Semi-scale Me-163 Komet slope glider with rocket boost

May 13 2005 edition
Steve Seibel
steve at aeroexperiments.org
www.aeroexperiments.org

 

Here are some photos of the author's Me-163 Komet slope glider with rocket boost, taken by Paul Naton: #1, #2, #3, #4, #5. (See more of Paul's work at www.radiocarbonart.com). More Komet photos: #1.

 

I modified a Dave's Aircraft Works semi-scale Me-163 Komet slope glider by adding provisions for aerial ignition of a rocket motor.  The rocket motor inserts into a plastic socket buried in the rear fuselage of the aircraft.  When a single motor is used, it protrudes about an inch beyond the rear of the fuselage, and when two motors are used, the lower-stage motor attaches to the aft end of the upper-stage motor and is completely external to the model.  The ignition system has both internal and external components: a servo inside the fuselage, operated on the throttle channel, closes a microswitch that allows a 9-volt battery mounted externally near the nose of the glider to deliver power to a standard Estes igniter inserted in the nozzle of the rocket motor at the rear of the glider.  For flying without the rocket motor, the 9-volt battery can be removed to lighten and balance the glider.  However, I find that I fly the model almost exclusively in conjunction with the rocket motor; if were rebuilding the rocket glider, I would include an internal, rechargeable battery to ignite the rocket motor.

 

The model is not intended to take off under rocket power.  Instead, it is launched by hand like any other slope glider, and then the rocket motor is fired whenever the pilot wishes.  The rocket motor is put to best visual and aural effect when it is ignited during a low, high-speed pass in strong slope lift.  (The effect is especially interesting at twilight, when the rocket's flame is most visible).  In weak slope lift, sometimes the lift profile is such that igniting the rocket will boost the glider to a high enough altitude to allow sustained soaring flight when this would not be possible at lower altitudes.  In calm air, the rocket motor can be fired shortly after the glider is launched, allowing the glider to gain enough altitude for a few minutes of flight--or necessitating a long walk to the bottom of the hill if the motor fails to ignite! 

 

To maximize the rocket motor's burn time I typically use an Estes E9-8 motor staged to an Estes D11-P motor.  The total burn time is still only a few seconds even with this double-stage configuration.  The long "smoke trail" stage that occurs after the first motor has exhausted its propellant, but before the upper stage is ignited, adds greatly to the overall visual effect.  So does the ignition of the upper stage, which occurs after most onlookers think the "show" is coming to an end, and is usually accompanied by a bright flash and by the visible ejection of bits of flaming debris from the rear of the model.  The lower-stage motor casing falls free of the model as the upper stage ignites, and the upper-stage motor casing remains with the model after the end of the burn.

 

As noted above, the lower-stage motor is completely exterior to the model.  It is simply butted against the protruding end of the upper-stage motor and secured in place with a single wrap of scotch tape, which is melted as the upper stage ignites.  Since the thrust load exerted by the lower stage burn has no sideways component, this primitive method of connecting the lower and upper stages is adequate. 

 

Normally, a rocketeer does not use a motor with a "smoke trail" section, followed by an "ejection" charge, as a lower-stage motor.  To the allow the E9-8 motor to reliably ignite the upper stage, it is necessary to scrape off the thin clay cap that lies on top of the "ejection" charge at the forward end of the E9-8 motor.  When this clay cap is removed, then instead of creating an explosive burst of hot gas, the ejection charge burns in a way that reliably ignites the next stage.

 

An alternative motor combination that creates less of a CG change during the rocket burn is an Estes D12-7 motor for the lower stage, coupled to an Estes E9-P motor for the upper stage.  This allows the larger, heavier motor casing to remain with the glider after the rocket burn is over.  However, the E9-P motors are a bit hard to find in local hobby stores.  Also, I usually prefer to have the longer E motor burn precede the shorter D motor burn, so that the model has lots of kinetic energy during the smoke-trail interval that precedes the ignition of the second motor. 

 

I've also experimented with the Apogee brand of long-burn, low-thrust motors, but found the ignition process to be too unreliable. 

 

As noted above, the motor is mounted in a simple plastic socket which is built into the EPP-foam of the rear fuselage.  To prevent hot gasses from blasting forward and destroying the model as the upper-stage motor nears the end of its burn, and also to prevent the motor from ejecting from the model at the end of the burn, the forward end of the upper-stage motor must be plugged in some way.  As noted above, I usually use factory-plugged Estes E9-P or D11-P motors for the top stage.   These motors do not include a "smoke trail" stage.  In the interest of having a "smoke trail" stage in the upper-stage motor as well as the lower-stage motor, I experimented with making my own plug for an Estes E9-8 or D12-7 motor for the top stage.  These experiments were not successful.  In one case, when the motor's "ejection" charge ignited, the plug remained in place but the motor casing split apart.  This allowed escaping hot gas to melt through the plastic socket in which the motor was mounted.  The hot gasses then blasted multiple large channels through the EPP foam that comprised the rear of the fuselage, so that this portion of the glider had to be re-constructed!  If I had scraped away the powder that comprises the ejection charge, and then plugged the motor, this problem might have been avoided: apparently the capped-off ejection charge created more internal pressure inside the rocket motor casing than the thrust charge did, unless it was just coincidence that the motor casing failed on this trial.  Also, if I had mounted the motor in a metal socket rather than in a plastic socket, the leak of hot gas might have simply ejected the motor from the model rather than damaging the model. 

 

I've found it important not to allow anything to touch the single wrap of tape that connects the lower-stage motor to the upper-stage motor. In once instance where the extreme forward end of this wrap of tape was accidentally overlaid with a second piece of tape, intended to hold the upper-stage motor firmly in its socket, both pieces of tape ended up serving as a "bridge" or "tunnel" for hot gasses. During the staging process, in the instant before the single wrap of tape burned in two and the lower stage fell free of the model, hot gasses tunneled forward under both pieces of tape and inflicted some superficial damage to the rear of the model.

 

Normally, the last step before launching the glider is to connect the external 9-volt battery into the firing circuit.  At this point the rocket motors are "armed", and are treated with extreme care. There is an inherent safety hazard created by the fact that the model is launched by hand after the rocket motors are "armed".

 

The ignition servo is configured so that it will not close the ignition switch until I have moved the throttle trim lever all the way down, and also have moved the throttle lever all the way down.  The throttle lever is normally held in the full up position by a strong rubber band attached to the transmitter, and I leave the throttle trim lever in the full up position until after I've launched the model.  Still, a severe radio glitch could, in theory, move the ignition servo to the limit of its travel and accidentally fire the rocket.  If this happened in the instant after my launch throw--which is the only time the armed motor nozzle ever points toward a person in close proximity to the model--I would be injured.  (I never allow anyone else to help me launch the model when the rocket motors are installed). For added safety, it would be better if there were two ignition servos and two ignition switches wired in series.  The second servo could be operated by moving the throttle lever all the way to the left, so the rocket motor would not fire unless both servos simultaneously reached the limits of their travel. 

 

The rocket motor must never be fired immediately after launch or any other time the motor nozzle is pointing toward a nearby person, because hot gas and burning particles are blasted for quite some distance out of the nozzle of the rocket.  Here are some other safety issues with the rocket glider: in theory a hard crash could disturb the ignition switch and fire the rocket motors.  During one twilight flight, I was surprised to see that small globs of burning material that were ejected from the model during the staging process fell all the way to the ground without consuming themselves.   For these reasons the rocket glider is only flown when the flying site is damp from rain.  (This describes most of the flying season for the south-facing butte that I frequent in Oregon's rainy Willamette Valley). 

 

Obviously, the Komet rocket glider is not AMA-legal.

 

Since the rocket burn involves the loss of all the propellant from both stages, as well as the loss of the casing from the lower stage, the CG moves forward quite a bit during the rocket burn.  After the rocket burn, the glider is rather nose-heavy, so a significant amount of drag is created by the resulting nose-up elevon trim that is required to deal with this.  In a custom-designed project where the motors were placed closer to the CG of the model, this would be avoided.  This would also allow the use of heavier rocket motors, or the use of the same rocket motors with a more lightly-built model, without creating unacceptable trim changes.  The overall performance of such a model under rocket power would be much better than the performance of the aircraft being described here: much higher climbs would be possible.  The goal of the present project, on the other hand, was simply to build a strong, durable, semi-scale Me-163 Komet slope glider from an existing kit, modified with a supplemental rocket motor for added scale-like excitement and fun.

 

Another modeler has created a much more sophisticated RC model Me-163 Komet that has a very scale-like appearance, and also has the capability to take off from a runway under rocket power. Let me know if you know url's for web coverage of this project, and I'll insert them here.

 

Here is a video (from RC groups, May 7 2005) of another modeler's S&B Me-163 Komet being given a bungee launch, followed by a rocket boost.