Project HALO Status Report

Rocket Ground Launch #1 -- Post-Flight Analysis

Held on Saturday, April 13, 1996 at the Barrens Test Range in Manchester, Tennessee

The following text was taken, with permission, from an article by Ron Creel, which was published in the May-June 1996 issue of the Southeastern Space Supporter, newsletter of HAL5.

The first ground flight test of the HALO rockoon was quite a sight and sound experience and a glorious success after long months of ground engine firings and launch preparations.  I have undertaken an attempt to reconstruct the flight performance of the HALO-1 launch vehicle.  This effort has been difficult because, unfortunately, the payload section in the nose cone was removed during the first few seconds of flight, and therefore no data from the flight accelerometer was recorded.  The wooden mounting plate which was used to connect the payload section to the forward mounting bulkhead on the N2O oxidizer tank was not sufficiently strong enough to endure launch aerodynamic loads.  Improvements in the mounting method for future flights are already in the works.

Taking the measured inert weights including tanks, valves, fins, electronics, parachute equipment, asphalt fuel, and an estimated amount of loaded oxidizer, (total launch weight of 36 pounds) combined with appropriate assumptions for thrust level, burntime, and specific impulse, have yielded the reconstructed performance plots shown in Figures 1 through 3.  This analysis was performed using a modified PC version of the U.S.  Air Force ROCKET trajectory code.  (ROCKET stands for Rand’s Omnibus Calculator of the Kinematics of Earth Trajectories -- a conveniently derived acronym.) High velocity rocket aerodynamic drag coefficient and standard atmosphere calculation subroutines were used for this analysis.

Launch Analysis

HALO-1 appeared to clear the launch gondola fairly well.  There may have been some torquing or spin induced as it cleared the gondola.  Shortly after this, a deflecting movement was observed, and it is believed that this correlates with the loss of the bulk of all three fins (which was observed in realtime and is quite evident on recorded videos.  It is appropriate to note here that the fins were purposely designed to be extremely light weight for operation at approximately 100,000 ft altitude for balloon launch at a much lower maximum aerodynamic load.

All three fins were removed when aerodynamic loads exceeded their strength.  The good news was that this removal probably tended to increase the spinning of the HALO-1 rocket, and thereby minimized adverse off-axis thrust effects and increased stability.  The vehicle was certainly very stable in flight after shucking its fins and appeared to correct itself and fly almost vertically.  We had previously debated the desirability of having fins on the HALO rockoon.  We certainly proved they were not required, and are already investigating an alternate method of spinning up the HALO rockoon for launch -- not using fins at all.

Motor Performance Estimation

Pre-launch predictions had shown that the expected burntime was on the order of 12 to 14 seconds.  However, observers of the launch and recorded video estimate that the burntime was about twice this duration.  Previous ground engine firings had indicated a specific impulse of about 210 seconds and an initial thrust level of about 320 pounds force for an initial oxidizer ullage pressure of 750 psia.  Indications are that the HALO-1 internal pressure at launch was about 680 psia.  This would have resulted in a somewhat lower thrust level.  In order to get a significant increase in burntime (i.e.  lower propellant flowrate) it is necessary either to use a lower thrust level or lower specific impulse.  I chose to use the former.  Reversing this would not significantly change the results of the analysis.

Therefore, assuming that we had a total of 19 pounds of propellant (16.2 pounds of N2O oxidizer and 2.8 pounds of asphalt fuel), specific impulse of 210 seconds, and sea level thrust of 200 pounds force during about 85 percent of the burntime (corresponding to the liquid oxidizer phase) and the remaining 15 percent of burntime using gaseous N2O -- we get the reconstructed thrust timeline shown in Figure 1.

I assumed that the effective thrust level would be reduced by a factor of three during the gaseous N2O phase from 15.75 seconds until N2O depletion.  The thrust might actually follow a curve like the dashed one in Figure 1.

Flight Performance Estimation

About 28.35 seconds of total effective burntime is the maximum I can reproduce at this time.  Note that effective thrust for the balloon launched HALO will be significantly increased with use of the high altitude nozzle extension.  Velocity versus time corresponding to the thrust curve is shown in Figure 2.  A maximum Mach number of 1.16 was calculated at the end of the liquid burn phase at 15.75 seconds at an altitude of about 12,000 feet.  A maximum altitude of just over 30,000 feet was calculated as shown in Figure 3.

I have no doubt that had we been able to launch HALO-1 from a balloon at 90,000 feet altitude, we would have certainly gone into space -- that is, above 50 nautical miles.  From a launch altitude of 100,000 feet we would have approached an apogee of 70 nautical miles.  We are ready -- Let’s Do It!

Ad Astra per Ardua -- “To the Stars by Our Own Hands”

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This file was last modified on Saturday, 15-Apr-2017 13:19:39 EDT