If it's hypersonic atmospheric cruise, then it would be for weapons development as there's no civilian use for that technology.
Satellites are launched first up and then horizontally outside the atmosphere.
Also with the size of the rocket the proportional air breathing machinery power scales between linear and square while rocket mass cubically, so a useful air breathing on say a Starship is hard to imagine.
Payload size is clearly a massive problem because airplanes are so volume limited. But there is another issue here: heating. In order for an air-breathing engine to provide useful thrust, you have to have enough air for it to breathe. That pretty much requires you to now have to deal with aerodynamic heating, which is a thing that most rockets try very very hard to ignore on launch.
The most prominent air-breathing to orbit attempt going right now, Skylon, seems to be viewed generally pretty skeptically by most people in the space community, from what I can tell. And even then, the log-line is different: instead of separate jet engines and rocket engines, the idea is to have a rocket engine that can make a mediocre jet engine for some portion of the flight. Most people outside RE don't think this idea will work very well (I recall people who ran the numbers pointing out that HOTOL- the program that Skylon grew out of- would actually have significantly more payload if it had just rocket engines) but Bond and co. continue on, with very small budgets, the sort of budgets suitable for long-shot technology development. Thus, it appears that the people who control the purse strings generally agree that this has a low chance of working out, even if it is worth trying.
I haven't done the math, but in general, the benefit of something like stratolaunch is much less than you'd expect
Here's a great (but complicated) reddit post on this topic:
https://www.reddit.com/r/KerbalAcademy/comments/63nzq6/how_d...
https://www.geekwire.com/2017/air-force-secretary-highlights...
This presumes, of course, that the cost of turn-around for reflight is <<< the cost of building a new one. This is obviously true for normal airplanes but was probably only barely true for the Space Shuttle. So there is a continuum here, a point where the between-flight inspection and cleaning costs do loom large enough to not be worth it.
The lift-to-drag ratio decreases with speed, but the limit is not zero, but rather 4.
https://en.wikipedia.org/wiki/Lift-to-drag_ratio#Supersonic
More precisely the L/D ratio as a function of the Mach number M is (4M+3)/M. Of course, that's the ideal maximum L/D ratio. But let's go with it.
So, let's see what happens if you travel at Mach 2, Mach 5 (lower limit of the hypersonic regime) and Mach 10 (which is around the reported speed of the Russian Zircon missile, which they tested a few days ago, on 28May2022). The L/D ratios would be 5.5, 4.6 and 4.3.
Let's say we envision a flight path of 1000 km (same as the said Zircon test). At Mach 10 that would take about 300 seconds = 5 min, and at the slower speeds of Mach 5 and Mach 2 it would take 10 and 25 minutes.
Of course, you'd like the missile to get to the target sooner rather than later, if you could help it. The paradoxical thing is that the faster the missile is, the less fuel it needs to burn.
You read that right. And this has nothing to do with air-breathing scarmjet engines. It's just old fashion rocket engines.
Here's how this works, in our example. At Mach 2, our drag is 1/5.5 = 18.2% of the lift, so your engines need to push with 0.18g to keep you at constant speed. They need to do that for 1500 seconds, so that translates into a delta-v of 0.18 x 10 x 1500 = 2.7 km/s. A typical kerosene-oxygen rocket engine has an exhaust velocity around the same, so the rocket equation tells us the ratio of dry mass over total mass (dry mass + fuel) is about exp(-1)= 37%. So, 63% of your missile has to be fuel.
At Mach 5 the drag is 1/4.6 = 0.22g. Delta-v becomes 0.22 x 10 x 600 = 1.3 km/s, and the ratio of dry mass to total mass completely reverses: now only 62% of the vehicle mass is dry, and 38% is fuel.
At Mach 10, delta-v is 0.23 x 10 x 300 = 0.7 km/s and the fuel mass is only 23%.
For comparison, the phenomenal Tomahawk cruise missile has a fuel weight of about 450 kg and a total mass of 1300 kg, so a fuel ratio of about 35%. A Mach 10 hypersonic missile could have a comparable fuel ratio as a Tomahawk subsonic cruise missile.
Of course, the Mach 10 missile needs a booster to bring it at altitude and cruising speed. But that booster can be a fairly dumb (and inexpensive) rocket. And in the era of SpaceX, who knows, that booster might even be reusable.