# ISRO's space plane on top of of a rocket - how unstable was it?

ISRO's space plane was launched on top of a rocket into a sub-orbital trajectory, with separation from the lift vehicle around 45 km and a maximum altitude of about 65 km, based on an analysis of the official ISRO launch video described in the ISRO fan reddit.

With wings at the top of a rocket, was this likely to have been extremely unstable? How was the flight controlled? Were there active surfaces on the space plane's wings that participated in stabilizing the flight?

above: From Ars Technica, photo credit: ISRO.

above: From NewScientist, photo credit: ISRO.

Just to correct original question in RLV-TD (Reusable Launch Vehicle Technology Demonstration) mission the winged body TDV (Technology Demonstration Vehicle) separated at 56 km altitude and proceeded to reach altitude of about 65 km before descent.

Yes RLV-TD configuration is unsymmetrical and unstable, former chairman of ISRO K Radhakrishnan briefly talks about that aerodynamic instability of RLV-TD, μα being 20 to 25 compared to 1 for PSLV/GSLV and 5 for LVM3 in this talk at 42 min 50 sec mark.

For control during ascent HS9 booster has RCS + SITVC system(Pitch+Yaw) for first few seconds of ascent, and later "fin tip control" as shown in following slide from presentation by M. V. Dhekane. Those four movable fins are attached to base shroud of HS9 booster. From what I have read in journal papers related to RLV-TD so far control surfaces on winged body don't play any role during ascent phase.

http://www.sc.iitb.ac.in/~pdsc2014/MVD_talk.pdf

Edit:

Official press release gives some of flight profile details. They also released a Time Vs Altitude visualization[MP4] based on telemetry from RLV-TD during flight.

http://www.isro.gov.in/launchers/rlv-take-videos

I have plotted it in graph form here.

A quote from paper titled "Particle Swarm Optimization Based PI Controller Design for Actuation System of Reusable Launch Vehicle"

RLV consists of a booster stage and a fly back portion. During the ascent phase, it will be controlled by using four fin actuators. During re-entry and return flight, the altitude will be controlled by two primary control surfaces, ie, elevons and rudder.

https://www.irjet.net/archives/V2/i5/IRJET-V2I594.pdf

• Thank you for the answer - I am still amazed every time I look at the launch photo, how such stable flight could be possible with that configuration. The attached pdf does a nice job of describing the complexity of the launch control system, and I think I can learn a lot from it; $\mu_{\alpha}$ is defined on slide 26 for example. Thanks also for the correction on the altitude - can yo help me find a link for that - I'll make a note of it in the question.
– uhoh
Mar 22 '17 at 0:22
• @uhoh I edited my answer to add few sources. Mar 22 '17 at 5:39
• Wow, looks really great!! I've corrected the altitude in the question and linked to Reddit, does that look OK?
– uhoh
Mar 22 '17 at 6:14

The fins on the HS9 booster are much larger than what you'd normally see on a rocket with a compact payload under a fairing, which contributes to stability passively, and according to this article, they are providing active steering as well:

https://www.nasaspaceflight.com/2016/05/india-launch-reusable-launch-vehicle-demonstrator-mission/