**Carmine Carmicino**

Department of Industrial Engineering—Aerospace Division, University of Naples "Federico II", Piazzale Tecchio, 80-80125 Naples, Italy; carmicin@unina.it

Received: 10 August 2020; Accepted: 13 August 2020; Published: 14 August 2020

Since the Editorial [1] of this Special Issue was published last 26 November 2019, I have had the pleasure to accept the submission of a couple of new articles which are now included in the Issue; moreover, for the sake of clarification, I want to mention that, over the revision process, the title of the paper [2]—which at that time was thought of as the last to be published—has been changed to more fairly comply with the revised topics.

The last two papers now appearing in the Special Issue deal, in the temporal order of publication, with the design and testing of a student hybrid rocket engine featuring an external carbon fiber composite structure [3], and with the development of an oxygen-methane torch ignition system designed for a hybrid rocket and later improved to be used in the testing of solid and liquid ramjet engines [4].

The former was developed in the framework of the German educational program Studentische Experimental-Raketen (STERN), by students of the Technische Universität Braunschweig, whereas the latter reports on a part of a research funded by the Foundation for the Scientific Research Support of the Brazilian Federal District at the University of Brasilia. I have particularly welcomed these two articles, in that, both being born in institutions recently involved in hybrids, they further prove the nice spread of the research activities in this subject across the world.

The aim of this short addendum is to give the final picture of the contents of the Special Issue, which collects 14 papers and 1 Editorial, 3 of which are review papers, 10 are original research papers, and 1 is a technical note.

The success of this experience has laid the groundwork for the "Hybrid Rocket (Volume II)", edited in collaboration with Toru Shimada and Arif Karabeyoglu [5], which, alongside articles addressing the advances in hybrid rocket technology and related analysis methodologies, will welcome papers dealing with novel space transportation systems, new flight systems, and mission concepts and optimization using hybrid rockets.

**Funding:** This research received no external funding.

**Conflicts of Interest:** The author declares no conflict of interest.
