Not So Surprised

“I am shocked, shocked to find that gambling is going on in here!” Capt. Renault says in “Casablanca” (1945).

In my case, I am shocked twice over to read that the Airbus A350-900 has a maximum ceiling (flight level) of FL303 at an assumed maximum gross weight for given conditions, as was illustrated on the specifications chart (AW&ST May 25-June 7, p. 59). 

Unless that is a single engine number, the performance would mirror that of the lower-altitude A300 of yesteryear. The A350, an otherwise spectacular aircraft, though apparently remiss of interactive sidestick controls, would be spending an inordinate amount of time seeking more efficient altitudes as it burned down fuel on a lengthy oceanic crossing.

Tom Carey

Freeland, Washington

Soup-Up The A-10

Single-mission aircraft abound in the military. The C-130, C-17 and the C-5 are dedicated to hauling cargo. The wonder airplanes—SR-71 and U-2—fly very high and take all sorts of “pictures.” The eternal B-52s mostly fly high and drop bombs.

The A-10 “Thunderbolt ll” (Warthog) basically provides ground attack against armored vehicles, and has its own armor that protects its pilot and engines—unlike any other U.S. Air Force aircraft. It fits well with its “wonder” brethren.

All of the other special-purpose aircraft are very different from the vehicles as they were first produced. The U-2, for example, is nearly twice the size of the original, with a much larger jet engine and two very large “sensor pods” added to the middle of each wing. It is a very different aircraft indeed. The odd reality is the keeping of the “same number” to help cover what is, in fact, a new aircraft.

The A-10 needs a new manufacturing run using the same name and numbers. First, extend the body and the titanium pilot armor for a second crewmember. Under his crew station, mount a chain-gun as on the Apache attack helicopter; the crewmember would wear a helmet-mounted sighting system for the gun as a self-defense system and for lateral attacks, as well as managing new sensors. 

Add a tail hook and nose gear launch with a beefed-up nose gear, and complete the makeover with a 4-6-ft. extension of the wings tricked out. Then, like the F/A-18 “Super Hornet,” the new version would be called the A-10 “Super Thunderbolt 2.”

Richard Neveln

Alameda, California

Power Nuances

“Electric Rocket” has a photograph of the Rutherford engine with its centrifugal pumps driven by electric motors (AW&ST April 27-May 10, p. 51).

Despite the motors replacing turbines, the terms “battery-powered turbopump” and “battery-powered turbomachinery” appeared repeatedly. 

A more descriptive term would be “battery-powered centrifugal pump.” True, this type of rotating pump with blades fits a broad definition of turbomachinery, but so does a propeller. No one would say “battery-powered turboprop” in referring to an electric airplane.

Reported promotional statements from Rocket Lab included an efficiency comparison between electric motors and turbines, impressive-sounding but neither new nor informative by itself. What really counts is the mass of the batteries expressed as a percentage of the propellant mass pumped. 

Most likely the batteries are much heavier than the small percentages typical of hot gas that powers turbopumps in a gas-generator-cycle rocket engine. The battery weight will impact the payload capability of the developmental Electron launch vehicle. 

Another unmentioned challenge is that the lightweight electric motors have to run the full duration of a launch without overheating.

John Whitehead

Davis, California

Coming to Terms

The terminology of helicopter and tiltrotor aerodynamics can be confusing, like rotorcraft aerodynamics itself. In “Alien Territory” (AW&ST May 25-June 7, p. 46), vortex ring state (VRS) was incorrectly referred to as a “settling with power” condition when the correct term is “power settling.” 

Power settling occurs when a helicopter or tiltrotor aircraft descends into its vortex ring, causing a high, uncontrollable rate of descent accompanied by partial or complete loss of control. The only way out is to reduce power and fly into undisturbed air, a maneuver most likely impossible if the aircraft enters VRS within a few hundred feet of the ground. 

“Settling with power” on the other hand, refers to a sink rate caused by insufficient power to sustain a desired flightpath or hover. 

Considering the aerodynamic and mechanical complexity of tiltrotors I’ll bet cost-conscious and safety-minded operators are more inclined to pursue the somewhat slower but far less aerodynamically and mechanically complex coaxial rotor helicopters under development.

Arnold Reiner

Pensacola, Florida