So why not air breathing?

In the last post we showed that if you could raise engine Isp by 20% you could double the vehicle mass and still come out ahead. Since that is the case, why don't all aircraft use air breathing which have much higher Isp?

The first issue is that even if you don't go to space you want to get above the thick atmosphere as quickly as possible. The atmosphere's drag keeps you from going as fast as you want. That means you can't really use air breathing engines for most of the flight - just at the start and end. So even a high Isp air-breathing engine may not raise the average Isp by 20%.

The other issue is the air-breathing engine tend to have pretty bad thrust to weight ratios. Since you want to get above the atmosphere as quickly as possible, you want to go pretty much straight up. That means your vehicle must have a thrust to weight ratio of at least one, and start from a standstill. Really good air-breathing engines that can do that only have a thrust to weight ratio of about 10:1. So your engine weighs 10% of your GLOW!

Unfortunately, that means that most air-breathing rocket assist engines do not work out.