Single-stage-to-orbit spaceplanes represent the ultimate design challenge in the field of aerospace engineering. The dual aims of maximizing both mass ratio - required for getting into orbit with a single vehicle - and payload capacity - the reason for going in the first place - directly conflict with one another. Although it is theoretically possible to achieve orbit with a single-stage vehicle, most designs result in a very small payload capability. By the time such a vehicle reaches low Earth orbit, the spacecraft is essentially a floating gas tank with a ridiculously tiny passenger cabin or cargo bay. Something like 90% of the vehicle is completely empty. This is the basic reason we are still stuck on vertical-staged rockets. Can something be done about this?
New vehicles require bold, new thinking. The first thing to realize is that disadvantages can be turned into advantages. As a simple illustration of this, consider the potential of a floating winged gas tank that just happened to be refueled in orbit. Never mind for now where the gas comes from. Just pretend for the moment that you are the pilot of a spaceplane that has just reached LEO and you are out of pro-pellant. You only have enough fuel to place yourself in an orbit that will allow rendezvous with a supply depot. Once fully replenished with fuel and oxidizer, what are your mission capabilities? The answer may surprise you.
Remember, the spaceplane you are piloting has a DV capability of 5 miles/s, fully fueled. You have used every ounce of this capacity to reach orbit, using a few other tricks to reduce propellant consumption, and minimize drag and gravity losses. So now you are sitting in orbit, you have just undocked from the depot, and you have a full gas tank. Your supply of onboard stocks is too small to attempt an interplanetary flight, since you would likely run out of food, water, and breathing air during the months-long transfer ellipse. But with full tanks, you are just itching to go somewhere. Fortunately, there is a large destination nearby: the Moon. It is only another 2 miles/s to achieve escape velocity from LEO, and returning to Earth is no problem, because you have wings. Getting in and out of Lunar orbit will not take much fuel, because the gravity field of the Moon is weak and you will have slowed down significantly during the uphill coast to the Moon. So you light off your engines once again, and you are on your way. Within 3 days, you, your crew, and your passengers are gazing down on the pock-marked surface of Luna from the cozy cabin of your sleek spaceplane. But wait a minute! You just remembered something important.
Your gas tanks still have the equivalent of almost 3 miles/s DV in fuel and oxidizer, and you will not need nearly that much to get back to Earth. It is downhill almost all the way, and the atmosphere is your free air-brake. Just then, you have an idea. The surface base has informed you that they are running low on fuel, and your passengers have been pestering you for a Moonwalk. But you cannot land the spaceplane on the Moon, because it is designed for landing only on Earth. You have no intention of making a tricky tail landing, because you do not want to tip over. So you give the crew at the base a call.
"Tycho One, Spaceplane Yeager." "Go ahead, Yeager."
"We've got a proposition for you. We just happen to have around half a tank of gas that we don't really need for the coast back. Seems we also have some tourists who've been itchin' for a Moonwalk, but they would need a lift to the surface and back. Could you maybe help us out?"
"Roger that, we'll rendezvous with you in two hours. Glad to help out." With that the deal is done. Two hours later, the spidery Moon shuttle docks with Yeager, hatches and hoses are sealed and safed, and passengers and propellant are promptly transferred. Another 24 hours at the Moon, space tourists back aboard, trans-Earth injection burn complete, and you're coasting home in the fastest plane ever built.
In a few days you will be entering the atmosphere at a searing 25,000 mph, make a few skip-entries to slow down and stay cool, and make your final entry and landing a week after you left the spaceport.
This little story serves to illustrate how versatile spaceplanes will be in transporting both passengers and propellant to the Moon. Because of the fact that any singlestage-to-orbit spaceplane will have large propellant tanks, it is automatically well suited for space tanker duty. In this way, the huge tanks can be useful not only in getting the vehicle into orbit, but also in supplying various bases and depots with precious propellant. Let us examine this idea a little more closely.
What if water is found on the Moon? Of course, it would exist as ice, probably in the environs of the Lunar poles. If this ice could be melted and electrolyzed into its constituent elements of hydrogen and oxygen, the Moon itself could supply large quantities of propellant for future space infrastructure. This completely changes the picture. The story would now have its heroes picking up Lunar propellants and delivering them to the Earth-orbiting depot, rather than the other way around. As far as getting to the Moon is concerned, the spaceplane would need only a fraction of the fuel that it needed to get into orbit. So it would fill its tanks to about two fifths of their capacity, fly to the Moon with a much lighter load, and then return with a full load of water, not rocket propellants. This is because water takes up less volume than the same material in the form of liquid hydrogen and liquid oxygen. When it comes to utilizing propellants, it is mass that counts. Once the Lunar water is delivered to the LEO propellant depot, it can be electrolyzed into rocket propellants using solar energy. This would be done just before the next scheduled arrival of a thirsty spaceship.
That is a peek at the future. But just how will these advanced spaceplanes be conceived, how will they be developed, and when will they be born? To answer these questions, we need to fully appreciate the baby's parents, its "mother" the airplane and its "father" the rocket.
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Do we really want the one thing that gives us its resources unconditionally to suffer even more than it is suffering now? Nature, is a part of our being from the earliest human days. We respect Nature and it gives us its bounty, but in the recent past greedy money hungry corporations have made us all so destructive, so wasteful.