From: ederd@bcstec.ca.boeing.com (Dani Eder)
Subject: Re: Black Horse: LOX mid-air fueling nontrivial
Date: May 17 1995
Newsgroups: sci.space.policy,sci.space.tech

cmeinel@auriga.unm.edu (carolyn meinel) writes:

>The problem with midflight transfer of LOX to Black Colt/Horse is that, 
>according to many sources, the first law of midair refueling is that the 
>tanker and vehicle have to be able to separate by default if anything 
>goes wrong. Under these circumstances some of the fluid being transferred 
>leaks. Even with JP5 this has killed people. 

The point of the mid-air refueling is so that the Black Horse vehicle
is starting fully fueled from above ground level with some initial speed.
This makes getting to orbit easier since you have lowered the velocity
required by the vehicle.

I have a suggestion that avoids mid-air refueling.  That is to use a
tow rope type catapult to accelerate the vehicle initially, and it
flies on it's own after that.

Here is an example:  Find two mountain peaks 10 km apart, call them
A and B, with A on the left.  On the left slope of A put a short ramp
up to the peak.  Put your vehicle on the ramp.  Run a high strength
cable up the ramp and over to B.  Provide a drive mechanism for 
winching in the cable (a big weight sliding down B driving a gear train,
so the cable will be pulled at more than 1 g).

If we limit the acceleration to 6 g's (for human endurance), we can
reach a little over Mach 3 over a 10 km distance.  The ramp on A
gives you some upward velocity initially, and once you get some
air speed, your wings provide additional lift, so you climb as
you get accelerated.  This way you clear the top of B by a good
margin and are generally climbing.  You unhook from the cable as
you near passing over B, and then light up your rocket.

Assuming for calculation purposes, a 50,000 kg vehicle (I don't
know the mass of the Blackhorse), an acceleration of 60 m/s^2
requires 3,000,000 Newtons (675,000 lb) of traction.  A carbon
fiber cable at 3 GPa stress (435,000 psi) would need 0.001 square
meters of cross section (1.8 cm radius).  The cable would mass
1.85 kg/meter or 18,500 kg overall, a reasonable overhead, since
you have to accelerate both the vehicle and the cable.  Assuming
the cable is moving 10% sidewise through the air (due to the
vehicle rising) and is around 2 km long with the vehicle moving
1000 m/s, we have a sideways drag component on the order of 200 KN,
which needs to be overcome by vehicle lift.  This is on the same
order as the cable mass when it is stretched out, so lift is not
a major problem.  Similarly, skin friction on the cable is reasonable.

Since you will be reaching high speed at not too much altitude,
dynamic pressure on the vehicle is a question, and 6 gs fully
fueled is more load than I expect the Blackhorse is designed for,
but it gets rid of the mid-air refueling problem.

Dani Eder