we have to remember that many russian sources are not translated and the US hates giving Russia credit for anything. i would say look at german sources for confirmation but i cant read greman. the show was secret weapons of russia/stalin (lil drunk) but they where taking about the i16 with the rocket in it and they said " it waisted to much fuel to fast and test pilot died in crash, rocket plane was abandon but many russian front line fighter were equipped with some kind of rocket booster for more speed". now i know this is some what a vague statement but there most be at least something more then an i16 rocket tests to this.
Google translate- Copy & Paste into English ie:
Jakowlew Jak-9
Typ: russisches Jagdflugzeug aus dem 2.Weltkrieg. Manche auch Jagdbomber, Panzerjäger oder Langstrecken-Begleitjäger.
Geschichte: Die Jakowlew-Jäger waren das am häufigsten verwendete russische Jagdflugzeug in Korea und dem 2. Weltkrieg, und das bekannteste war die Jakowlew Jak-9.
Auch der zahlenmäßig am stärksten vertretene Typ der Jakowlew-Jagdflugzeuge, die Jak-9, wurde in einer Reihe von unterschiedlichen Versionen bis in die 50er Jahre des vergangen Jahrhunderts hinein produziert. Die Jak-9P, die letzte und beste Version, wurde von den Nord-Koreanern zahlreich im Korea-Krieg eingesetzt.
Yakovlev Yak-9
Type: Russian fighter plane from World War 2. Some even fighter-bombers, tanks and long-range fighter-escort fighters.
History: The Yakovlev-hunters were the most frequently used Russian fighter in Korea and the second World War II, and the most famous was the Yakovlev Yak-9.
The numerically most abundant type of Yakovlev fighters, the Yak-9 was produced in a number of different versions until the 50s of last century. The Yak-9P, the latest and greatest version, was abundant from the North Koreans during the Korean War used.
It's this easy... PS I found nothing which said Booster rockets=Booster-Raketen, or Raketenantrieb anywhere on the Yak-9/9U
http://weltkrieg2.de/Waffen/Kampfflugzeuge/Jagdflugzeuge/russische/Jakowlew/Jak-9.htm