The build-up approach is a fundamental principle in flight test, an essential strategy that ensures risks are mitigated wisely. It suggests beginning a flight test or evaluation from the least risky condition, usually in the “middle” of the envelope.
After performing flight test techniques at these “safe” points, then testing proceeds slowly towards the unknown, and the size of each successive step is generally smaller as the boundary, or the condition with highest risk, gets closer.
|For example, in the NASA Orion CPAS flight test, the build up approach involved testing the parachute assembly first on a special drop platform that ensured a “sure thing” deployment of the parachute. This was followed by an airdrop with the CPAS drop test vehicle that simulated some ballistic characteristics of the Orion Crew Entry Vehicle (CEV) but at a weight less than the actual CEV. Airspeed and weight of each subsequent airdrop also increase to test the parachutes at successively higher dynamic pressures.|
NASA has an experimental technology known as the hypersonic inflatable aerodynamic decelerator that has been tested recently on a rocket launched from Wallops Island, and they have just released a flash based, interactive simulation here that we can use to illustrate the build-up approach.
The very first “maneuver” you will accomplish in this simulation is to separate the spacecraft from its docked position on the International Space Station with a mouse click (or a screen tap on a mobile phone). Then you must perform a de-orbit burn with a mouse click of varying length.
Here are some suggested ways to experiment with your de-orbit burn, using the NASA simulation, and employ the same kind of build up approach that you would find in flight test.
1. Begin with a very short de-orbit burn. Update your position with short burns.
2. Use longer burns to update your position.
3. This is where it really counts–now that you have practiced, assume that you only have one, short deorbit burn. (After all, you may not be able to relight the rocket.) Try to hit the re-entry window with just one click. Did you make it?
In the world of flight test–in some of the most dangerous tests–a test pilot in an experimental aircraft may only have one chance to hit the “target.”
So, based on your experiment, in what position should the space station be in order to hit the re-entry window optimally? How long should the burn be? What’s the optimal number of burns?
Do you see that there’s more than one way to solve a problem?
On the last Friday of each month, the column What is an FTT describes some of the fundamental maneuver building blocks performed by test pilots to gather data during flight test missions. An FTT is in some sense a description of an experiment. It is a key element of the scientific method applied to aerospace sciences, engineering, and aviation.
You can access all of these posts by clicking on the FTT category hyperlink below the post title.
Each Friday, @FlightTestFact will deliver examples, definitions, and explanations of flight test techniques for the entire day. You can view these tweets by searching for #FTT and #flighttest as depicted below. You can also click on the picture below to be taken to the twitter search results. What FTT would you like to know more about?