Recap of EOSS-50


EOSS 50 Pioneer Astronautics automatically deploying Mars Micro Balloon test flight, 4/21/2001

Notes regarding flight planning and actual flight performance.

Background: this flight was intended to test the Mars Micro Balloon concept during a dynamic descent. The experimental methanol-filled black plastic balloon was stowed in the spring loaded canister and had a tension switch attached to circuitry which would sense the loss of lift after burst of the main balloon and deploy the black plastic bag out of it's canister 6 seconds after burst. To simulate a space probe entering the Mars atmosphere, test of the inflation of the balloon with methanol vapor is desired at 20-30 meters per second descent rate. Three grossly under-filled 600 g balloons (nicknamed 'grapes') are attached to the flight string to slow the descent of the MMB experiment after burst. [Parachutes large enough to give the desired descent rate at 100000 are too big to be practical, plus yield an unacceptably long descent time.] The 600g balloons were filled to -50% free lift for this flight, i.e. filled with enough helium to lift about half of their weight.

Flight planning: total mass of the camcorder, the experiment, the GPS tracker and APRS and the 'peanut beacon', and the 3 x 300 g equivalent of the 3 'grapes' was 8.86 lbs. We planned for an aggressive 40% free lift, which would be obtained by filling the 3000g main balloon to neutral buoyancy under 15 lb sandbag-ballasted nozzle. With 40% free lift, we expected an ascent rate over 1100 feet per minute and time to apogee of about 100 minutes. The calculated aerodynamic drag of the 'grapes' was small and considered negligible if they were in free air. But since the grapes fly in the wake of the much larger main balloon, thus have even less drag than if they were in free air, the decision to ignore the aerodynamic drag of the 'grapes' was made.

Actual flight: Wind over the top of the main balloon at the launch site created false lift and the main balloon was under filled. Although the sandbagged nozzle was apparently levitating under the lift of the main balloon, we were at that time experiencing about 5 pounds of false lift (!). With the severely under filled main balloon, we achieved an actual average ascent rate of 559 feet per minute, which equates to about only 8%, or 562 grams, of free lift. It was known before the flight that premature burst of the 'grapes' would effectively add mass to the flight string because their helium would be lost. During ascending flight, the 'grapes' hang down and hit the main flight string and suffer stress concentrations on their tightly stretched skin as their diameter increases at altitude. For that reason it is hard to predict the burst altitude of the 'grapes' and redundancy is desired. If we were anywhere close to our planned 40% free lift, an additional 300g, 600g or even 900g of flight string weight would not have had a significant impact on our ascent rate, in the worst case of all three 'grapes' bursting early, the attendant 900g equivalent mass added to the flight string would have brought the free lift down to 24% and an expected 950 feet per minute ascent rate. Apparently, one 'grape' burst around 92000 feet and added about 300 g mass to the flight string. The ascent rate between 92k feet and 103k feet was about 368 feet per minute, which is consistent to adding 300g on to the EOSS 50 flight string. This equates to a small, 4% free lift. And later, at about 103000 feet, a second grape burst. With the additional 300 g load on the flight string, the balloon was then very close to neutral buoyancy. The payload struggled slightly upward over the next 18 minutes averaging a miniscule 48 foot per minute ascent rate until the main balloon burst at around 103900 feet. Immediately after the burst of the main balloon, the descent rate was just under 20 meters per second, indicative of one 'grape' remaining on the flight string. The small 600 g 'grapes' are about 12 feet diameter at this altitude.

Summary and recommendations: 'false lift' is a problem anytime there is wind at the launch site, and for EOSS-50 it presented a significant issue in the slow ascent rate and resulting longer flight path over the ground. Fill of a balloon in a still wind hangar environment seems desirable. Lacking a hangar at the launch site, waiting for still air seems prudent. Another option is to estimate the amount of helium loaded into the main balloon by means of taking differential pressure readings off the helium K-bottles. Regardless, on EOSS-50 we had a significant error on out main balloon fill and that negatively impacted the mission.

ed note: here are two charts provided by Mark from THIS EXCEL SPREADSHEET