The Space Review: you have done orbital refueling, but have you considered 'sub'orbital refueling?

[DDE]

https://thespacereview.com/article/3875/1

[sevenperforce]

I always had a soft spot for Black Horse. This seems better.

[Dirkidirk]

very kerbal, I approve 

[sevenperforce]

Now to build and test in KSP...

You'd need planes trimmed to lift off automatically at a certain speed so you could get them off the ground together. Then it would be a matter of choosing pre-set trim action groups so they would essentially fly themselves into a suborbital parabola.

I really like the idea of dumping precooled oxidizer into the intake of a COTS fighter jet engine in order to squeeze out extra performance. Wouldn't be as efficient as SABRE but would be considerably cheaper, easier to achieve, and more robust.

[hms_warrior]

Maybe i am reading this wrong but...isn't that just a complicated way of Crossfeed? With Planes instead of Falcon Heavy?

[magnemoe]

2 hours ago, hms_warrior said:

Maybe i am reading this wrong but...isn't that just a complicated way of Crossfeed? With Planes instead of Falcon Heavy?

Yes, if you imaging cross feed on an reusable falcon heavy, you will drop the boosters a bit earlier but the core will be fully fueled at this point. 

The problem with suborbital refueling is the short time you have to do it. It will require you to fly pretty much in formation up to suborbital. 
I would also have the tanker and orbital plane of different design since the tanker don't have to handle orbital reentry. but has to have the tanker systems. 

if you go for an spaceplane route this makes some sense. 
You would want to do mid air refueling after takeoff anyway as your landing gears and engines don't need to designed to take off with full load. 
 

[DDE]

On 2/7/2020 at 12:35 PM, magnemoe said:

You would want to do mid air refueling after takeoff anyway as your landing gears and engines don't need to designed to take off with full load. 

And SR-71 actually leaked a lot until it had heated up by going supersonic.

[Cheif Operations Director]

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Edited February 8, 2020 by Cheif Operations Director

[magnemoe]

4 hours ago, DDE said:

And SR-71 actually leaked a lot until it had heated up by going supersonic.

I know, note that they you have both fuel and LOX tanks that plan has some issues 

[Guest]

This seems to introduce much more risk and complexity than simply staging a single craft. 

[magnemoe]

16 hours ago, Dale Christopher said:

This seems to introduce much more risk and complexity than simply staging a single craft. 

Agree, not sure about the math here but I think that if the carrier plane carrying the orbital plane it would not be much worse than carrying only its fuel. Now you can drop the atmospheric engines and the air intakes on the orbital one also you can further slim down the wheel, also the wheels does not need to be raised. You also has the option to replace the spaceplane with an disposable second stage to carry heavier payload. 
You also avoid doing docking and refueling in an hurry. It still makes sense to refuel the carrier plane before it goes supersonic and start climbing.