A far light in the sky, a faint rumble like distant thunder… soon drowned out as the klaxons screamed again over the hard-packed clay of Isahan Interplanetary.
“Oddly Specific Impulse, nuclear heavy-lifter out of Vevery, grounding! All personnel, clear pad eight! I repeat: Oddly Specific Impulse, nuclear heavy-lifter out of Vevery, grounding! All personnel, clear pad eight!”
A series of thuds and locking clanks betokened the closing and sealing of the access ramps. For a minute more, the landing pad was silent, a dark-gray disk of layered graphite, sapphiroid and cerametal slabs nearly a mile wide within the ring of its earthen berm. From that far back only the most discerning eye could make out the lines between the slabs, so carefully were they cut and fitted together – even, or perhaps especially, those concealing the accesses and other pad facilities.
Much easier to make out were the rail-less “hot” shaft at the center of the pad, ringed with black and yellow caution markings, and the giant, blocky digit-eights and inward-pointing arrows at each cardinal point, inlaid in white metal. That did not require a discerning eye, merely an incautious one; the prompt radiation dose you’d receive from a vantage point atop the berm wouldn’t kill you, but the vomiting and blue-blotch syndrome was unlikely to be pleasant. Nor was the rest of what an octet of nuclear lightbulbs at full thrust would do to your senses.
The light dropped lower. With suddenness it flared bright, and thunder bloomed across the plains. It had come close enough to the ground that our hypothetical watcher – or the screens at port control – could make out the polished metal of Impulse’s forward hull, a curved silver bullet, but her aft hull remained hidden. In space, the superheated hydrogen gushing from her roaring drives would be invisible, but down in the atmosphere it ignited as soon as it mixed with the air; Impulse descended on a column of flame, which washed back up around her as she descended, wrapping her aft hull in a fiery cloak through which only the edges of her tailfins showed.
Lower and lower she dropped, almost imperceptibly slowing, as if she would dash herself to pieces on the ground. Her pilot was of no mind to waste reaction mass, and had saved all his deceleration for the last seconds of flight. Her flame touched the pad, gushed sideways, kinked as a last-minute side-slip properly aligned her drive plume with the “hot” shaft which swallowed it whole, leaving only a few curls of fire to wash out over the width of the pad. Above, aligned on the lee edge of the berm, the tall radiator fins which carried away drive heat from pad and shaft alike burst into carmine life.
Down further she sank, crossing these last few hundred feet as slowly as the thousands before them had been swift. The roar of the engines eased a little. Down, and down some more. Contact. Impulse thudded onto the pad, resting on the reinforced trailing edge of her tailfins, and her pilot expertly killed the drives, thunder disappearing into echoing silence. Without sound, it seemed, Impulse dropped her drive shroud into position, a cylinder of lead-composite to confine drive radiation to the shaft where it belonged, and from the edges of the pad the sprinklers rose and fired, drenching heated pad and searing hull alike with water that turned almost instantly to steam.
From far away the announcer spoke again.
“Oddly Specific Impulse, nuclear heavy-lifter out of Vevery Station, has now grounded at pad eight. Service team stand by. Rad-check team, commence sweep. Disembarkation may commence in twenty minutes.”
Is the exhaust radioactive? I was under the impression that the nuclear lightbulb design used a closed cycle. Or is it simply that the engine itself is giving off radiation?
You’re quite right – it’s a gas-core closed-cycle design, so the exhaust itself is clean, just superheated hydrogen.
But when running at high (thrust) power, the reactors in the lightbulbs themselves do still produce an amount of prompt gamma radiation that it would be Very Unhealthy to stand next to, and which would require an inconveniently high amount of mass to shield against – especially since for most of the flight cycle you’ve got plenty of empty space around the drives to let the inverse-square law do its thing. It’s easiest, the designers concluded, to keep the pad shielded during the launch/landing itself, and then shield the engine enough – with the drive shroud and hot shaft – for safety while running at warm idle and no more.