12
$\begingroup$

With a million pounds of thrust and many g of acceleration and a significant fraction of a g of vibration, connecting the engine to the rocket can't be trivial.

At launch the mass is very high and maybe inertia keeps the low frequency vibrations down, but as propellants run out, maybe that becomes less effective.

Is the thrust from the engine transmitted through the nozzle directly to the rocket frame? Is there any attempt to dampen the vibration?

I just found this video after watching the one in the answer by @jlansey below. You can see the engine move vertically but the test frame doesn't. At least in this test set-up, there are shock-abosrbers somewhere.

In those old-style gasoline-burning automobiles of the late 19th, 20th and early 21st century, motors had motor-mounts - rubberized bushings or brackets or something similar that allowed the motor some degree of motion/vibration. Do rocket motors on crewed vehicles have something analogous? I'm guessing that vibration limitations for crewed launches are tighter than for non-crewed missions.

I found this answer about vibration isolation of SRB's to be potentially used as crewed launch vehicles interesting and related, but it's focus is certain characteristic vibration modes of SRBs that are nearly finished and basically hollow tubes.

Improve this question
$\endgroup$
14
  • 2
    $\begingroup$ I am pretty sure they don't but I can't confirm it well enough to write an answer. If you look at my answer to this question space.stackexchange.com/questions/13691/… you can see the gimbal bearings called out on the schematics and the metallic mount points where they fit in the photograph. $\endgroup$
    – Organic Marble
    May 30, 2016 at 16:31
  • 2
    $\begingroup$ You might want to look at pogo oscillation mitigative strategies. It doesn't seem quite like what you are asking for, but certainly related. $\endgroup$
    – user
    May 31, 2016 at 11:22
  • 5
    $\begingroup$ @MichaelKjörling yikes! Reading this vibrationdata.com newsletter from 2008 (found in your linked Wikipedia article), it says “The Apollo 13 vehicle had a sever pogo vibration with the center engine during second stage burn. The engine experienced a 34 G vibration at 16 Hz, flexing the thrust frame by 5.2 inches peak-to-peak… The vibration was apparently localized to the engine frame… The astronauts did not report feeling any corresponding vibration…” the frame itself was absorbing a 13cm p-p motion of the engine! Wow! $\endgroup$
    – uhoh
    May 31, 2016 at 13:21
  • 1
    $\begingroup$ @uhoh The Apollo 13 second stage center engine also cut off early during ascent (thankfully; apparently it was unintended), at about 00:05:32 GET. That's even mentioned in Howard's movie. $\endgroup$
    – user
    May 31, 2016 at 13:59
  • 1
    $\begingroup$ I've read, contra that article, that the pogo-ing likely did cause the early shutoff -- when the engine was traveling backward relative to the fuel system, it dropped the pressure far enough to trip the "out of fuel, shut down cleanly now" subsystem. So the problem did luckily create its own solution. $\endgroup$
    – Russell Borogove
    May 31, 2016 at 14:20

4 Answers 4

Reset to default
9
+100
$\begingroup$

I have managed to confirm that at least the SSME mounting to the Space Shuttle Orbiter does not have any shock absorber, as shown in this diagram.

enter image description here

The top of the device bolts directly to the Orbiter thrust structure. The bottom bolts directly to the SSME powerhead. The interface is a "simple" spherical bearing. There is no provision for any kind of compliance.

The drawing is from the Rocketdyne Space Shuttle Main Engine Pocket Data Book RI/RD87-142, page 2-78.

This document has more on the gimbal bearing.

The gimbal bearing provides a means of attaching the engine to the vehicle while allowing the engine to be pivoted (gimballed) around its two axes. This is necessary in order to point the engine thrust vector for vehicle steering, in the manner of a ship’s rudder. The gimbal bearing is bolted to the vehicle by its upper flange and to the engine by its lower flange. It supports 7,480 pounds of engine weight and withstands over 500,000 pounds of thrust. It is a ball-and- socket universal joint in which concave and convex spherical surfaces on the seat, body, and block intermesh. Sliding contact occurs between these surfaces as the bearing is angulated. Fabroid inserts located at the sliding contact surfaces reduce friction that occurs during gimbal bearing angulation. The bearing, which is installed during engine assembly, measures approximately 11 by 14 inches, weighs about 105 pounds, and is made of a titanium alloy.

I cropped this picture to show the engine interface area. enter image description here You can see the vehicle side of the spherical bearing in the center of the opening. The bolt pattern matches the drawing!

The green pushrod devices are the thrust vector control actuators, the red circles are covers for the holes where the low pressure turbopumps mount.

Edit: info on the thrust structure from here:

The internal thrust structure supports the three SSMEs. The upper section of the thrust structure supports the upper SSME, and the lower section of the thrust structure supports the two lower SSMEs. The internal thrust structure includes the SSMEs, load reaction truss structures, engine interface fittings and the actuator support structure. It supports the SSMEs, the SSME low-pressure turbopumps and propellant lines. The two orbiter/external tank aft attach points interface at the longeron fittings.

The internal thrust structure is composed mainly of 28 machined, diffusion-bonded truss members. In diffusion bonding, titanium strips are bonded together under heat, pressure and time. This fuses the titanium strips into a single hollow, homogeneous mass that is lighter and stronger than a forged part. In looking at the cross section of a diffusion bond, one sees no weld line. It is a homogeneous parent metal, yet composed of pieces joined by diffusion bonding. (In OV-105, the internal thrust structure is a forging.) In selected areas, the titanium construction is reinforced with boron/epoxy tubular struts to minimize weight and add stiffness. This reduced the weight by 21 percent, approximately 900 pounds.

Finally found a decent picture of the thrust structure. It's from the Dennis Jenkins book Space Shuttle, the 1992 edition, page 140.

enter image description here

Improve this answer
$\endgroup$
4
  • $\begingroup$ Wow that is beautiful! Do you know if there is vertical transfer of the thrust directly behind the bearing (towards the spacecraft - into the page)? I'm asking because to my untrained eye this looks like it could be some kind of flexure mount. For example just below the left bearing in the photo there is something that reminds me of a hinge. $\endgroup$
    – uhoh
    Oct 1, 2016 at 2:33
  • $\begingroup$ Updated to add details about the thrust structure. Everything I read about it says it was engineered for stiffness - the last thing they wanted was any compliance or flexing. $\endgroup$
    – Organic Marble
    Oct 2, 2016 at 4:14
  • $\begingroup$ I'm slowly understanding the page 140 image - are these three points the bearings in question? i.stack.imgur.com/02OHL.png If so, it sure looks like the thrust is transferred directly without any significant intentional flexing. $\endgroup$
    – uhoh
    Oct 2, 2016 at 9:09
  • 1
    $\begingroup$ Yes, I think you have correctly id'd the 3 mounting points. $\endgroup$
    – Organic Marble
    Oct 2, 2016 at 10:01
4
$\begingroup$

The process of starting a rocket engine is highly controlled and choreographed. It doesn't start up from 0 to several g's in an instant. Usually it takes a second or two. The burning itself is also very optimized to stop any instabilities from building up – so minimizing the vibration. Here is a video from the Space Shuttle where they also start each of the 3 engines at a slightly different time (don't know if this is common.

Another final reason they probably don't have shocks is efficiency, there is no way to absorb the force without a loss of efficiency usually as heat. Same reason high-performance bicycles don't have shocks.

Improve this answer
$\endgroup$
4
  • 1
    $\begingroup$ These are interesting points! I'm just asking about during-flight vibrations after liftoff, not engine starts. That's a really nice video of the shuttle launch! Excellent tutorial! $\endgroup$
    – uhoh
    May 31, 2016 at 4:25
  • 1
    $\begingroup$ Also, think the part about loss of efficiency needs to be looked at more carefully here. Loosely speaking, rockets "go" by conserving linear momentum. If you think a shock absorber would reduce thrust and acceleration, you'll have to explain why they actually reduce the exhaust velocity significantly. The bike thing happens because the tire is in contact with the earth, which can absorb momentum. Rockets aren't "touching" anything besides their own exhaust (once outside most of the atmosphere). $\endgroup$
    – uhoh
    May 31, 2016 at 4:33
  • 1
    $\begingroup$ ...also, true shock absorbers tend to be a combination of storage and damping - or springs and dashpots (analogous to reactive and resistive (ohmic) elements in the case of circuits). So there is even more wiggle room for the designers. $\endgroup$
    – uhoh
    May 31, 2016 at 4:59
  • 1
    $\begingroup$ "don't know if this is common" The Saturn V (strictly speaking, the SI-C first stage) did that too. Wikipedia claims that the center engine ignition occured at T-8.9s, followed by one outboard pair at T-8.6s and the other pair at T-8.3s (actually, the claim is that the outboard pairs were started at 300 ms intervals). This was done in order to "reduce the structural loads on the rocket". Once the onboard computers confirmed that the engines had correct thrust, the hold-down mechanics released the booster and liftoff occured at T-0. $\endgroup$
    – user
    Jun 10, 2016 at 12:23
-2
$\begingroup$

Marshall and North American engineers devised three changes to the second stage. They installed a helium gas accumulator in the LOX line of the center engine. This reservoir served to dampen fluid pressure oscillations, keeping them out of phase with the vibrations of the thrust structure and engines.

Improve this answer
$\endgroup$
1
  • 1
    $\begingroup$ Uh, the second stage of what? $\endgroup$
    – Nathan Tuggy
    Sep 21, 2017 at 2:08
-3
$\begingroup$

Yes they do use shock absorbers. In this case it's called an "accumulator." The problem arose during the Apollo 6 launch." The "resonance effect" or "pogp oscillations" abruptly appeared during the first 2 minutes of flight. Pogo arises fundamentally because you have thrust fluctuations in the engines. Those are normal characteristics of engines. All engines have what you might call noise in their output because the combustion is not quite uniform, so you have this fluctuation in thrust of the first stage as a normal characteristic of all engine burning.

Now, in turn, the engine is fed through a pipe that takes the fuel out of the tanks and feeds it into the engine. That pipe's length is something like an organ pipe so it has a certain resonance frequency of its own and it really turns out that it will oscillate just like an organ pipe does.

The structure of the vehicle is much like a tuning fork, so if you strike it right, it will oscillate up and down longitudinally. In a gross sense it is the interaction between the various frequencies that causes the vehicle to oscillate.

https://en.wikipedia.org/wiki/Apollo_6

Improve this answer
$\endgroup$
1
  • 6
    $\begingroup$ Accumulators aren't really shock absorbers in the sense that the question asked. They alter the mode frequencies of the fluid systems so there isn't harmonic interaction between structural vibration and the fluid system that would result in resonance, followed by catastrophic failure. There really aren't shock absorbers in the structural sense. The thrust plate of a rocket engine is hard-mounted to the thrust structure, which is designed to withstand the impulsive loads from startup and shutdown. $\endgroup$
    – Tristan
    Sep 20, 2017 at 20:26

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge that you have read and understand our privacy policy and code of conduct.

Not the answer you're looking for? Browse other questions tagged or ask your own question.