How does Spinlaunch manage the counterweight right after launch?

Spinlaunch has successfully launched a suborbital test vehicle from a 1/3 scale demonstrator, see for instance aviationweek.com or space.com.

While spinning up the arm, perfect balance is achieved thanks to a counterweight opposite the payload. What puzzles me is, what happens right after launch, when the counterweight is still here and imparts a huge force on the axle?

A comment on this answer How might SpinLaunch actually spin something fast enough to launch it into orbit? suggests to release the counterweight at the same time, into a deep hole: "To avoid disbalance, two objects of equal mass should be launched simultaneously in opposite direction. One up into the sky and the other one down into a deep hole into the ground.". One answer of the same question notes that the amount of energy is on the order of 10³ kg of TNT.

However, looking at the images on their site, there is only one exit tube, to the sky. Even if the counterweight is closer to the center, it has a lot of kinetic energy and creates a lot of disbalance.

So, how do they avoid the counterweight destroying the axle through intense vibrations?

• Very interesting question. I don't have an answer, but I did come up with some numbers worth sharing. Of course, there's a bucketload of speculation here! Their website says the projectile will exit at 5,000 mph, the acceleration is 10,000 g's, and they will launch payloads up to 200 kg. I guessed that they would need 2,000 m/s of delta-v to make orbit and that their structure etc would be only 20% of the mass of payload and propellant (very aggressive, I think). I estimate a total launch mass of ~ 500 kg, a spin rate of 423 RPM, and unbalanced load of 50 tons. Apr 11, 2022 at 3:34
• To be cynical, I get the impression that it's something that they're brushing under the carpet at the moment - it's likely a huge technical challenge, and some sort of capture mechanism/tunnel would ruin the sleek look of their concept. Apr 12, 2022 at 8:33
• Also, the launch speed is ~2.2km/s, suggesting they need ~6km/s to reach orbit, so I would expect the mass being released to be >2 tonnes Apr 12, 2022 at 8:37
• The way I'd approach it: the arm would have a "slider" actuator that can shift it relative to the axis. Right before release of the rocket activate the slider, shifting the counterweight closer to the axis, the rocket away from it (with rocket's angular momentum slightly higher than the counterweight's that doesn't require any force input) and release the rocket as soon as the slider reaches end of travel. When the rocket is released, the arm will be again in balance, the (now longer) length of the arm that used to hold the rocket with same momentum as the counterweight on (now shorter) arm.
– SF.
May 5, 2022 at 9:33

In this video from Real Engineering youtube channel, it's been said that: "the ideal solution would be to release a counterweight in the form of another launch vehicle after a single half rotation of the tether. The oil-filled journal bearing the massive axle sits upon should be able to absorb the force of this imbalance over a period of time this short"

Following Christopher James Huff great comment, some heavy counterweight on a very short arm could roll inside a cage.

When spinning up the whole thing is balanced, the counterweight spins and rolls with no load nor friction against the cage.

When the payload is released, the counterweight keeps following its circular path inside the cage, yet it is suddenly loaded on the inner surface, and slows down due to friction* on this massive cage. The cage has to withstand this enormous force, still at least there is no impact nor sudden stop that's calculated in TNT units.

The connection between the shaft and the arm could be an elongated hole, allowing a small offset without damaging shaft bearings when payload is released.

*If not by friction, somehow, this system could allow to recover part of the energy used to accelerate the whole thing, while the releasing counterweight method makes the recuperation impossible.

• Clever design, cleverly explained, and Spinlaunch hasn't published photos of their device's interior that could disprove that this is what they're doing. +1. May 16, 2022 at 17:10

Spinlaunch releases the counterweight at the same time as the vehicle. Their patent says:

The launch vehicle 105 may be released from the launch vehicle tether... Simultaneously, the counterweight 135 may be released from the counterweight tether 130.

...The circular mass accelerator structure 150 may comprise a second exit port directly opposite the exit port 115 to capture the counterweight 135 that is released simultaneously with the launch vehicle 105 to minimize an imbalance on the motor at the time of release.

(To comprehend these excerpts as an engineer rather than a lawyer, for "may" read "will.")

The patent's figures do not show a second exit port, but one figure does show the released counterweight 135. Nothing is said about what happens to the counterweight after release or after reaching its exit port.

Maybe a separate question: how is the released counterweight's kinetic energy dissipated? Fermi estimate: the released vehicle (5000 mph, 11 T) has 30 GJ energy. Say the counterweight has a tether half as long, so it's 2500 mph and 22 T, or 15 GJ. A car on the highway has about 1 MJ, which in a crash is dissipated by 20 "impact attenuators" (barrels of water or sand), so 50 kJ each. 15 GJ / 50 kJ = 300,000 barrels. Each about 0.3 m^3, so you'd need a cubical pile 45 m on a side.
Are there any photos during construction of a large pit, or a long line of dump trucks removing dirt?

• I guess "may" is used in the patent to say that the design could do this, but if it doesn't do this and does something else, it doesn't invalidate the whole patent. May 5, 2022 at 8:25
• Likely, but what an overreach. What else could "The launch vehicle 'may' be released from the tether" mean! May 5, 2022 at 15:16
• @CamilleGoudeseune Trying to make the claims as broad as possible seems to be standard practice for patent lawyers. On my last patent, the lawyers seemed very good at taking my very clearly written technical paper and making it as broad and confusing as possible. This doesn't seem much different. May 12, 2022 at 6:58
• If the payload is released straight upwards, the counterweight should be released in opposite direction straight downwards for symmetry reasons. There should be a very big and deep hole under the spinlauncher. But how to prevent an earthquake?
– Uwe
May 12, 2022 at 21:41
• @TrySCE2AUX energy losses are pretty far down on the list of problems with this proposal, but: the counterweight is on a much shorter arm in the illustration, so it could be a much higher mass at lower velocity. Remember that momentum is proportional to velocity while kinetic energy is proportional to velocity squared. Doubling the counterweight mass and halving its velocity means halving its energy. May 14, 2022 at 12:55

This August 2022 Real Engineering video has an answer:

• currently on the scale model, they do release the counterweight at the same time, not into a hole, but into an armored section of the vacuum chamber
• the plan is to have two satellites to launch, one at each end of the rotating arm. They release one, and half a rotation later they release the other one. The bearing should be able to withstand the imbalance for a time this short.
• Yes, more plausible than a sliding arm. But then why is the arm slotted, I wonder? Aug 20, 2022 at 15:21

SF's comment of May 5 2022 "the arm would have a slider actuator" might be close.

The brief cutesy animation that ends Spinlaunch's June 16 2022 video, 1:42-1:44, shows that the arm has a slot. The slot is also barely visible, albeit without a callout and without any mention, in the patent diagram in my May 4 answer. (The animation lacks both the counterweight (#135) and the payload, but never mind.)

After launch, to dissipate energy, the arm could then slow down at its leisure due to friction from the air that just entered its container.

Naively, the arm's 15 GJ would heat the container's 360,000 kg of air (from 300,000 m^3) by 15e9 / 360000 = 4e5 K, ten times hotter than the melting point of anything we know of. Maybe they spray in water, or gradually open more vents for convection, or use regenerative braking to charge batteries for the next launch.

• "the arm's sudden imbalance would fling the counterweight end outwards until the arm regained balance" - could you elaborate on the mechanism here? Aug 9, 2022 at 23:52
• Thanks for the obeservation. Details about friction, I remember reading about high performance doors that shut right after launch to prevent air entering the container, otherwise the shockwave would be devastating. Aug 11, 2022 at 7:07