For your ordinary pen if you try to write against gravity then the pen will stop working soon after you begin writing.
The space pens actually contain pressurised air which pushes the ink out while writing. So the pen writes smoothly even against gravity. They tend to be expensive because of the high quality seal at the tip which prevents leaking and maintains the pressure for years.
For our pen we will pressurise the refill and seal it. Modern ball pens have a good sealed tip too. They might not be as good as the nasa pen, but still can be pressurised with little or almost no leakage.
Step 1: 1.Materials
1- A ball point pen of your choice ( dont use gel pen because they will leak).
2- Baking powder.
4- Instant glue.
5- Drinking straw.
6- Strip of rubber( for sealing the refill).
Step 2: 2.Prepare the Seal
Step 3: 3.Assembly
First of all put a little bit of water into the refill. Now because of high surface tension the water will not go in easily.You can use a syringe to put in the water. If you dont have a syringe then use a drinking straw. First pull the drinking straw so that it becomes thinner at the middle. Then cut it at the thin region. The straw will now fit in the refill. Put a tiny amount of water in the refill.
Now put a tiny amount of baking powder in the refill. You can just scoop up the baking powder with the end of the refill and then push it inside with a stick.
Now put a bit of glue at the end and put the rubber seal you made earlier. Hold it tightly until the glue dries. Once the glue is dry, shake the refill so that the baking powder mixes with the water and produces carbon dioxide which will automatically pressurise the refill.
Step 4: 4.Finally
It might not be as good as the nasa pen, but it still works and it works really well. So make your space pen and have fun.