starship-design: FYI black horse

[KellySt@aol.com]

I thought I'ld foreward some stuff you might be intersted in, or might be 
worth working into a updated site.




Date:  Mon, Sep 16, 1996 11:40 AM EST
From:  MbClapp
Subj:  Answer to your question on Usenet RE Airdrop vs. Retanking
To:      Kelly St

     There has been a great deal of work done on 
examining space access concepts that are based on air 
launch as a means of reducing Delta-V to orbit to a 
level that reduces the performance requirements on a 
launch vehicle. Essentially, subsonic staging provides 
benefits over single stage to orbit in several areas. 
The gravity losses are reduced because the vehicle is 
in horizontal flight, supporting itself by aerodynamic 
means rather than by engine thrust. The drag losses 
are reduced as well. Above 18,000 feet, over half the 
atmosphere is beneath you. The back pressure losses on 
the engines are also reduced because the limit for no 
separated nozzle flow permits larger expansion 
engines. Finally, there is the possibility of staging 
above weather, an operational advantage.

     Relatively little work has examined the 
possibility of inflight propellant transfer as an 
alternative to air launch, however. The reasons for 
this are unclear. The inflight propellant transfer 
concept does offer five distinct advantages over air 
launch.

     First of all, the experience base in military 
aviation with inflight propellant transfer is 
enormously greater than that for air launch. Perhaps 
four hundred manned aircraft have ever been released 
from beneath other aircraft. A similar number of 
inflight propellant transfers are performed each day. 
And the number of stores in excess of 50,000 pounds 
that have been released from aircraft total a few 
dozen at most. Every modern military aircraft can be 
refueled in flight, and for many missions it is 
critical. "Take off, top off and continue with the 
primary mission" is an everyday operation in the US 
Air Force.

     Second, the separation of two large objects in 
flight is an inherently risky maneuver. Stores 
certification history for military aircraft is full of 
examples of released objects striking the parent 
aircraft and causing major damage. The risk can be 
minimized by a number of means, including captive 
carry testing, wind tunnel work, and build-up flight 
test, but at some point the certification program must 
commit to releasing the object – an all-or-nothing 
affair. This level of risk can be managed, but doing 
so drives costs up. Propellant transfer, on the other 
hand, can be certified by slowly and incrementally 
flying formation, then near the tanker, then in dry 
contact with the tanker, then with increasing amounts 
of propellant transfer, opening the envelope in a very 
gradual fashion. Inflight refueling accidents are 
unheard of in flight test. It is a safer activity to 
certify.

     Third, the performance of an air-launch system 
is subject to some important limitations. Because two 
airframes are under the influence of one set of 
engines, the aircraft cannot climb quite as high for 
separation as an inflight propellant transfer concept 
can. The interference drag between the two airframes 
also limits the envelope of the ensemble to some 
degree. The effect is not enormous, adding up to an 
advantage of perhaps 250 ft/sec of Delta-V to the 
inflight propellant transfer concept, but it is 
noticeable. 

     Fourth, the inflight propellant transfer 
concept offers some important advantages in 
flexibility. The orbital aircraft has the capability 
to fly suborbital missions without propellant 
transfer, to distances of 3,000 to 6,000 nautical 
miles, depending on the aerodynamic configuration. 
This capability exists because the airframe is capable 
of independent takeoff and landing. This offers a 
transcontinental range for a number of alternate 
missions that are difficult to imagine for an air 
launch concept. For the same reason, the tanker and 
orbital aircraft may be based at different locations, 
and interfaced only in flight. This offers more basing 
flexibility and removes the requirements for 
specialized facilities and ground support equipment 
such as that needed to mate the Shuttle orbiter to its 
carrier aircraft.

     Finally, the carrier aircraft for an air 
launch concept must be either an entirely new aircraft 
or a major, airworthiness affecting structural 
modification to an existing aircraft (unless the gross 
weight of the orbital segment is very small). The 
carrier must bear not only the weight of the 
propellant, but also the empty weight of the aircraft 
as well as its payload. The orbital aircraft must be a 
great deal smaller than its carrier for an air-launch 
concept, while it can be larger than the tanker (or 
tankers) for an inflight propellant transfer concept. 
This drives the designer to very large carrier 
aircraft, which can be expensive.

     

     
Mitchell Burnside Clapp