Here’s a great engineering project for kids – build a rocket powered LEGO® car!

How to Build a Rocket Powered Lego Car

The Idea

A couple weeks ago, I saw an idea on Pinterest to build a rocket powered Lego car.  It sounded like something that would be right up our alley!  The instructions said to use a film canister filled with Alka Seltzer and water to power the car.  The Alka Seltzer and water release gas, which builds up until it blows the lid off of the film canister.  Since film canisters are no longer readily available, my husband suggested using mini M&M’s tubes.  We tried it!  And it flopped. 🙁  The cap shot off the mini M&M’s container with a lot of force, but all of the energy went into the cap and did not propel the car forward more than about 8 inches.

My dad is an engineer, and we discussed what might fix this project.  My dad suggested drilling a small hole in the cap so that only a little bit of gas would be able to escape at a time, thus giving the car a more ongoing source of power.  However, we talked ourselves out of that idea because we figured that the car needed a burst of force to get going.  The hole in the cap would allow gas to escape as soon as we dropped in the Alka Seltzer, so no big burst as the pressure finally blew off the cap.  I tried gluing the cap to a cardboard box so that the cap would remain stationary and hopefully propel the car forward when it blew, but that didn’t work either.  The car still only traveled a few inches.

On Sunday, I asked a friend at church if he had any ideas.  He agreed that it was a pretty awesome idea, but wasn’t sure what to do.

On Monday, I checked my e-mail and found a message from our church friend – he had a working prototype!

How to Build a Rocket Powered LEGO® Car:

This activity requires adult help and supervision.  You know best whether your children are old enough for this project or not.

Using his design, Aidan was able to build two different rocket powered LEGO® cars that both travel over 20 feet.  Our longest run was 34.5 feet!

The key to success is to use an empty Tacky Glue bottle (we used the 4 oz. size).  Fill the bottle about 2/3 of the way with water.  Drop in two Alka Seltzers (we cut them in half to fit them in) and quickly screw on the lid.

The pressure builds for about 30 seconds and then the cap blows off the tip and the car shoots forward!

Car #1:

How to Build a Rocket Powered Lego Car

Here’s a view of the car without the Tacky Glue bottle.

How to Build a Rocket Powered Lego Car

We attached the glue bottle with a rubber band.

Car #2:

How to Build a Rocket Powered Lego Car

How to Build a Rocket Powered Lego Car

TIPS:

  • Make sure the car is lightweight.
  • Make sure that the car is strong enough to handle having the glue bottle attached with a tight rubber band and strong enough to withstand the launch.
  • Make sure that the wheels of the car still spin freely when the Tacky Glue bottle is filled with water.  The water adds quite a bit of weight, and if it touches the wheels or causes other pieces to touch the wheels, the car won’t go very far!
  • You want the nozzle of the glue bottle to be as low to the ground as possible so that the force is located on the same plane as the wheels.  This helps to overcome the friction between the wheels and the ground.
  • Put a towel behind the car to catch the cap when is comes off – otherwise you might be hunting for it for awhile!
  • Quite a bit of water sprays out along with the CO2 gas, so this is a good outside project.
  • You may need to dry off the bottle in between runs.  We found that if the bottle was wet, it was harder to screw the lid on tightly and the lid would end up leaking.  If it leaks, you won’t get enough pressure built up to blow off the cap.
  • We had two Tacky Glue bottles, and one worked better than the other.  The one that worked the best was an older one that we had on hand.  Our new Tacky Glue bottle would sometimes swell from the pressure, and yet the cap had still not released after two minutes.  We ended up just releasing the cap ourselves.  I really don’t think that the plastic bottle would explode, but just be aware.
  • Have a video camera ready.  It’s super fun to re-live your best launches!

How to Build a Rocket Powered Lego Car

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3 Comments

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  1. Kristina Barnaby Mar 20, 2015

    I would love to make these for my girls summer STEAM camp, but I do not have any Tacky Glue bottles on hand. Do you have suggestions for other bottles that would work -- like an empty craft bottle? I could always buy the glue and dump it, but that seems like a waste.
    Thanks.

    Reply
    1. Sarah Mar 20, 2015

      Hmm, good question! The key is to find a bottle with a small nozzle with a small cap. Something like a film canister won't work. If you do end up emptying out the glue, the 4 oz. size Tacky Glue is less than $2 per bottle, so at least it's not a huge expense. I wonder if the the glue could be saved in a container of some sort?

      Reply
      1. RO Aug 19, 2017

        Perhaps these would work: https://www.amazon.com/Plastic-Squeeze-Dispensing-Pinnacle-Mercantile/dp/B016910ROM

        Reply
        1. Vicky May 14, 2019

          These don't work. We've had failure with the new tacky glue bottles as well. The tip doesn't pop off and the bottle swells. We took off the thread of the nottle and the cap blows off before we get it to the ground. Next, we'll try taking off a section of the threads in hopes that the tip will blow off. We also found warm water makes these blow faster than cold water.

          Reply
    2. Engineer the Dad Apr 15, 2015

      Fantastic! I'm so impressed with those. I'll be building my own with the kids soon. FYI - it's the water being expelled that provides most of the thrust. It's much heavier than gas, and so provides thrust at much lower nozzle speeds than you would need with gas. It's counter - intuitive, but the more water you have pushed out, the further the car will go. Tilting the bottle down a bit might give you more distance, as the gas will need to expel more water before it can escape.

      Love your site!

      Reply
      1. Science Mom Sep 8, 2015

        Does the smaller cap work better due to the slower release of gas, or because it pops off easier? Thanks.

        Reply
        1. Vicky May 14, 2019

          Because it pops off easier.

          Reply
      2. Lilly Nov 10, 2016

        Do you have any alternatives to use instead of legos?

        Reply
        1. Sarah Nov 23, 2016

          Not that I can think of... Sorry!

          Reply

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