Claim
The Adaptive Nut Can be Reused
Evidence
In the reference implementation of the VPSL system, the adaptive nut separates from the launch sled and spacecraft at the end of the forward acceleration section. While the sled and spacecraft continue to coast at full speed up the ramp, the pitch of the screws transitions from "variable and gradually increasing" through "constant" to "variable and rapidly decreasing". The adaptive nut adapts to this change by repositioning its grappler pads to the opposite sides of the screw flights. The "variable and rapidly decreasing" pitch of the screws decelerates the adaptive nut, bringing it to a stop near the downrange end of the ramp. Because the ramp is much shorter than the acceleration section, the adaptive nut must decelerate at approximately 820 m/s² (about 84 g) to stop before reaching the end of the ramp. Although this is a high acceleration for a payload, the adaptive nut is a purpose-built mechanical assembly engineered to tolerate such loads.
The current mass budget allocates 10,000 kg to the adaptive nut, 1,000 kg to the sled, and 24,940 kg to the fueled spacecraft including the payload. While the nut experiences a higher deceleration rate than its earlier acceleration, the total mass being decelerated is smaller.
The screws in the deceleration section are similar in construction to the screws in the acceleration section but incorporate a more rapid change in pitch to achieve the required braking rate. In the deceleration section, inside each screw, flywheels start out spinning more slowly than the screws. As the adaptive nut threads along the deceleration screws, its linear momentum is converted into rotational torque on the screw shafts, and that torque is transmitted through electromagnetic clutches to the flywheels, causing them to accelerate. In effect, the nut’s forward kinetic energy is absorbed as an increase in flywheel angular velocity, with the flywheels acting as temporary energy reservoirs during braking.
At the end of the ramp, the adaptive nut disengages from the guideway and is set aside, clearing the system for the next launch. Multiple adaptive nuts are employed when several spacecraft are to be launched in rapid succession. During pauses in launch operations, the adaptive nuts are reattached to the guideway and returned at low speed to the starting position in preparation for subsequent launches. Each adaptive nut descends the ramp under gravity and then coasts along the maglev guideway, with linear motors spaced at intervals of several kilometers providing propulsion assistance to maintain speed throughout the return journey.
Following each launch, electric motor-generators recover the flywheels’ stored rotational energy, converting it to electrical power while slowing them down for reuse. The recovered energy is then supplied to the motors in the acceleration section to re-spin-up those flywheels in preparation for the next launch cycle.
Reviews
The following reviews are limited in scope to the validity of the claim made above, and do not imply that the reviewer has taken a position regarding any other claim or the overall feasibility of a concept that is supported by this claim.
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Reputation: 0Verdict: SupportsGraduate studies in physics and math at Michigan State University Honors College.“Possible in principle and critical to feasibility, but will require extended runway to brake from high velocity.”
The stated mass budget allocates 10,000 kg to the adaptive nut, 1,000 kg to the sled, and 24,940 kg to the fueled spacecraft. It's not clear whether the 24,948 kg for the fueled spacecraft includes a payload mass of 10,986 kg (as stated in the 2024 IEEE conference paper on "Revolutionizing Space Launch". I'll assume that it doesn't. That would mean that the VPSL system would be accelerating 46,926 kg to 11,129 mps in 774 km of submerged tube. I believe that works out to a bit over 14 G's of sustained acceleration. That's too much for a crewed vehicle, unless the crew's lungs and body cavities have been emptied of all air and gasses and the crew are immersed in a water bath. Either I've miscalculated somewhere, or the numbers for the different sections of the launch tube are off.
The point I was going to make, however, is that if the adaptive nut is 21% of the mass accelerated, then the same drive force applied by the screw for accelerating everything should be able, when used in reverse, to brake the adaptive nut to rest in 21% of the distance. However, per the numbers in the IEEE paper, the ramp distance (83 km) is less than 11% of the submerged acceleration distance (774 km). The braking force will therefore need to be more than twice the acceleration force. The actuators must then be designed to transmit more than twice the net braking force as the net accelerating force. That's a major design complication. To make things even more complicated, the pads that follow the screw threads will be following the opposite face of the drive threads than the face used for acceleration.
I'm not prepared to say that arresting the adaptive nut can't be done; physics doesn't prohibit it. But it may stress the limits on strength of materials and is at best a very challenging mechanical design problem. I'm inclined to question whether the benefits of a variable pitch screw are enough to outweigh the disadvantages of a fixed pitch, variable speed design.
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