On the design of rodent scale Pulse drive
Single Stage to Orbit (SSTO) vehicles
(AKA The bits you didn’t see in ‘Out to Launch.’)
By: Stainless Steel Rat
"The most famous example of a pulse drive is the Orion. It was developed during the 1960s as an alternative to complex space planes or ridiculously expensive and inefficient staged rockets for getting a large vehicle or cargo into orbit. Test flights were even made using scale models with chemical explosives. In essence the design was ‘Built your vehicle on a large solid plate with shock absorbers in between, somewhere where the land values are low. Light an atom bomb, coated with some absorbing material under the plate. The plasma from the explosion hits the plate. Plate gets pushed up and transfers momentum to vehicle through shock absorbers. Keep on throwing atom bombs under the plate until you reach orbit.’ The concept works because although the peak temperature and pressure of the reaction is above the melting point of the plate, it’s only at that temperature for a fractional part of the cycle. The same trick is used in internal combustion engines."
+++
Gadget made a last minute check of her controls, such as they were. The chains with their magnetised nuts were secure in their compartment. They would be deployed in orbit to control the fire extinguisher Orbital Maneuvering System. The rest of the orbital instruments on the console to one side, were dark. The only thing active thing was the digital watch face set into the ceiling of the control cabin. Five minutes and they would launch. The boost to orbit was automatic, the only thing she’d be able to do was watch the thrust gauge, a spring balance that would show the size of the pulses of acceleration.
Once again she ran over the numbers and proved this was the only way. Nothing else could have been built in time. Given her druthers and a couple of days to work she’d have tried to duplicate the Space plane in rodent scale, complete with the hypersonic tanker that refueled it in mid air and allowed it to go to orbit. As it was she’d been lucky to find the supply of construction material at this government scrap yard, including some explosives.
The upper garbage can that made up the ship comprised three sections, an airlock under the lid, the control cabin with it’s port hole and padded floor and ceiling, made out of large saucepan that fitted inside the main hull, and the service deck with it’s batteries and a small salvaged oxygen cylinder. It was open and directly joined to the HMX storage bay, the lower can, and not designed to hold pressure. Adding shock absorbers between the crew can and the lower one would have cost them one more orbit of the suit, and according to her calculations exhaust the oxygen inside it.
The lower can was her main worry. Underneath it, and the metal plate welded to the bottom, were five sticks of HMX. Similar to dynamite in appearance but far more powerful. Inside the can were about 80 more. When Zipper connected in the onboard batteries it would be down to the digital watch to trigger the initial explosion. They’d be kicked upwards by the shock-wave hitting the plate with a pulse peaking at a painful 20 times gravity. The ship's structure could take it, and so would they, though it might cause hyper-elastic deformation. It would hurt, but wouldn’t do any permanent harm, any more than the same effect in the centrifuge had caused Chip and Dale any lasting discomfort.
‘The gravity feed system will work!’ she thought to herself, ‘It has to!’ That first blast would kick them off the launch pad and force the first inboard stick into the launcher where it would be lit and flung out of a one way trapdoor in the pusher plate. Fractions of a second later it would explode and the shock-wave would send them higher. The process would be automatic and self sustaining from then on. Eighty blasts later they’d be travelling at 4.5 miles per second, heading up towards on a trajectory that would peak at the orbital height of the errant space suit, and a few miles behind it. Then they could use the OMS to regularise the orbit and carefully home in on it. She’d made sure those CO2 jets would give plenty of velocity change.
One minute. She’d deploy the tether and fly over to the suit with her inhaler MMU to attach it. With the suit and ship connected she could take her time with the transfer. Monterey was still outside. "Remove the gantry for launch!" she called through the external speaker. There was cry of "Roight!" and a crash from outside. "Commence ignition sequence!". she announced next, telling Zipper to connect the battery wires for the igniter.
Fifteen seconds. "You can start the countdown!" she heard from outside. Monty dropped down through the door to the airlock compartment. He reached up and wound back the sardine can lever that sealed the door closed behind him. A light on the panel showed the nose hatch was sealed, so she just motioned him to lie down.
Ten seconds. She started counting down. The igniter continuity light was still dark and the hatch to the service deck was still open, right next to her. Where was Zipper? Then she realised he must be waiting the end of countdown to connect the wires. She suddenly realised even as she counted.
"Nine…" ‘Ohmigosh he’ll be flattened and the hatch will still be open…’
"Eight…" ‘And when we get higher the cabin will depressurise…’
"Seven…" ‘and we’ll be immobilised by the thrust and unable to…’
"Six…" ‘Of course!’ "fivefourthreetwoone."
"Zokay!" she heard from the service deck along with the crackle of sparks as Zipper twisted the two wires together. He zipped into the control room and on to the matting beside her. She slammed the hatch shut about half a second before the timer really hit zero.
WHOOOM! Squeeeeeeeeeeeeeeeze.
WHAM!
WHAM!
WHAM!
The pulses of weight felt like Fat Cat was repeatedly slamming a door shut on her, with the expected result. The reflector in the ceiling showed all three of them looked like Cheese flaps a la Monterey, literally flattened into the padding. She concentrated on watching the thrust gauge, designed to be in line of site even in this extremity, but could only think one thing.
WHAM!
‘Next time…’
WHAM!
‘… I add…’
WHAM!
‘… shock absorbers…’
WHAM!
+++
"Homing in on a target in orbital space is not easy. No air friction and the vagaries of orbital mechanics can trip up the best atmospheric pilot. When in a stable orbit, thrust in the direction of motion, you fly higher, and as a result your speed actually drops, in the same manner that a stone spun on a string makes slower circles with a longer string. In elliptical orbit the situation is similar, but there you’re forever trading speed at a lower altitude for height at a higher one.
To locate something as small as a space suit from even a few miles away is a trick in itself. Consider the size of a man at that distance, add an absolute black background and a glare in sunlight, or a background of a million points of light in a planet’s shadow. Instrumentation is vital, both to give relative location and relative speed, because it may be travelling in a completely different direction."
+++
WHAM!
WHAM!
Nothing.
Clunk. The pulse drive can separated from the main ship and dropped away in silence. Inside the cabin it was also silent, until there was a trio of popping sounds as the three intrepid space travelers inflated back into their own shapes and floated up into the air of the cabin. Gadget’s body ached and her eyes were bleary but her mind had popped back into focus with her body, and it was sending her warning signals. Her first act was to seal the Service deck hatch, already held closed by the air pressure in the cabin.
"Strike me starkers lass! Next time I’m gonna walk." "Zyeahhh!"
She pulled herself over to the port hole using the floor padding and broke out navigational instruments from a cubby, taking a measurement on the limb of the earth with a micrometer. "No time to talk guys, I think we have a problem. The first explosion pegged the thrust meter, meaning it went higher than 30 Gees. There must have been some ground interaction effect I didn’t take into account. The additional thrust has thrown our whole trajectory off. We’re lucky the ship didn’t crumple."
She was double checking her mental calculations on a note pad, which gave her an answer she didn’t like. This meant she didn’t see Monty’s sudden guilty look, or Zipper doing a perfect ‘Chip bonking Dale’ impression on him.
She turned looking worried and moved to the console, letting down the pull cords that hung towards the steel plate. "We were going to come up on an elliptical orbit with it’s apogee, sorry peak, several miles behind them and a velocity too low to maintain that orbit. I’d have used the OMS to speed us up and creep up behind them carefully, matching orbits precisely. My best guess is our new orbit has a higher peak and it’ll pass in front of them."
Monty looked stricken. "You mean were going to miss em?"
"Not if I can help it! There may have been a slight anomaly, but I’m going to correct it!"
She held on to the floor padding with her hind paw toes, and pulled on two cords with her forepaws. The cabin seemed to rotate through about 45 degrees. She shifted to other pull cords and Monty and Zipper floated into the ceiling padding, and she suddenly felt bat-like, hanging from the floor, as the low deceleration pulled on her.
"I think I know from my measurement of the size of the Earth how much higher and farther we came. I’m slowing us to counter that. It’s a bit inaccurate though. I just wish I knew exactly how much bigger that initial explosive effect was."
Monterey pushed himself back towards the floor and grabbed on as Gadget was doing. He hesitated and said, "Ehhh. You could try… twenty sticks."
Gadget, nodded, watching the digital watch face and not his face, which would have given away everything. "You know, that sounds about right. Golly Monty, I didn’t realise your tail was a gravimetric sensor as well as a weather station. In that case… 3, 2, 1, Off." She let go the cords and the pull stopped. "That should do it. Unfortunately we’re now effectively in a steeper climb, allowing us to meet them, but not match speeds. The best I can do is slow us down. Our only option is to deploy the tether and try to lasso them as we pass. But we’ve only got two shots, one on the way up, the other that I can make on the way down with an OMS burn."
Monterey and Zipper both looked a bit dazed, but Monty asked, "Anything we can do?"
"Of course. Pull those fridge magnets off the wall opposite the port hole." The two travelers looked puzzled but did as they were told. They’d have been less mystified if they’d been outside. Of course they’d have been more worried about not breathing, but then life is a series of tradeoffs.
On the outside of the hull, between the two fire extinguishers, a steel fairing, no longer held to the aluminium garbage can by the magnets inside, fell away, allowing a spring loaded boom to fold up and out. On it’s end were a pair of servo motors, forming an ‘L’ shape. On the top of the ‘L’ was a loop antenna that uncoiled like a watch-spring. It should have been accompanied by symphonic music, but Gadget hadn’t had the time or spare payload to add a sound system.
Gadget flipped a switch and the console lit up. A cross made of LEDs, 5 in each arm started doing a two dimensional Knight Industries 2000 impression and a low tone came from the loudspeaker. "This will allow us to home in on the suit. The astronauts on the Spaceplane would have triggered the suit’s automatic beacon so they and ground control could keep track of it. The antennae outside will scan back and forth until they each find the strongest emission on the right frequency band. The offset of the servos feed back into the ordinate and abscissa display. I adjust our attitude to counter the offset. By adjusting the gain on this dial I can make the increments quite accurate…"
The tone gained a second higher pitch and the LEDs stopped sweeping, low and right. This stopped her explaination in mid flow and probably saved the other two's sanity. "Contact! But we’re going at quite a high speed. The base tone is mixed with the Doppler variation between the suit beacon carrier wave and an equivalent tone generator in the console. The greater the difference, the greater our relative speed." Even as she explained she was pulling cords, tweaking direction and velocity against a target she couldn’t even see. Occasionally she glanced into a periscope arrangement that lead to the hull.
Monty just held on, switching looking out of the port hole and back at the mouse mechanic, intent on chasing her electronic quarry. The tone dropped and the LED positions stopped swinging back and forth as she worked. Zipper hovered by him, flapping his wings to counter whatever alterations in gravity were occurring. All Monty could do was hold on and not get seasick.
"Zipper-me-lad, I haven’t had a ride like this since I flew a hand glider in a hurricane!" Even as he gave forth this antipodean aphorism, Gadget called out. "I see it! There’s something up ahead, twinkling. We’re in vacuum so that means it’s an irregular rotating object. Deploying capture device!" She turned a knob on the console and on the service deck a section of floor snapped open, allowing the thick cord of the lasso to uncoil behind the ship.
Monty watched as a space suit swept past, only to snag by one foot in the rope that trailed behind. He felt the slight jerk as the line went taut and an even bigger jerk for making the mistake that made the whole business necessary. Gadget, meanwhile, yelled "We’ve got ‘em!" and floated over to the porthole.
The ship and suit were now at rest relative to one another, whatever their absolute motion was. She’d take the rescue bag, a heavy duty zip-lock bag with a windsock like tube of plastic, and seal the tube over the suits visor. Chip and Dale could transfer into the bag and seal it behind them. Drag it back to the airlock and repressurise it from the cylinder. Simple.
She could disengage the sink plunger and release the heat-shield filler held inside. The heavy liquid would foam out and form a hard pumice-like layer over the entire top of the ship. That and the air-break of the airlock compartment would provide the necessary thermal protection for the control cabin. The fire extinguishers had plenty of pressure left to act as retros for the de-orbit burn. She started thinking out loud.
"All we have to do is fire the retro-rockets and use the parachute to splash down in the ocean." She floated away to suit up. While she was outside she’d check the parachute was still properly stowed on the underside of the service deck.
Then Monterey interrupted her plans. "Eyahhh… Parachute? Just how big a splash were you thinking of making, luv?"
This did not bode well. She slowed to a float in front of the console. "You didn’t do something with the parachute, did you?"
As he spluttered, she was already furiously thinking. Without the parachute for terminal deceleration, the phrase would be all too appropriate. "Let’s see…" Of course, the Space plane! It had shifted to a higher orbit in the same orbital plane. She mentally plugged the time of contact with the space suit into her equations and brightened up. Their erroneous high trajectory was suddenly an advantage, putting a rendezvous with the Space plane well within the limits of their remaining OMS power.
"Well, we’ll just have to go home in the Space plane. I always wanted to ride in a real space ship." She’d alter the frequency modulator on the homing system to the Space Plane's beacon frequency and use her original homing strategy. She turned a dial and hauled on the pull cords and the ship blasted onwards. Towards the Space plane and an unexpected rescue for the Rescue Rangers.