CONTRAIL QUESTION

Your recent article on curbing contrails (“Methods of Curbing Contrails Are Taking Shape,” Jan. 10-23, p. 32) was both interesting and informative, but confusingly it failed to describe just how it is that contrails, and their cirrus cloud result, produce radiative forcing.

I always understood that clouds generally reflect sunlight back into space, thus performing primarily an Earth cooling effect. So how is it that contrails and cirrus clouds do the opposite?

Chris Skillern, San Diego

Editor’s note: Contrail cirrus produces both a cooling and a warming effect—cooling by reflecting sunlight back into space, warming by trapping the Earth’s heat inside the atmosphere. The scientific consensus is that the overall impact is a warming effect.

EXPENDABILITY REVISITED

I published a commentary in AW&ST in 1991 that argued for reviving the Saturn V to support the Space Exploration Initiative (May 20, 1991, p. 67). I proposed reengining the Saturn’s upper stages with space shuttle main engines (SSME) to improve its performance. For that I was chastised in a response letter to the editor for being foolish enough to suggest wasting reusable SSMEs on an expendable rocket.

Thirty-one years later, the Space Launch System is ready for launch using four SSMEs (now renamed RS-25s) as the main propulsion engines (along with the solid rocket boosters)—on an expendable rocket (“SLS at the Launch Site,” Dec. 20, 2021-Jan. 9, 2022, p. 54).

I suppose I can claim some modicum of vindication here that the reusable SSMEs are now acceptably expendable. At any rate, here’s hoping for a successful launch and that the SLS will enjoy a longer operational lifetime than the Saturn V’s all-too-short career.

Thomas J. Frieling, Tallahassee, Florida

CORRECTION

The credit on the concept image with “ARRW Revealed” (July 26-Aug. 8, 2021, p. 48) should have read “Air Force Magazine/U.S. Air Force Concept.”