Rockets Class
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Lesson 3: Pump Rocket
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Introduction: Pump Rocket

Let's move onto a larger rocket with a few more components. The humble pump rocket actually has a lot going on, using water to displace air inside the rocket housing to build pressure. This pressure will cause a cork to eject and then use the water as a propellant to blast off. In this class we'll cover how to make this wonder of a rocket, and explore what goes into making it airborne.


The best part of this type of rocket is that you probably have all the materials around your home already! Gather up these supplies and we can start construction.

Supplies:

If you're not in the mood to make your own, they sell lazy rockets if you don't want to DIY, which are screw-on water rocket fins to a soda bottle.


Cork Stopper

Take a wine cork and stick it into the soda bottle's opening to test and make sure that it fits in snugly. The seal has to be tight, so if your cork doesn't fit then look for a larger cork - or you can wrap the cork with tape to slightly increase the diameter.

Remove the cork from the bottle and use a sharp knife to cut the cork short enough so the Schrader (bike pump) needle is longer than the cork. If you look closely at the top of the inflation needle you can see a small opening that needs to stay exposed.


Drill Into Cork

Using a drill bit that is slightly smaller than the inflation needle, drill into the center of the cork all the way through. If you don't have a drill you can punch through the cork with an awl or a nail. The important thing is that the drilled hole is smaller than the Schrader needle.

Push the Shrader needle through the drilled opening to check that needle eye is completely unobstructed when inserted into the cork.


Fit Cork

This rocket works by building pressure inside the bottle, with the cork being ejected when the pressure builds up. The opening where the cork was will be where the thrust for the rocket will be expelled. Because of this, the cork needs to fit very snugly in the bottle opening. Remove the needle from the cork and insert the cork into the mouth of the bottle and check the fit.

If your cork is slightly smaller in diameter than the bottle opening you can wrap it with tape to make the diameter larger.


Add Fins

Fins on a rocket help keep it steady in flight. The fins are lined up parallel to the body of the rocket so that when the rocket is in flight the fins will direct air, keeping the rocket pointing in the right direction during flight.

Rocket fins can be made of almost any rigid material and take almost any shape you like; I made these fins out of an old cardboard box. I wanted these rocket fins to be large and rigid enough to hold the rocket up off the ground, allowing enough room for the bike pump valve while keeping the rocket perpendicular to the launch pad (the ground).

Once I was happy with the sizing and shape, I first cut the rocket fin out using a sharp hobby knife.

Then, repeated the same process for a few more fins. Your rocket can have as many fins as you like, but I recommend 3-4 fins (a good compromise between weight and stability).

With the fins cut out, I taped them onto the bottle at the height that would have them hold the bottle opening up off the ground. Hey, it's looking like a rocket now!


Tape Cover (optional)

The rocket is functionally complete, but could use some treatment to look more like a rocket.

Borrowing from NASA's space program I decided on a white covering for my rocket. I used white duct tape, since it has the added benefit of being able to secure the fins together while covering them, but paint could also work.

When covering the bottle I deliberately left a section running the length of the bottle uncovered, allowing a window into the bottle so I can see how much water ballast to add later.

The fins were covered in the same manner and reattached to the body.


Decoration

It's been scientifically proven that a decorated a rocket directly proportional to how far it will go.

Since there was already a white covering on the rocket I decorated it with NASA's logo and a few other rocket-like markings.

While I was at it, I used the uncovered window I left to be my water gauge, applying incremental markings with a permanent marker. Your rocket is all ready to go! Head outside with your rocket, bike pump, some water, and get ready to launch!


Pre-Launch + Launch

Before blasting off you're going to want to source an area outside, since it's going to be a messy launch. Find an open location away from people, power lines, buildings, and trees, and make sure you're able to get the area wet (like a field or the driveway).

Invert the rocket and fill the bottle about 1/5th full of water. The water in the rocket is the reaction mass that will help launch it high into the air.

To prevent any water loss, it's a good idea to insert the inflation needle of the pump into the cork first and then install the inflation needle into the bike pump before plugging the bottle opening with the cork. This ensures a very snug fit before upending the bottle.

Turn the rocket right side up and place on a level surface in preparation for launch.


Science Moment

Why add water?

This rocket works on pressure, specifically the pressure difference between what's inside the bottle (water and pressurized air) and the air outside the bottle (the atmosphere). When we use the bike pump we're forcing more air inside the bottle, creating pressure. Air can be compressed, but water cannot. By adding water as a reaction mass (like the ballast we used in the Stomp Rocket Lesson) we're reducing the air space inside the bottle. When more air is forced inside the bottle the pressure builds up, and when there's enough pressure the air will try and escape the bottle through the easiest exit - the corked opening. Since there is water in the way of the air escaping (the reaction mass) the air will push against the water, forcing the cork out of the bottle and expelling the entire contents of the bottle (compressed air and water) in one explosive motion.

This explosive exit of reaction mass will cause the rocket to shoot up into the air, this is because of Newton's 3rd Law of Motion"For every action, there is an equal and opposite reaction".

Regarding center of pressure; the pressure difference inside and outside the rocket is resolved at the narrow bottom of the rocket, making that the center of pressure. Since the center of pressure is at the extreme bottom of the rocket, the center of gravity will have to be somewhere above this point, therefore we know we have a good rocket design.

{
    "id": "quiz-1",
    "question": "Good rocket design has:",
    "answers": [
        {
            "title": "The center of gravity above the center of pressure.",
            "correct": true
        },
        {
            "title": "The center of pressure above the center of gravity.",
            "correct": false
        }
    ],
    "correctNotice": "Yes! Rockets work best when the pressure is pushing the center of gravity.",
    "incorrectNotice": "That's incorrect."
}

If we were to attempt a launch with just air the cork would explode out the bottom of the rocket when the pressure had built up but the rocket wouldn't go very far, the air would just rush out. There's a balance for how much water to add to the rocket to achieve the best compromise of water and air space, which you can clearly see by how high the rocket goes. Try filling about 10% with water, the 50%, and then 90% water. What did you notice?


Pump + Blast Off!

When you're ready to launch, start pumping the bike pump to build up pressure inside the bottle. You'll hear the bottle bubble and fill with air. With enough pumps, the pressure inside the bottle will build to a point where it overcomes the friction holding the cork in place and jettison the cork and inflation needle, sending the rocket skyward! You may need to hunt for the cork and needle if it becomes detached from the bike pump after launch.

Troubleshooting:

While pumping you may hear an audible hiss, this means the cork doesn't have a tight seal and the rocket is losing pressure. Depending on how much pressure is being lost, the rocket may require more pumps in quick succession to compensate for this small loss in pressure, or you may have to abort and start over, making a better seal around the cork with tape.

Pump rockets are a very satisfying and fun way to demonstrate how rockets move as a result of pressure difference.


With the basics of rocketry covered, we're ready to move onto the final frontier of introductory rockets: solid-propellant rockets. These types of rockets are a BIG step up from the rockets we've made so far, using a solid fuel mixture to provide thrust for lift-off. These types of rockets are very reliable and are what many early rockets were made from before the switch over to liquid-propellant rockets, which are what all space-faring vessels like SpaceX and NASA use today.

Let's reach for the stars in our final lesson, Model Rockets!

CLASS PROJECT

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