Blaster Master is a pretty tough game, but Jeff Rovin's How to Win at Nintendo Games #2 would have you believe that it's a lot tougher than it actually is.

In Area 4 you come across an area that is blocked by two doors, with a keyhole on the outside of each

And the strategy for getting on the other side is as follows from page 31 of How to Win at Nintendo Games #2

"Here's a special tip: When you come to Stage Five at the end of Four, there are two Doors , and you need a Key for each. If you only have the one you got for killing the long-tongued, fire-breathing Frog Mutant at the climax of Stage Four, don't despair! Open the first Door, get out of the ol' Rover, and climb the ladder all the way up. Take a mighty leap from the top, staying close to the wall on the right; if you land on the lock, you'll open the Door. You'll lose that Jason's life ... but it's better than standing around with your hands in your pockets, unable to continue!"

So, first you have to beat the boss of the Area to get a Key, no problem there.

Then you come back to the doors and open the first one.

Climb out of your tank, climb the conveniently-placed ladder to the top of the precipice.

Then jump off, hugging the left wall, and aiming for the lock.

It opens, you die. Totally right!

Except...

One could also just climb down the series of ledges slightly to the right, then walk over and unlock the door with the same key (it doesn't disappear after being used, which is different from a lot of games).

That way you can open the door to continue while not costing yourself one of your precious extra lives.

So it's totally true as presented, but there's a better way.