from Come Fly With Me – Exploring Science through aviation and aerospace concepts.
GROUP SIZE: Large
TIME: 45 minutes
TYPE OF ACTIVITY: Teacher Demonstration
TEACHING STRATEGY: Guided Discovery
CONCEPTS: Ablation Regenerative cooling
SKILLS: Observation Inference
Objectives: To demonstrate methods of cooling a spacecraft or aircraft traveling at high speed in the atmosphere; to learn the concepts of regenerative and ablation cooling.
Materials: Two ring stands or tripod supports; two burner triangles; two small paper cups; two candles; water; heavy plastic cup; plastic cup which has been dipped in melted wax repeatedly until a thick (1/2″ or more) coat of wax has built up on the outside; matches.
Teacher Background Information:
The problem of dissipating the heat build-up caused by friction when a plane or spacecraft travels through the atmosphere has been a hard one to solve. Three main ways of cooling are used now. One is ablation. Ablation makes use of a melt away covering to carry the heat away with it. The covering is sacrificed and enough must be put on a craft so that it does not all melt before the craft slows to a speed where heat is no longer a problem. This cooling technique was used on spacecraft through the Apollo missions.
Another technique is that of regenerative cooling. This method makes use of a super cooled fluid passing through pipes near the surface which is being heated. The fluid absorbs the heat and carries it away from the surface. This method is used in some planes and in many rocket engines where the fuel, liquid Oxygen and/or Hydrogen, is circulated around the rocket nozzle and preheated before being burned. The third method uses material which actually stores the heat and releases it slowly so that nearby surfaces do not overheat quickly. The Space Shuttle uses such a material. ‘
- 1. Set up two ring stands or tripods and place a triangle on each ring. Place a candle under each. Fill one paper cup and place it on one of the stands, above the candle. Place an empty cup above the other candle. Light the candles. What did you observe happen? Why do you suppose that is? (The empty cup will burn quickly. The filled cup will not get hot enough to ignite because the water carries the heat away or stores the heat.) This is similar to the Shuttle tile function.
- 2. Take another cup and place a small amount of water in the bottom or fold an index card into a flat tray shape with a small amount of water in the tray. Heat the cup or card the same way as before. The water will boil carrying the heat away in the steam. Of course, the card will not ignite until the water has all been evaporated. This is an example of regenerative cooling.
- 3. Hold the wax covered cup up with a stick over a plate. Light one of the candles or a propane torch. Aim the flame at the cup and observe. What happens? Why do you suppose the cup behaved the way it did? (The wax melts away carrying the heat with it.) If you try a control experiment with an uncovered cup, what do you suppose will happen? Try the same experiment with the heavy plastic cup. What similarities or differences were there between this cup and the wax coated cup? USE A WELL VENTILATED AREA. This is an example of ablation.
Try to obtain other materials which will respond to ablation. A chemistry teacher may be able to suggest some plastics or other substances which will work. It is very difficult to get a sample of a Shuttle tile but you might try. The Space Mobile demonstrators bring a piece with them when they visit schools for workshops; you may be able to see a demonstration of the tile’s ability if you have such a visit. The tiles, by the way, must be cooled by air conditioning within 15 minutes of touchdown or the heat “soaks” through to the Shuttle skin which is only aluminum.