Baking is not cooking, the same way rocketry is not flight. Or at least, that would be a first impression, to constantly hear about the extreme precision required to get to orbit or anywhere after. In contrast, right after takeoff, an airplane can lose an engine, or even both, only to glide along and land in the Hudson. The passengers end up safe and sound, with a story they will tell the grandchildren, over and over. Flying along, then suffering an explosive decompression, Aloha Airlines Flight 243 still made it to a runway, minus its upper front fuselage. It’s assumed, flight can be forgiving in ways rocketry is not. Rocketry is different, like baking, requiring we follow a recipe precisely because close enough equals disastrous. Everything must be just right. And yet, is there more leeway in rocketry than we think?
Right now, two giant rockets will soon take their first test flights. The NASA Space Launch System completed (more or less) a wet dress rehearsal last week. Cryogenic propellants were fully loaded, as if mere seconds away from an actual liftoff. The precise sequence of tests began days before, but it pales compared to the strict requirements of everything in the minutes going to orbit. Also, just the week before, the SpaceX Starship obtained an approval (more or less) from the FAA for its first test flight. Unlike the first SLS rocket, SpaceX already built and tested multiple Starships, learning from each, including test flights of the Starship (upper portion), loading fuel and oxidizer, and firing up engines. One Starship even left the ground twice in the same day (sort of).
“The more you throw backward, the faster you move forward.”
Here we have two rockets, trapped in the same physics, with wildly different ways to get to orbit. The SLS is an expendable rocket, throwing everything away to get its payload to a destination. The Starship will try to get even more delivered but return everything to Earth *spaceship included* to reuse next time.
It’s said that adding any math equation to a story means halving the potential audience. At the risk of losing some readers, it’s worth putting “the rocket equation” here, if only to ask, what does this say in plain English? Admittedly, translating a famous equation (well, famous in my part of town) is not novel. It’s common to see this version of naming a tune in as few notes as possible to get us thinking. (For example, Einstein’s space-time and everything equation “T’=T√1-V2/C2” often goes under, “the faster you move in space, the slower you move in time.”) Perhaps we can read the rocket equation “Δv=u ln(mi/m)” as telling us, “The more you throw backward, the faster you move forward.”
Staging, discarding, or separating part of your rocket, is always a great way to get rid of weight along the climb. Whatever push you provide after that point keeps you moving faster. The SLS takes this to heart, with large solid rocket boosters providing lots of push at the git-go, when the entire vehicle weighs the most it will ever weigh. Even more weight-efficient, these are soon discarded, this time not to be recovered as the Space Shuttle did. No parachutes for recovery, less weight to push along.
Starship, however, takes an entirely different route – figuring if you want your rocket back in one piece to use again, but you also want a usable payload, start by super-sizing the whole thing. SpaceX will also catch the Starship on its return, using a “mechazilla” contraption to avoid carrying legs or landing gear all the way to orbit and back. To top it off, the Starship will refuel in Earth orbit, a way to get more stuff to throw out the back instead of only relying on what initial fuel you hauled up.
We haven’t yet figured out a working spaceplane recipe, but there is no shortage of saying the measurements must be exact!
Spaceplanes, too, are more than familiar with this territory of tough decisions combining ingredients to satisfy the math. There are so many possible routes for a spaceplane putting a payload in orbit that even with the vast computing resources we now have, we still struggle to calculate the best path. A spaceplane is an initial decision – let’s grab air along the way as part of what I throw out the back. A spaceplane’s tough choice is deciding how much time it spends flying at around 100,000 to 150,000 feet, between about Mach 5 to 20. You want to grab as much air as possible to combine with fuel and hurl out the back so you move forward faster. Yet the longer you stay gathering air, the hotter it all gets, so you must carry more thermal protection. And all that free air fights you too, meaning more drag passing through that air, slowing you down. Now you end up spending more time accelerating, not yet ready to head up to enter Earth orbit. We haven’t yet figured out a working spaceplane recipe, but there is no shortage of saying the measurements must be exact!
Rocketry seems very precise, with little to no room to maneuver, except most of the time, we see something very different. Raised on “the tyranny of the rocket equation,” my fellow cult members once said reusability will never make sense. Then it was full reusability will never make sense. Some even said the complexity and difficulties of the Space Shuttles proved this to be the case. Why carry an entire spaceship to orbit, like the Shuttles, when it’s just the crew or the cargo that needs to get there.
For the same reason, we are told spaceplanes will never make sense, even though one day we are debating how much maneuverability as airplane-like cross-range they might have. You might land here or far over there. Airplane-like operations, flexibility, and much more margin for error don’t make sense?
It’s in the rocket equation. There are no ifs, ands, or buts about this, leaving almost zero room to imagine anything other than a retreat from the dream of reusable Shuttles. Voyages to Mars are also enamored with the rocket equation – if at times to close off possibilities. Ironically, we are now where hypersonics, which holds the promise of spaceplanes, is worked in the realm of weapons. This is happening while rockets, which look like missiles, move ahead in the civilian world.
So, the rocket equation is not as tyrannical as it seems. There is much more latitude for the imagination than we thought. We are free to at least try and cheat in the Kobayashi Maru test, because who likes to lose? Rocketry may be between baking and cooking, with latitude for a little of this, or more of that, after all.