, the last pulse would be transmitted at an exact inteval before cutoff, representing the time it takes the flight termination system to flag because of loss of signal (which is probably triple the pulse interval to prevent cutoff due to casual interference with the radar.)