HORIZONTAL TAKE OFF AND LANDING
Triton Systems, LLC
From time to time when attending an aerospace conference and making a presentation about the Stellar-J, a concept for a horizontal take off and landing reusable first stage, I would pose a question to the audience. Most of the audience was usually involved with operations of vertical launch, expendable systems or were pitching plans for vertically launched reusable systems. “How many of you from out of town arrived here via a HTOL aircraft? … And how many of you arrived via a VTOL?”
Surface conveyance accounted for some portion of the attendance, of course; and when one exchanges travel anecdotes, it’s clear no one is enjoying trips to the airport anymore, but it is not because the passengers would have preferred vertical launch over a takeoff roll. The point is that we as a nation have been going subsonic and stratospheric with turbojets and turbofans for over half a century, logging personal air miles so high that most don’t even bother to count. If millions of us migrate to grandma’s house for Thanksgiving or Christmas over a twenty-four hour period, then how many of us at a given instant are at 10 kilometer altitude traveling at Mach 0.8? Could it be 100,000? It would take 500 jumbo and mid-size aircraft each with an average of 200 passengers.
Now if transport of that nature is so routine, then what’s the problem with going any higher or faster? How much more risk and expense is assumed by incorporating conventional aircraft flight procedures and proceed to higher heights like the old N-104s, the rocket powered F-104 Starfighters or a variant of the X-15? How does the expense or risk increase if we were to reach Mach 5 or 6 or apogee ten times higher than airline cruise? Well, why do it in the first place?
Either one intends to fly an attached payload or passengers – or else one intends to release a payload or passengers ( i.e., launch a satellite into orbit). In the latter case, a rationale would be to find a more expeditious means than launching and descending vertically – or not recovering a vehicle at all. Some years ago ( 2007) William Bauhaus, then the president of the Aerospace Corporation, at the annual AIAA Aerospace Convention in Long Beach, CA, spoke of the need of a hybrid vehicle: partially expendable and partially reusable. He was speaking in the wake of cancellation of the X-30 program to build a prototype for single stage to orbit operations. A reusable first stage was recommended to reduce development and operational costs, but the mode of operation remained vertical launch. Whether it was all rocket like the Shuttle Orbiter which took off vertically and landed like a glider, propulsion was to be determined, but not the launch mode. That was vertical. In our case, we recommend a further hybridization: 1st stage has both air breathing propulsion and rockets operated in sequence in support of horizontal launch and landing.
There are two corresponding metrics for performance for rockets and jet engines: respectively, specific impulse ( ISP : pounds of thrust per pound of propellant consumed per second) and specific fuel consumption ( SFC: pounds of fuel burned per hour per pound of thrust). One might prefer metric vs. English measures and argue as well that there are other dimensional and measurement units, but a little back of the envelope calculation indicates that burning carried propellant with SFC = 0.5 is equivalent in rocket ISP terms to 7200 seconds. In space propulsion that’s equivalent to very efficient electric propulsion, but with orders of magnitude better thrust to weight ratio – and that’s more than twenty times higher than the ISP of an F-1 engine that drove Saturn V rockets through the sonic barrier and into the stratosphere. Moreover, when you consider what Tsiolkovskij’s rocket equation expresses, it’s not the difference in velocities at start and finish, but also total velocity required to overcome physical losses from drag, gravity and turning once you ignite a rocket and head to an orbital target. To make that story short, the principal velocity loss for ballistic rockets headed to orbit is “gravity” or that resulting from climbing to altitude. It can be calculated a priori as Delta-vGRAV = sqrt( 2 g delta-h). Does this remind you of the concept of “work”? When you are flying under jet power you are not nearly as concerned about drag, because you are not changing velocity but flying in a nearly unaccelerated equilibrium. You can also turn to a desired azimuth with little penalty and cancel out some of the ideal velocity requirement with aircraft airspeed; you can even do switchbacks and experience little penalty when attempting to align with an orbit inclination, ascending node or even rendezvous path. Ballistic vertical launches are limited to “dog leg” maneuvers in powered flight.
Back in the late 1980s and then again in the late 1990s, I had the privilege of participating when NASA examined replacement of the Space Shuttle Solid Rocket Boosters with boosters running on liquid engines; the first time with expendable liquid rocket boosters (LRBs) and the second time, with liquid fly-back boosters (LFBBs). The second study moved beyond expendable booster concepts to recoverable vehicles, boosters with wings, jet engines and landing gear. True, the Shuttle looked enough like a cathedral with flying buttresses already; and made for an even more exotic flying machine with two more sets of wings (some designs had canards as well). But still, there were significant arguments in favor of the idea:
Solid rockets provided for a severely rough ride; you could see it starkly in videos of the astronauts jerked around on the flight deck until staging. Solid combustion chambers and propellant “tankage” were much the same; whatever structural strength was needed to store propellant; the combustion chamber pressure had to be built in; and that was about 50 atmospheres for the Shuttle solids. They were also hot. The solids were also very difficult to shut off – unless they were hybrids as well with a fluid oxidizer run over a fuel or second propellant. Solid rocket motor casings were “recovered” from the ocean, which was technically reuse, but only minimally when one considers how fuel was re-packed and segments were rejoined. Many veterans of the solid rocket program wondered if recovery was any benefit at all. A liquid rocket could be re-fired on a test stand a lot more easily than a solid motor…
In other words, many liquid engines are inherently vetted for multiple firings and even multiple flights as proven by performance on test stands. Beside the Space Shuttle Main Engines and those of the X-15 rocket plane, there came available after 1990 a number of durable, high performance engines from the former Soviet Union, developed at un-accounted cost. In 1990 I went to the Soviet Union and got near enough to touch an RD-170 at Baikonur; so near that the authorities just about threw me out of the processing facility. I was transfixed; I was staring at the future.
Dumping liquid rocket turbo machinery in the Atlantic or Pacific is not the most effective way to recover them. So, how do US buyers of the RD-180 ( a two chamber version of the RD-170 four chamber and nozzle package, developed specifically for the US market) recover their engines? And how about the NK-33 first stage engines on the Antares, or the RD-170s used on the Sea Launch Zenit boosters? How did they all address this problem? They installed the engines on expendable stages and left them to sink in the ocean bottom after a single flight or 1/20th or 1/30th of their rated operational lifetime. The Energomash and Kuznetsov engines are propulsion state of the art, staged kerosene engines that could run dozens of times at 200 atmosphere pressures ( 3x domestic hydrocarbon-fueled engines ) or more, providing 30 to 40 seconds more specific impulse in tighter dimensions and higher thrust to weight than our gas generator cycle competitors.
It must be expediency. Development and operational costs are examined by the customer ( the government or a comsat manufacturer) and an agreement is reached based on an expectancy of a limited number ( a dozen?) of flights. The taxpayer or the impatient stockholder pays the up-front costs of continued use of a 1950s solution to space flight over here, but the technology remains over “there” awaiting for “their” capital investment for exploitation.
The LOX-hydrogen equivalents of the Soviet staged combustion engine families (!) were the Space Shuttle Main Engines (SSMEs). They were recovered by flying them attached to the Shuttle. The demise of the Soviet Shuttle ( Buran), perhaps, was that the 20 nozzle ( 4 Zenit boosters and four core engines) configuration that the Buran lifted off with – of those 20, not a single engine or combustion chamber came back with it. Reusable high performance engines and wings need each other.
So what does this mean? I suspect that few readers of this are in a position to buy RD-180 or NK-33 engines, but they might have connection to a small satellite payload. Judging by small satellite conferences, there are a large number of people in that category. For some time the space “architecture” in the west has been built on Cold War heritage military surplus and Ariane-style communication satellite launchers delivering 5-10 ton government and commercial payloads worth hundreds of millions or even a billion dollars. A dozen launches seems to satisfy everyone involved in this process, says the circular argument, but it does not constitute much space access.
Whether from Marshall Spaceflight Center or Wright Patterson Field, development plans to rectify or overturn this state of affairs are ostensibly several decade long treks toward technology readiness level (TRL) 9. If they involve hypersonic scramjets, I suspect that munitions studies are more the intended payoff. But the half life of these programs is about 2 years or a Congressional term. I do not see much hope of salvation in this form, even if I live to be a centenarian.
But I do see merit in horizontal take-off with air breathing engines on a winged vehicle which can ignite rocket engines at cruise altitude and speed, to burst out of its air-breathing performance envelope. After shut-down from a burn with a gradual pitch up ( e.g., constant rate), the vehicle separates a single or two stage package which fits within a defined volume above the aircraft fuselage. This configuration will depend on mission requirements, just like many of the things that are displayed at air shows as accessories fitted under fighter aircraft wings attached to pylons. The high performance rocket engines attached to the winged HTOL first stage will pay for themselves by re-use; the jet engines and wings will save rocket propellant and pay for themselves as well. Rocket thrusts levels will be smaller because the vehicle will never be balancing itself on its tail and its highest flight path angle will be about the same elevation as the North Star is above my house in Houston.
Wes Kelly is President and Chairman of Triton Systems LLC of Houston Texas. He is a veteran of NASA shuttle and flyback booster programs and now advancing the Stellar-J launch vehicle
Triton Systems LLC
17000 El Camino Real
Houston, TX 77058
Phone: (281) 286-3680