If the problem of access to space were to be solved by increments, what would those increments be? 

 

A launch vehicle to reach space must fly at least ten times higher and reach speeds over twenty times faster than the commercial aircraft. This has been done with reusable spacecraft such as the Space Shuttle, but not frequently nor inexpensively.  Yet in contrast, commercial flight is done routinely by millions of individuals. 

 

Certainly, space flight is more difficult than commercial airline operations, but what is striking about these two contrasts is the vast empty reach between the two.  Should there not be more routine access to high altitude and high velocities?  And could this not be used to provide a hybrid launch into space from a reusable craft with an expendable upper stage?  And if so, what is a modest, economical approach to obtain such a means?

 

Since the 1950s military and commercial air traffic has routinely cruised at nearly the speed of sound around 40,000 ft.   Beside military traffic, commercial supersonic transport passenger service has crossed oceans at twice the speed of sound at altitudes around 65,000 feet, (e.g., the Concorde SST).  These means of travel are both fundamentally based on the capabilities and limitations of conventional jet engines and the realm of space has been attained almost entirely through the use of rockets.    To give serious consideration to improving space access by means derived from aviation, it is necessary to discuss the strengths and weaknesses of the two engine types.  From that perspective we discuss the two below, leading to what we believe is a logical next step for space transportation.

 

JET ENGINES

The main technical differences in commercial subsonic passenger flight over 50 years have been changes in jet aircraft propulsion: conversion from turbojet engines with high fuel consumption rates to turbofans with high “by-pass ratios” but greater fuel economy.  Control system, aerodynamic and automation advances have made flight more efficient as well, but conventional passenger air-cruise has remained locked at 80 to 90% of sonic speed ( Mach number M=1).  In both instances, subsonic and supersonic, passenger flights fly within a distinct envelope of flight cruise speed and altitude: an equilibrium of gravity countered by lift and thrust countered by drag with little or no net sensed acceleration.

 

When human flight patterns from near the ground all the way up to low orbit are considered as plots of altitude vs. speed, the regions of exploitation appear almost like ecological niches.  The top 50 airlines of the world each operate fleets of from 30 to 300 planes.  In 2005 their revenue passenger millions of miles varied from 11,116 (Qatar Airways) to 138,221 (American Airlines).  At any instant in the day hundreds of passenger aircraft are aloft around the world transporting the equivalent population of a mid-size city.

 

If air traffic is thick at subsonic speeds between 35,000 and 45,000 ft and regular but sparse at 65,000 ft and inclusive of supersonic aircraft, why don’t we see much of any activity at 80,000 or 100,000 ft with speeds triple or quadruple the speed of sound? 

 

Initial impressions of this issue could be formed by two different sets of circumstances leading to optimistic or pessimistic expectations.  Attainment of higher aircraft speeds and altitudes in the mid-twentieth century led to expectations of a continued trend.  Yet costs for space transportation systems, the efforts for safeguarding their flight and the incidents of failures led to a sober view of further flight progress as well.  We say both impressions are justified, but the performance desert between the stratosphere and space is permeable, a barrier that should not remain for much longer.

 

For jet powered flight, these two thresholds of equilibrium or cruise flight are based on the momentum or velocity imparted to air taken into the engines and then ejected in exhaust through combustion.  A jet engine’s exhaust velocity must be faster than the intake and surrounding stream of air  – or else jet thrust becomes negative.  Air velocity coming into a jet engine depends on how fast the jet aircraft is already moving (or when it’s not, amounts of air sucked in by compressors).  But since the aircraft experiences atmospheric drag as it moves, there must always be a net positive increase in exhaust velocity ( imparted by combustion with fuel), not only to maintain exhaust velocity equal to or above airstream velocity, but sufficient to overcome drag forces as the aircraft moves at fixed speed.  Acceleration for a jet aircraft will fade out when exhaust velocity cannot overcome drag as well as exceed the airstream velocity. 

 

The expression for aircraft drag force is ½ r(h) C_D(M, a) v² S.  Rho represents density at altitude h; C_D is the drag coefficient which will vary with mach number and angle of attack (a); S is a reference surface area and v is the relative velocity in air.  Were it not for the fact that the drag coefficient varied with M, drag would increase indefinitely as relative velocity increased.  Fortunately the drag coefficient reaches a local peak near sonic speed and then drops off significantly at supersonic and hypersonic velocities, barring problems with aircraft design characteristics.  As a result, supersonic aircraft have tended to punch through the sound barrier with high performance jet engines using devices such as afterburners that increased thrust and exhaust velocities at the expense of fuel efficiency.  Beyond the “sound barrier” they could throttle back somewhat to fly in supersonic cruise, exemplified by the Concorde SST.

 

In both these of instances, subsonic and supersonic, passengers are neither screened for medical problems nor trained for emergency procedures before boarding – because risks aboard these flights are minimal.  Yet even though most of a passenger flight occurs in cruise- equilibrium conditions, an aircraft must accelerate to climb or reach cruise speeds.  Jet engine accelerations are highest at low altitude and low speed where they are most efficient at accelerating exhaust jets of air.

 

ROCKET ENGINES

As it is often pointed out in basic physics classes, rocket engines are not limited in their acceleration by their exhaust velocities, but rather by how much propellant they carry with them in flight.  While a rocket powered aircraft will experience drag, the rocket engine’s thrust is unaffected by the speed it is traveling through the atmosphere because it is not taking packets of air from it and attempting to impart additional momentum to it like the jet engine does.  Under rocket power, a spacecraft or aircraft  is pushed forward by the expelled propellant.   While the efficiency of the rocket engine is related to the exhaust velocity (specific impulse), the rocket thrust will not turn negative if the vehicle’s atmospheric velocity exceeds its exhaust velocity.   As a result, rocket engines have plenty of reserve punch for pressing through the sound barrier and velocities far beyond, provided they have enough propellant.  

 

Should we wonder why there have been so few flights with rocket powered aircraft, flying at speeds faster than the Concorde SST?   There have been Space Shuttle flights flying into orbit more than 100 times – and there have been about 200 flights of three X-15 rocket planes about four decades ago.   Both these vehicle types reused their rocket engines – and, of course, their airframes.  The X-15 flights were tests ultimately reaching conditions similar to the staging conditions at which the Shuttle sheds its solid rocket booster motors.  Some of these were at higher speeds and climbed to higher altitudes than the Shuttle Solid Rocket Booster (SRB) motors.  Other flights involved speed runs at moderate altitudes where the heating was higher than climbing spacecraft would normally experience.  X-15 results were interpreted as successful since they encouraged a commitment to Space Shuttle development in the next decade, a much more ambitious program.

 

One explanation for the lack of rocket aircraft activity in comparison to supersonic or subsonic jet activity is that jet engines are more useful for sustained cruise in the atmosphere.  If a conventional chemical rocket accelerates to high speed and altitude, it will do so in a matter of minutes at the most – and then it will be out of both fuel and oxidizer and coast like a glider or rock, depending on the vehicle design.  Jet aircraft burn propellant in a more miserly fashion in part because they do not carry oxygen or other oxidizers to burn with fuel.  In conventional rockets, the oxidizer can weigh from 2-7 times as much as the fuel. 

 

Despite such terms as “aerospace” technology, applications of jet and rocket engines have been compartmentalized so that their distinguishing characteristics are obscured for comparisons.  Specific fuel consumption (SFC), the measure of jet engine efficiency is expressed in terms of lbs of fuel consumed to produce lbs of thrust per hour.  Values greater than unity typify turbojets, reaching  2.0 or more with afterburner operation.  Turbofans in cruise flight can reach values as low as 0.3.  Some modern turbofans with low bypass ratios (small inlet cross sections for the fans) can run about 0.5.

 

In the rocket business, efficiency is measured in specific impulse (I_SP) or lbs of thrust produced per lb/sec of propellant consumed ( both fuel and oxidizer).  Rockets burning oxygen with fuels composed of hydrocarbons or hydrogen tend to range in efficiencies between 250 to 460 seconds rating.  If you multiply these ratings in seconds by the gravitational constant (32.174 ft/sec2), it is a good indicator of rocket exhaust velocity. 

 

If we convert jet engine efficiency ratings to rocket terms, turbo fan engines (eg., SFC=0.3) have astounding efficiencies in terms of rocketry.  Values of SFC from 0.5 to 0.3 represent specific impulses of  7200 to 12000 seconds. Only exotic forms of space propulsion systems such as ion drives can produce such efficiencies, but not very much thrust.  Conventional jet engines produce about 5 times their installation weight, the turbojets tending to be more powerful per pound than the turbofans.  Rocket engines produce 50 to 150 times their weight in full throttle thrust.  But in terms of specific fuel consumption (SFC) their scores would be ten or twenty times higher than jet engines.  Though pilots think of jet engines flying under afterburner power as fuel hogs,  they pale in comparison to rocket engines carrying both fuel and oxidizer.

 

Given these characteristics of rocket and jet engines, there are still numerous yet untested ways that they can be applied to designs for transport.  For this reason we will not claim from the evidence cited above that rockets can not be adapted to intercontinental travel, but instead we would recommend attention to how the combination of jet engines and rockets can provide improved access to low earth orbit.  We believe that subsonic and stratospheric first stage flight should be performed with jet engines and then rocket engines should be ignited for climb to hypersonic speeds and altitudes several times higher. At this point a recoverable first stage shuts down releasing an upper stage or stages.  How high or fast the first stage should go would depend on a number of issues: cost, operability, technology… But we are certain that such stages should fly higher and faster than conventional commercial traffic flies today.  The nature of the upper stages should only be constrained by what can be integrated atop of the first stage.