X-37C

The X-37C is a crewed version of the X-37B, which flew successfully a couple of times.

The crewed model is said to be up to 80% bigger than the current one, which masses at 11k pounds.  If it can be kept within those parameters ( by mass ), then it may be possible to mate this with a rocket and launch it on a StratoLauncher, as envisioned in the prior post.

The original goal for the StratoLaunch was for 500k pounds to be strapped on the Big Bird ( birdzilla ) that would taxi down the runway and take off like a conventional airplane.  The spacecraft would be attached to the wing structure.

I think such a configuration could fit under the wing.  The Dragon capsule originally planned ( and scrapped ) is 12 feet in diameter.  This X-37B model is a bit smaller, but a bigger one hopefully won't be too fat.

If the weight and sizes can be kept in check, then it could all be feasible.  The key is the SABRE engines would have to be imported into the design so as to achieve the payload fraction needed for this design.  The booster would also be reusable, which means that the lifting body concept must also be employed for it.

After lifting the X37-C to orbit, the lifting body booster would do a once around the earth and then land like an aircraft.

Total wet mass could be comparable to the Falcon 5 configuration originally planned for the StratoLaunch.

If mass cannot be minimized enough, perhaps an airbreather booster could be employed to get it up to a first stage altitude where it could be released, then the airbreather could land.  The StratoLaunch can position it so that it has minimum need for propellant for a glide back home.

The X-37C is not considered as a prospect for the StratoLaunch at the current time, so I could be just talking out the wazoo again.  Also, an airbreathing rocket like the SABRE is not being developed in the USA, as far as I know.  There is another airbreather, but I don't like it.  But it may work for getting the bird up to near orbital velocity, while the final burn is performed by the orbiter, like the Shuttle did.  A conventional rocket like Musk's is probably the best bet.

Frankly, I'd like to see an airbreather used on the first stage at least.  This saves mass while gaining altitude and velocity.  The second stage may be expendable, or an attempt at reuse could be employed eventually.

You could use an airbreather SABRE powered first stage, a SpaceX type reusable second stage, plus a Gemini type capsule which would be under 10k pounds.

Thus, this requires a belated speculation alert.