I am new to this group, and so glad I found it.

Rockets use thrust vectoring, fins or both for flight control. Lets say when SpaceX launches a Falcon 9, from liftoff until MECO, when is thrust vectoring most active? Meaning, obviously the thrust vectoring is active throughout the ascent, but is thrust vectoring most active at the lower altitudes or upper altitudes? I am working with a group of students to develop a rocket that will be launched at 100,000 after being towed up to that altitude under a weather balloon (Rockooning, etc). We are not trying to make it to space. The first launch is scheduled for June 8,9 of 2019 at Spaceport America. The first launch is a test of telemetry, video transmission and control. I am trying to work out the problem of thrust vectoring. Here is the web-page for our project. It is "Mission 2".Thanks everyone! enter link description here

  • $\begingroup$ Welcome to Stack Exchange! We try to keep it down to one "question" per question. I see several here: "when is thrust vectoring most active?" "Will thrust vectoring be needed for a launch from that altitude? " Especially not a good fit for Stack Exchange is "Any thoughts on this subject?". This is most definitely not a discussion forum as we look for questions that have a single definitive answer. The reason this is important is that someone might be able to answer one of your questions but not the others. Take some time to edit your question down - you'll get better answers. $\endgroup$ Commented May 8, 2018 at 17:57
  • $\begingroup$ You could look at my answer to this question space.stackexchange.com/questions/23137/… to see a real world example of "when is thrust vectoring most active" but for a non-typical vehicle. $\endgroup$ Commented May 8, 2018 at 18:00
  • $\begingroup$ We were totally considering PID if needed. We use PID controllers in our quad copters for stabilization. We have not tested it out yet, but I think the PID controllers that we use in our quad copters could be adapted to controlling thrust vectoring. Thanks you for the link to the prior discussion. $\endgroup$
    – Paul Kaup
    Commented May 8, 2018 at 18:09
  • $\begingroup$ Great edits! Again, welcome. $\endgroup$ Commented May 8, 2018 at 18:12
  • $\begingroup$ I am just curious as to whether or not we will need to worry about thrust vectoring for this project. launch from 100,000ft asl with fins for flight control, or will we have to do some sort of thrust vectoring. our planned apogee is about 160,000ft asl. The atmosphere is extremely thin at those altitudes, but with enough velocity, we may have enough airflow to provide for an adequate center of pressure. although the forces exerted at the CP will diminish rapidly during the ascent. Just my thoughts. $\endgroup$
    – Paul Kaup
    Commented May 8, 2018 at 18:14

2 Answers 2


Rockets use thrust vectoring, fins or both for flight control. Lets say when SpaceX launches a Falcon 9, from liftoff until MECO, when is thrust vectoring most active? Meaning, obviously the thrust vectoring is active throughout the ascent, but is thrust vectoring most active at the lower altitudes or upper altitudes?

Early rockets used roll to generate stability and modern rockets use a little bit of roll to orient the occupants for orbital insertion. Vectoring is usable near the surface of the Earth and in space while fins require a thicker atmosphere to operate efficiently.

Thrust vectoring is great for fighter jets and missiles that require extreme maneuverability with a rocket going into space the use of thrust vectoring only adds unnecessary complexity, weight and expense.

Information about the SpaceX engines was provided in the answer to this question: Falcon 9 Merlin engine thrust vectoring.

Use of thrust vectoring immediately after launch and continued use of it suggests that you are undecided about your course, that doesn't happen with a space launch. Use of vectoring is the only choice once you're in space. The Falcon 9 uses grid fins for much of it's descent, they are stowed for ascent; it's more a case of "launch time" controlling the direction of flight than one of firing the rocket at a random time and direction, then relying on mid-course correction to fix everything.

So the answer to: "... is thrust vectoring most active at the lower altitudes or upper altitudes?" is: upper altitudes, but you would want to calculate to use the minimum amount in any event. Force applied off-axis is less efficient, great for tight maneuvers.

It's easier to use thrusters for minor corrections and that's what is used for guiding and landing reusable sections. In one of the videos on your webpage you can see that Van Allen's rocket only used fins (despite their inefficiency the design is easier to implement).

" I am working with a group of students to develop a rocket that will be launched at 100,000 ... I am trying to work out the problem of thrust vectoring.".

Much like Van Allen's rocket you would want to avoid it for simplicity. You will need electronics that can work in that environment and calculate direction outside of the usual permitted altitude of GPS satellites (they don't like guiding missiles in space), in reduced gravity. Probably easiest to simply ensure that it's nose up and not aimed at anything, then fire and hope for the best.

You would need cameras and flight radar in the nosecone, telemetry to Earth, and make split speed decisions using a flight control computer if you expect to actively guide the rocket.

"Working out the problem of thrust vectoring" is hopefully a point of interest included in your question, and not a years worth of work you are asking for help with. I'm happy answering the first question and leaving additional questions for others to tackle.

Here's some more information about launching a rocket from a balloon:

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    $\begingroup$ Rob, first off "Wow"...thank you for all of your insight...tons of really good info. Thank you for the feedback. $\endgroup$
    – Paul Kaup
    Commented May 9, 2018 at 18:34
  • $\begingroup$ @PaulKaup - You're welcome. I've added a few links to the bottom of my answer - everything from another SE question to getting up there (and staying there for a while) for $ or free, along with a link to the Stanford rockoon. $\endgroup$
    – Rob
    Commented May 10, 2018 at 3:01
  • $\begingroup$ We are actively developing a thrust vectoring system (for models). Just trying to decide whether or not to apply thrust vectoring as part of the solution to maintaining stability for our "Rockoon" launch. This first flight next year is really a big learning project. Maybe we will try a simple point and shoot and keep the systems simplistic. Our main objectives with the project next summer are to maintain positive launch control (launch, abort, and cut-down we do this with Dragon link and Eagle Tree Vector), and Tx and Rx of Telemetry and Video on 1.3ghz, $\endgroup$
    – Paul Kaup
    Commented May 10, 2018 at 16:56

I agree with @Rob, effective thrust vector control is not a simple problem. Your best approach is probably to use fins, and "ensure that it's nose up and not aimed at anything, then fire and hope for the best." But there are some specific things you can do to up the probability of "the best".

Since you'll be launching from a balloon, and you don't want the rocket to be aimed at anything, you won't launch the rocket exactly vertically upward—at the balloon! There'll be a non-zero zenith angle to the rocket's initial pointing. If the rockoon isn't in a turbulent region, that angle is easy to control via the design of the launch platform suspended beneath the balloon. A nice long tether between the balloon and the platform decreases the angle subtended by the balloon as seen from the platform, and that allows launch at a smaller zenith angle. As we'll see, larger zenith angles are not your friend.

Few rockets without thrust vector control systems have their thrust vector perfectly aligned through the rocket's CM. In the absence of a control system this causes a torque that turns the rocket, in pitch or yaw or both. Fins are such a control system, at least to a point. They provide a stabilizing moment (negative feedback) that gets larger with higher speed. With fin stabilization, before a thrust offset can cause large angular deflections, you must get the rocket's speed up to where the stabilizing moment is equal (and opposite!) to the offset thrust's moment, with a very small angle of attack. At high altitude that speed is higher than at low altitudes.

Getting to high speed as quickly as possible sounds like an easy approach, requiring only a large thrust-to-weight ratio, i.e. high thrust. But for a non-spinning rocket, if the inertial moment of the rocket stays the same, and the thrust vector offset angle stays the same, the angular acceleration caused by the offset thrust is increased by the same ratio as the increase in linear acceleration, so you don't "win". Also, fixed fins cannot completely correct for offset thrust! If there is an offset, once effective the fins will limit the pitch/yaw rate, but the offset thrust will always cause a pitch/yaw moment and thus a net angle of attack. The simplest way (without active control systems) to mostly counteract the offset thrust is to spin the rocket around the roll axis. I say mostly because in the initial few moments of the launch, before the spin-up occurs, some turning can occur.

Spinning a rocket with a video camera is probably not what you had in mind; you don't want viewers to get vertigo. During the boost phase spin might be unavoidable to limit the zenith angle. But if you want to teach the students some control theory and control systems, you could have them build moveable fins, or moveable control surfaces on the trailing sides of the fins, to provide spin-up to a pre-determined rate during boost and then cancel the spin when boost is over. You can also propulsively spin (see below) before the fins become effective, then use the fins to spin down after boost.

On to the effects of a flight path at a non-zero zenith angle. If you launch vertically (zenith angle = 0), the vector sum of the acceleration vector due to the engine (assumed aligned with the rocket's long axis, call it the z axis, and the CM is in that axis) and the acceleration vector due to gravity are parallel so there's no pitch or yaw moment, and the rocket doesn't turn. But if you launch at a non-zero zenith angle, the acceleration vector due to the engine and the acceleration vector due to gravity are not parallel, so the vector sum of acceleration is not along the z axis; it is biased toward Earth. As a result of the offset net force vector, there is a component of acceleration earthward from the initial flight path, changing the flight path to a higher zenith angle. Fins turn the rocket to the new flight path angle, into the relative wind. But since gravity is still acting, the net force vector is still groundward of the new flight path angle, so the rocket winds up turning even farther from vertical. The larger the zenith angle (up to 90°), the faster this turn goes, because the gravitational acceleration vector is farther from parallel to the thrust acceleration vector.

So as I said above: large zenith angles are not your friend. Two things contribute to large turning angles from this effect: long duration burns, and low thrust-to-weight ratios (acceleration magnitude). Long, low-thrust burns can have the rocket headed almost straight down at burnout.

The conclusions from all this: 1) launch with as high an initial acceleration as feasible. You can use a technique the modern Atlas rockets use: thrust augmenters ("strap-ons"). Small, short-duration but high-thrust augmenters might be very useful here. If you're launching with, say, a K-class engine, strapping on 2 long core-burner H's might help a lot. 2) Canting them ever so slightly to provide roll moment might help too, giving the rocket a spin before the fins become effective. Be sure the strap-ons and fins spin the rocket the same direction!

Getting those strap-ons to jettison after burnout would also be good, so you're not dragging along extra mass during most of the K (or whatever) burn.

@PaulKaup , I hope this is helpful!

  • $\begingroup$ We always hope our posts are helpful, but I don't know if using the @ in the body of a post (as you've added in the recent edit) has any particular messaging effect. If you't like a user to be flagged, it should be used in a comment. $\endgroup$
    – uhoh
    Commented May 10, 2018 at 2:18
  • $\begingroup$ Tom,Spinning initially may be a good solution. Dual stage the rocket with the first stage of fins designed to give the rocket spin about the roll axis. Then design the second stage fins without spin. Interesting problem to engineer our way through. Second stage fins would need to have more surface area to counteract the lack of air density, but enough velocity may make up for the lack of air density. hmmmm? $\endgroup$
    – Paul Kaup
    Commented May 10, 2018 at 17:03
  • $\begingroup$ @PaulKaup This would probably work. You'd lose a little efficiency by having the 2nd stage fins fighting the 1st stage fins during spin-up but it wouldn't be much. Be sure the 1st stage thrust is really high. If it's a cluster, think about canting those engines just a hair—0.5° or so— for a spin moment coming off the rail, not having to wait for speed to give a q (dynamic pressure) the fins can work with. You can have the students calculate how much cant angle will impart how much spin, and how long it would take the fins to give that much spin. $\endgroup$ Commented May 10, 2018 at 17:19

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