I used a vise and a couple of 2x4's to simulate a hard to reach pipe. As a plumber I encounter this a lot. They do make a cable saw for this but, they don't last. I'll show you a way you can cut it easier than a cable saw.
Step 1: Get Nylon String
The string on the left is nylon . It's very common string. The string on the right is not nylon it's cotton and will not work.
Step 2: The Process
Wrap the string around the pipe as shown. I don't cut my string; that way I can position myself to a comfy spot while cutting the pipe.
Step 3: Back View
After the string is in place pull on the string lightly to keep it in place to where you want to cut it; at the same time saw with both ends of the string back and fourth.
Step 4: It's Working!
This is sched 40 solid pipe. It works even easier with foam core. With just a few pulls you will get a groove started.
Step 5: The Finale.
You will quickly get a rhythm going and before you know it you will have cut a straight smooth cut.
Step 6: Done
It takes about 20 seconds to cut 1-1/2" pipe. Works every time!