Static Balancing of a Payload

by Bill Claybaugh, Reaction Research Society

Dislocations in the Center-of-Gravity (Cg) of a rocket with respect to the vehicle centerline can lead to “coning” in flight.  This coning is visually characterized by a corkscrew exhaust trail when it occurs during thrusting. Whenever it occurs—and it is likely in all rockets that are not axisymmetrically balanced—it increases drag and lowers performance.  In cases of “extreme” unbalance (which can occur for axial offsets of Cg of as little as a few tens of thousandths of an inch for small diameter rockets) the vehicle can be torn apart in flight by the aerodynamic forces created by the Cg imbalance.

For rockets that are carefully designed to be axisymmetric, the only important issue to address in assuring balance of the rocket is to make certain that the fins all have the same weight within a small margin (removing mass from the heavier fins is best done at the fin Cg).  If all other components are axisymmetric—including the propellant grain—then the only part of the vehicle that will require formal balancing is the payload:

I Longitudinal Balancing

The image below shows the set up for longitudinal balancing of a payload:

Dual-weight scales with precisely placed mounring locations

Two identical scales are slightly modified to support the payload so that the longitudinal axis is parallel with the base of the scales and level.

After carefully measuring the distance between the centers of the two scales (6.760” in this example) the weight of the payload supported by each scale is noted in two orthogonal planes.  Using a moment balance calculation, the center of gravity is determined in each plane:

Averaging three measurements in plane 1, the scales showed 6.953 lbsm on the left side and 4.523 lbsm on the right scale.  Creating an arbitrary starting datum for calculation that is 0.7125” to the right of the right scale center point (that is, at the bottom of the payload base plate) allows the following center of mass balance:

Cg = (4.487 lbsm * 0.7125 in.) + (6.967 lbsm * 7.4725 in.) / (4.487 lbsm + 6.967lbsm)

​​​​= 4.824” from the datum.

Rotating the payload by 90 degrees and weighing again allows calculating the Center of Gravity in that plane, in this case, that value was:

​​​Gg = 4.881” from the datum.

Thus, the Cg appears—on average—to be offset by 0.057” between the two planes.

Balancing: Subtractive and Additive

Balance can be achieved either by subtracting weight from the top of and in the plane that shows the longer Cg distance (Plane 2, in this case) or by adding weight to the top of the plane that has the shorter Cg.  Conversely, weight could be added at some bottom location of Plane 2 or deducted from a bottom location on Plane 1. Note that any of these actions have effects on the rotational balance, which we will address in a future article.

Subtractive Balancing

We can estimate the amount of weight that needs to be deducted from the firstplane by deciding that the weight will be removed at 9.375” from the datum (a location at the top of the conical aluminum section, just below the fiberglass structure that supports the flight computer / transmitter in the image).

The mass required to move the Cg by 0.057” is then:

Mass Lost (or Gained) / New Total Mass = Change in Cg / Distance to New Mass

Adjustment Mass / (11.451 – Adjustment Mass) = .057 / (9.375” – 4.881”)

Adjustment Mass = 0.142 lbsm

This mass must be removed at equidistant locations from the rotational axis, that is, we will require two 0.071 lbsm axisymmetric holes in opposite sides of the payload at the required location.

We can estimate the drill depth assuming the use of a mill cutter.  The required cutting depth is then the cross-sectional area of the mill times the drill depth times the density of aluminum:

​​Mass to be removed = 0.098 lbsm. / cu. in. * (pi * r^2) * h

For a 0.5” mill cutter that depth is:

​​​0.071 = 0.098 * 3.1416 * (0.5 / 2)^2 * h 

​​​​​h = 3.69”

Obviously, this is impractical as the depth of aluminum at the subject location is only 0.125”.

Additive Balancing

The relatively large cutting operation required for subtractive balancing is due to the low density of aluminum.  Alternatively, we can estimate the additional mass required in the shorter of the two planes (Plane 1). Because we can use, for example, tungsten for the weight adjustment, it is possible to move the shorter Cgforward with much smaller but higher density balance weights.

Adding mass at a greater distance from the Cg will lower the mass required to achieve the needed shift.  If we consider adding mass at a location 13.375” from the datum location (on the cross bar near the top of the fiberglass structure that supports the flight computer / transmitter) we can calculate:

Adjustment Mass / (11.454 + Adjustment Mass) = .057 / (13.375” – 4.824”)

Adjustment Mass = 0.0763 lbsm

Noting that one-half of this is 0.03815 lbsm or 17.3 grams, we can observe that a standard tungsten weight used in “pinewood derby” cars of 0.25” diameter and 0.5” length weighs about 14 grams. Thus, drilling two axisymmetric holes at the required location and gluing the tungsten counterweights in place will produce a Gg offset close to the 0.057” adjustment required (see the comments below on the resolution of this system for locating the Cg).  Note that the two balance weights, in final form, need to weigh the same within a small margin or they will introduce a mass imbalance in rotation. We should also note that we are not here accounting for the mass removed to make the holes for the counterweights.


The final step is to remeasure the Cg in both planes and verify that they are within the resolution of the measurement system.  If not, another, smaller, adjustment in mass in the longer plane or an adjustment of the balance weight in the shorter plane may be required.

A Note on Accuracy

The relativity low cost scales used in this example resolve weight to 0.10 grams. However, multiple measurement in the same plane (achieved by gently pressing on the payload to reset the scales) shows that the scales are in fact accurate to about +/- 10 grams, about 0.3% of the total weight of the subject payload.

This means that the true resolution of this system with regard to differences in Cg between planes is about 0.015”, thus, once the Cg between the planes is within 0.15”, further adjustment is unlikely to increase accuracy using these relatively low-cost scales.

MTA work event, 2022-10-20

by Dave Nordling, President, RRS.ORG

The Reaction Research Society has built a new and larger pad to the west of our current launch pads at the Mojave Test Area (MTA). This has been due to the generous donation of lifetime member, Bill Claybaugh, both of his time and resources. This 12-foot square pad is angled to the southwest and is intended for larger launch vehicles soon to be launched from the MTA. This work started a few weeks back with the initial excavation which was delayed due to the extreme heat this summer.

This work event took place over a few days starting Thursday, October 20th and going to Saturday, October 22nd. Bill and the society would like to thank fellow society members, Dimitri Timohovich, Rushd Julfiker and Joe Dominguez for giving their time and sweat to this facility improvement,

Excavation and rebar supports in place.
Add gravel
Spread to an even layer
Adding water is important.
Add concrete
Leveling the pour
Finishing the edges

This is only the first steps to building a useful platform for launching larger rockets from the MTA. The pad must cure over several weeks during these cooler months before temperatures drop below freezing at night, Next steps will be preparing the surface for anchors then adding Bill Claybaugh’s launch rail which is in work right now.

This work will continue to be documented in our work event reports. We are a member driven society and deeply appreciate the hard work it takes to improve and upgrade our site to support our mission as an organization.

Our next monthly meeting is always on the 2nd Friday of the month at the front office of the Compton/Woodley airport at 7:30pm. Come join us on November 11th.

October 2022 meeting

by Dave Nordling, President, RRS.ORG

The society met at the Compton Airport front office again for our monthly meeting on October 14th. Mike LaGrange joined us by teleconference for the first time. Joel Cool-Panama was welcomed as our newest administrative member.

We first reviewed past events,

  • USC RPL sample grain burns
  • pad anchor modifications
  • restroom progress at the 10/1/22 work event

We spoke of pending events

  • Bill Claybaugh’s new pad, 10/20-10/22
  • Aerospace Corp launch of experimental motor, 10/20
  • next work event, 11/5-11/6, more plumbing
  • USC RPL static fire on 11/12
  • Events subject to change, public calendar soon to come online with the new website

The RRS reported quarterly dividend from

Further concrete repair work is needed. Four of the 28 mounting holes need more work. This will likely be done by the society. We need to determine best way to extract old anchors and patch the holes with sufficient time for curing before redrilling. USC RPL will assist by drilling final anchors once repair work is done. USC has been very helpful in making this important site improvement.

A budget update on restroom with expenses to date and how much to finish. Last estimate showed we’re on target. An update is coming before next month. Our goal is for the restroom to become operational by year’s end.

The society is working hard to help other members to become pyro-ops. More pyro-ops means safer operations and greater flexibility in future events. Frank and Bill Inman have their letters and are working on their applications. The best way to start is being active with society events and apprenticing under experienced and licensed individuals.

RRS has updated their flight waiver with the FAA to 100,000 feet. Some planning and advance notice required. Laminated copy of our waiver and instructions will be put in the Dosa Bldg. Also, launch requirements available from the RRS president.

Patrick Finley of the Collegiate Propulsive Lander Challenge attended. He explained his foundation to encourage propulsive landing technologies among university groups. Five technology prizes. The RRS is a supporter of this initiative. He can be reached at “”

Wolfram Blume has been working on the Gas Guzzler over the summer. Fuel pump and flameholder fixes. Next flight could be in December?

Bill Inman is building the 2nd generation Solar Cat with fellow member Dale Talcott in Nevada. Subscale prototype built in the summer had excellent focus and heating. Next test at MTA could be December, perhaps January.

John Krell’s avionics chips have had some improvements. Now can do 16 channels at over 1 kHz all on a chip fitting in a slim alpha payload tube

First meeting with LACMA by the president and VP on a STEAM project focused on the arts. Further discussions will better define the RRS role. 

2023 Symposium will celebrate our 80th anniversary, tentative date is April 8. Researching locations, food service and potential speakers. Official launch will be in the new year.

New launch rail design proposed for high power rockets using 24-feet of 1515 launch rail and a 1-ton hand-winch to operate. It will be an outdoor fixed asset and supplement existing launch rails. Need to get a materials list and drawings made for a contractor quote.

Larger 60-foot launch rail for liquid rockets is in the planning stages. The RRS has had several entities interested in using such an asset.

We will soon be starting the effort to sort out the old equipment in the north yard, determine the purpose, origin, and dispose or refurbish each item. We must use our space wisely and not store articles without a relevant purpose. Junk will always fill a void.

RRS may be getting a new pair of storage containers and fire-fighting equipment. Council is in discussions and supportive of this site improvement. More details later.

Some at the meeting indicated strong interest in learning how to weld. The RRS is seeking an instructor which may give us training for a nominal fee. More on this next month.

3D printers were discussed near the end of the meeting. Several members have the devices, but design tools are just as important. Google Sketchup, Solidworks and CATIA are options but costs on some can be prohibitive. This would make a fine topic for a future article on RRS.ORG

Nominations for executive council offices will be at the November meeting next month. An election chairman will be appointed who is neither a current office holder or a candidate for office.

Next meeting at Compton Airport front office on 11/11/2022 at 7:30pm.