Rockets vs Space Planes

Rockets vs Planes

This excellent article was extracted directly from the reference web site!!!

Everyone's seen pictures of rockets taking off - both real ones and imaginary ones. And everyone's seen pictures of spaceplanes taking off - but they're all imaginary - because they're impossible! (or at least, be prepared for a long wait). The basic problem for designers of reusable space vehicles is finding enough speed to reach orbit without carrying so much fuel the vehicle is either too heavy to get there or able carry anything other than fuel. So the answer is either to make the vehicle very light, or to find a way around having to carry all that fuel.

There's more than one way to do it

1 - HTOL 2STO

The problem with planes is that wings contain very little fuel, but they have a big surface area, so they're heavy. Their advantage is that they can generate lift in the atmosphere so that a launch vehicle uses much less propellant in the first part of its trajectory. But that isn't enough of an advantage to overcome their weight problem - so HTOL (Horizontal Take-Off and Landing) SSTO (Single Stage To Orbit) is impossible with existing or near-term materials and propulsion technology.

However, HTOL is possible with a little help: various kinds of assisted-launch HTOLs are possible - like a rocket-powered launch-sled or dropping off from a balloon at high altitude. Unfortunately these are not at all convenient for operations! Alternatively TSTO (Two Stage To Orbit) or 2STO, and also 1.5STO, are possible, and may well become attractive systems. Good examples of these are the Spacecab/Spacebus 2STO design which has a first stage like a large Concorde that takes off using existing jets and then uses existing rockets to climb to high altitude (about 40 km) where it releases the upper stage already almost outside the atmosphere. The upper stage or orbiter is like a smaller, blunt (good for re-entry) first stage, using existing rockets to get to orbit, and then small jet engines for landing.

1.SSTO is the approach by the Black Horse and Pioneer Rocketplane team. A vehicle looking rather like the Spacebus upper stage takes off from a runway with almost empty propellant tanks. Then it refuels in mid-air, using technology that's been used for decades by Air Forces. It thereby gets to "launch" fully fuelled at high altitude, neatly getting round the impossibility of HTOL SSTO.

Another ingenious 1.5STO is the approach by Kelly Space Technology. Their winged spaceplane will take off horizontally, fully fuelled but unpowered, being towed behind another aircraft. In this way they too can get to high altitude before they have to use any of their propellants. Neat - we wish them luck!

2 - VTOL SSTO

The advantage of VTOL is that it's structurally simple - or can be if you design it right. Circularly symmetric (forget all the complexities of wings!), a "flying propellant tank", inherently stable on reentry like the early space "capsules", and known to be possible with 20-year old technology. Several different design approaches are possible, using different types of rocket, and also innovative approaches like the ROTON rotary rocket.

3 - The Duckbilled Platypus option

The X-33 takes exactly this approach, of course, and it should be noted that one of the losing entries, the DC-X, was a VTOL SSTO proposal and the only one to have actually had a prototype build and flown. Strictly speaking there's also VTOHL (Vertical Take-Off, Horizontal Landing), like the space shuttle. We can't take that seriously at Space Future because it's not a good design, as it has a number of fundamental problems. The key difficulty of SSTO is to build a light vehicle, and so the structure must be as efficient as possible. In a VTOL vehicle the loads are basically along the vehicle from bottom to top. In a HTOL vehicle they're mainly across the vehicle like an aeroplane. But a VTOHL has to support stresses both along and across the vehicle - so it's heavier. Secondly, once a VTOHL vehicle has taken off, there's a period of time during which it can't easily recover from a failure. This is contrary to the fundamental idea of aviation safety, namely the need for "continuous intact abort". At any time it must be possible to save the vehicle and passengers from an accident. The Challenger disaster was a vivid demonstration of this problem of VTOHLs, and they're unlikely ever to be satisfactory for passenger-carrying. But convince us if you can!

Another problem is this: wings allow you to use aerodynamic lift at take-off, so you don't need to use so much thrust and propellant. (The thrust of a VTOL rocket's engines is typically 1.4 times its mass, while the thrust of an airliners' engines is only about 1/4 of it's mass - big saving!) So to build wings and just carry them vertically to orbit is shooting yourself in the foot! Finally, an unpowered high-speed landing is very dangerous. Airliners can rev their engines at any time and "go round again" if there's a problem before landing - but the space shuttle would be destroyed. Even Chuck Yeager didn't like doing "dead-stick" landings in a fighter plane, because of this risk.

4 - The Rest of the Field

In addition to the above, there have also been proposals for two stage vertical launch vehicles, and even a single-stage-to-orbit space station that launches itself!

In the blue corner...

Now, there are dedicated, professional, convincing (even noisy!) supporters of both HTOL and VTOL approaches, and both are certainly possible - in the right configurations. Rather than take sides, Space Future would like to see both getting funded and being built and put into operation. The competition between them will be just great to watch, and it'll help to improve the designs, drive their costs down, and speed things along nicely!

The war's over

Unfortunately the space industry suffers from the problem of having been a government activity for decades during the "cold war". As a result there are numerous research establishments with expensive equipment in many countries which these institutions want to use, because it's embarrassing to them to admit they're a waste. So they're always coming up with ways of spending money on anything except passenger launch vehicles.

Projects working on scramjet (supersonic combustion ramjet) engines are a good example of this. They sound great, taking oxygen from the air instead of carrying it with you - but they're essentially pointless technology, at least for civilian uses. They'll certainly never be used in any commercial vehicle. Think about it - Supersonic Combustion Ramjet means that it can't operate below supersonic speeds. So it needs another engine to take off with. And it's an air-breathing engine, so it can't operate in space either! So you need rockets on board. So you need 3 different propulsion systems - heavy! Worse, experiments have shown that originally planned to go to Mach 26 or Mach 20, they can't work much above about Mach 11. And to use them the launch vehicle has to cruise in the atmosphere at that speed (like standing in front of a welding torch!) instead of just getting out of the atmosphere as quick as possible. But there are expensive hypersonic wind-tunnels which "have to be used", and so instead of trying to make a launch vehicle that might actually make money (what a thought!), government institutions keep thinking up plans why they should continue this work instead.

Well, if these organizations want to waste taxpayers' money, what's new? "Who cares?" you might ask. What must not be allowed is for these organizations to block the start of the space age by justifing projects like this with the ludicrous claim that "This is the way to open the space frontier." - it isn't. The way to open the space frontier is to build passenger vehicles with existing technology and to start passenger services. It's as simple as that.

Keep it simple, stupid

Henry Ford didn't wait for the V-8 engine to be developed before he started selling cars! He made them with the engines he could make at the time, and he got started! And then with the money he earned he improved his products step by step. It's going to be the same with space travel. It's going to start with what's available - not with what might be developed decades from now, if enough billions of taxpayers' money is used (and probably not even then).

And so that means making and operating VTOL SSTO vehicles, and 2STO/1.5STO HTOL vehicles. So we're looking forward to seeing vigorous competition between different models of these different vehicles, and to seeing which companies and which countries play a significant role in humans' approaching space future - and to seeing which ones keep their heads in the sand, and their people in the dark, until the race is lost!

Reference: Reference:

http://www.spacefuture.com/vehicles/rocketsvsplanes.shtml

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