For that matter, why can’t jet planes take off vertically? I mean besides the Harrier.
The Skylon concept mentioned above claims to address all these points and to offer a genuine working SSTO concept with a considerable cost per kg to orbit benefit. They claim to have just made a major breakthrough in their engine design using lighter materials.
As mentioned in this thread over a decade ago, they obviously can, but keeping aloft using thrust alone is far less efficient than using wings to create lift. The AV-8B can’t even lift off with a full payload in VTOL mode, which is why it’s mostly used as a STOL aircraft.
Well, since this thread has been revived, Paul Allen’s Stratolaunch Systems deserves a mention. Working in conjunction with Elon Musk’s SpaceX along with Burt Rutan’s Scaled Composites, they planning on gluing together two 747’s and carrying a rocket booster between them for earth orbit launching. I’m excited for them!
There were attempts to launch horizontally. Here is a quote from the wiki page on Gerald Bull:
(see Gerald Bull - Wikipedia). From the stories I’ve heard from people who knew him at McGill, he was obsessed with things that go bang. However, the project never went anywhere. I imagine the real obstacle is overcoming air resistance and the best way to do that is to launch vertically.
Excellent concept. By making the the jet engine also a rocket engine, they eliminate a significant amount of dead weight from the unit. The linked press release mentioned helium cooling, but I gather from Wikipedia that the long term plan is to use the liquid hydrogen fuel. SSTO is the ultimate holy grail… but again, notice on the Wikipedia diagrams how much of even this vastly more eficient craft is all fuel tank.
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The trouble with dropping a rocket from an aircraft is just that - you go through all the trouble of giving a craft altitude and speed, then you drop it so it loses some of that just to get separation, to avoid frying the launch vehicle.
Another issue is engine efficiency, for standard rocket engines. IANARocket Scientist, but I recall reading the shape of the rocket engine bell vs. external pressure detemines how efficient the rocket is. An engine that is efficient in the thick atmosphere is less efficient at close to zero atmosphere, and vice versa. This is why 3-stage rockets are a good idea. Each stage’s engines are tuned to the atmospheric pressure it will operate against.
There have been some novel designs, like aerospike, that try to make the same engine fairly efficient at all speeds and pressures.
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There’s a reason why this is called rocket science.
Have no fear Sloth, here in General Questions you are guaranteed a few mostly straight answers to any question before things start to go off the rails. Even in zombie threads. Especially in space zombie threads.
I’m amazed they don’t have more funding considering the current crop of super-billionaires pouring money into private space vehicle development.
Is it because they are British or because there is something fundamentally flawed in their plan?
Rocket (and jet, and any other reaction type ) engines have poor efficiency at low speed. To minimize fuel consumption, you need to get the speed up as quickly as possible. That means you need to get out of the atmosphere, or the densest part of it anyway, as soon as possible. This means you want to go straight up at first. The space shuttle actually had to throttle back during one middle point of the launch because the structure couldn’t handle any higher aerodynamic loading. Once you are above the bulk of the atmosphere, you turn so that you arrive at the desired altitude on an orbital path.
I think what many of us may be missing here is the fundamental difference between power and energy. An object at a given altitude and velocity has a potential energy: the kinetic energy it will release upon falling. More than that much energy must be given to the object in order to get it to that point. Using air foils may require less power but will consume a comparable amount of energy to vertical launch. The large amount of power in a vertical launch appears to be significantly more, but in the end, it may not be. You might be able to build a very tall launch tube (cannon) that could offload a large amount of the fuel (effective payload mass) from the vehicle, but then you are faced with the problem of how to optimize launch acceleration for best effect without destroying the payload. Ideally, you could build a projectile shell with near-perfect aerodynamics to fire the ship up through the densest part of the air where the shell would open and the ship would (hopefully) continue the journey under its own power.
Yes, it’s the secondary effects that require energy.
A ship launched vertically must not only get the kinetic energy of final velocity, it must also balance against the pull of gravity (i.e. energy required to hover) during vertical ascent. It must also overcome air resistance.
An aircraft launch, ie. taking off horizontally, means that forward motion, requiring less initial energy, can create the lift to balance gravity. However, it runs longer, so basically what horizontal lift does is expend about the same amount of energy, but spread over a longer time with weaker engines; possibly more because you are fighting air resistance for a longer time while going horizontally.
Don’t jet engines need air to work? There isn’t any in space.
PS: My husband IS a rocket scientist and I make sure to clean his slide rule weekly 
If he screams at you, pretend you’re space, where they can’t hear him.