I’ve started watching ‘movies of the games’ downloaded from Youtube and some of them are actually good. At the moment I’m watching one of the Metal Gear* game-movies and at one point there is a discussion of a fictional Soviet missile launch system, basically IRBM’s / ICBM’s fired from a tank.
It’s mentioned that to boost the range of the missile they accelerated the launch platform horizontally along a runway with rockets to 300 miles per hour and then launched the missiles which increased their range from 2500 to 6000 miles.
Leaving aside the feasibility of such a system, it’s only a game after all, would accelerating the launch platform have such an effect? I doubt it but what do I know.
*I was surprised at the gentle humour in the series and how Solid Snake is such a goofball at times, I can see why they’re so popular!
Any impulse you can impart to a boost vehicle along its trajectory certainly adds to the lift capacity (or for a ballistic missile, range) just by virtue that it doesn’t have to expend propellant to achieve that amount of momentum, and specifically doesn’t expend propellant just trying to hold itself up against gravity (referred to as “gravity losses” or sometimes as “gravity drag”); the faster a booster can get moving into its trajectory, the smaller those losses are which is why solid propellants are frequently used to increase the boost capacity of a space launch vehicle, and is part of the reason why they are used in compact ballistic missiles even though their mass efficiency (specific impulse) is poor compared to typical liquid propellants. Most of the impulse in a booster is devoted to accelerating the mass of the propellant that it will use along its powered flight, which is why rockets and missiles are mostly propellant by mass and volume.
How much an impulse sufficient to get a booster going to 300 mph would increase the range would depend upon specifics of the booster and trajectory but it would be measurable. However, the complexity of such a system and the losses from aerodynamic drag at near sea level make this impractical. Upon review I see that the system described is accelerating the booster horizontally (i.e. tangent to the Earth); ballistic missiles achieve their range largely by going straight up, and the additional horizontal velocity isn’t going to add that much (as a first order approximation you can just multiple the 300 mph by the duration of the vehicle’s flight—typically ~30 minutes for an ICBM—and see that it doesn’t do much). For an orbital insertion trajectory, that velocity essentially adds to the orbital speed (again, minus aerodynamic losses) so it would be significant; however, it would also increase your max-Q alpha loads (the point of maximum aerodynamic pressure times the angle of attack) because the vehicle would achieve that condition sooner and in a thicker part of the atmosphere, again increasing both aeroloads and the aeroacoustic-driven vibration environment, which is definitely undesirable.
SpinLaunch is actually a real thing. Well, real in that they are testing it. Whether it pans out is a different question.
Basically, they take a rocket and spin it really, really, really fast and then let go and fling it towards space. This fling is not strong enough to get it all the way to space so, part way up, a rocket ignites and pushes it the rest of the way.
I think this meets the criteria the OP is asking about even if this particular method was not in mind. SpinLaunch is basically accelerating the launch platform to add energy to the rocket so the rocket itself can be smaller (and presumably less expensive). If you believe their marketing it also means they could, in theory, repeatedly launch rockets much more quickly than any other system (like SpaceX or NASA).
It also occurred to me our planet is a moving launch platform. There is a reason rockets tend to be launched as near to the equator as possible. The rockets gain momentum from the earth’s spin. The close to the equator, the bigger the boost.
The French didn’t build their rocket base in French Guiana because they like the climate. They put it there to get the most boost from the earth as they could when launching a rocket. The US launches from Florida (Hawaii is closer to the equator but not enough to make it worth shipping rockets out there). The Soviet/Russian launch facility (Baikonur Cosmodrome) is almost as far south in Russia as you can get. There was even a ship you could launch rockets from that sail onto the equator to maximize this benefit (or there were plans for one…not sure it ever got into service).
That’s a good point, and as for the Spinlaunch thing isn’t there a problem that the g-forces created by the spin mean the kind of satellite that could be launched in that way is limited?
I would imagine there are many limitations on this system and it is only usable for some small payloads to get to orbit (assuming they get this all sorted…still being developed). Something like the James Webb Space Telescope would never, ever be shot out of something like that (if nothing else it was far too big but also it is a delicate instrument).
SpinLaunch will not put normal rockets out of business. We’ll still need the big ones. This will nibble at the bottom range of payload sizes if it ever gets fully operational (still a big question mark but they are having an honest try at it).
Playing around with some simulations (read: computer games) over the years, I have noticed that if the missile (rocket) is barely able to lift the payload, any boost of speed early does make a big difference. Think about it this way, if the rocket can just get off the ground, a starting velocity of 200 fps, means that rocket will be making 200 feet every second + the acceleration of the rocket (which is very low) progress in gaining altitude when before it might gain 10 feet or so, so it can get higher much faster. But as the rocket increases thrust this gets diminished, as the rocket quickly attains speeds that makes that initial boost much more insignificant (but still helps). This gives a higher achievable altitude which would equate to a longer range, and I can see the OP’s numbers working out in some situations.
Now for horizontal launching, which I have done far less ‘simulations’ with. with aero effects that can be used to move to the vertical, or at the least allow a steeper vertical accent as it starts with free horizontal speed. But sometimes, with enough power one can launch and ‘fly’ horizontal till the curvature puts one into space. But that’s at the other end of the equation, one must have a very powerful engine to do this which negates much of the advantage of the launch boost (and adds quite a bit of complication). Also engines in the horizontal direction do not fight gravity, so a weaker engine can do more in acceleration. Directly opposing gravity gives the engine 1G penalty that it has to produce just to keep it even.
We had such a proposal at NASA in the 1990s. Basically like the one shown from When World’s Collide but powered by linear electric motors. I believe it was designed more for sounding rockets rather than space exploration.
As I understand it, the main range factors for a warplane, for instance, launching missiles, is the platform’s altitude and speed. The higher the jet is flying and the faster it’s going, the greater the range of its missile, such as an AMRAAM.
Well yeah; it’s not a lot different than why shot-putters are typically tall (Ryan Crouser is 6’7"!). Essentially the higher the shot is when it leaves the shot-putter’s fingers, the farther it’ll go. Assuming that it falls at the same rate, and travels laterally at the same speed, then the higher that it starts, the longer it has to fall, which translates into a longer distance.
Launching missiles from a higher altitude works the same way.
Watching the Apollo launches, I was always surprised at how long it took for the rocket to get moving at any decent speed–it’s burning tremendous amounts of fuel and climbing at a brisk walking pace.
I assume this is partly an optical allusion, kind of like how an oncoming locomotive looks like it is traveling much more slowly than it actually is.
Nevertheless, I have wondered if there would be any benefit of putting some platform in a deep well that could use terrestrial energy sources to heave the rocket up to that walking/running pace, possibly up to height of the top of the gantry, so it could save all of the fuel normally needed to clear the tower.
To be clear, I’m not talking about a cannon that is trying to impart full escape velocity; just a fast elevator that would give the thing a good shove and save on fuel needed to be carried aboard.
Surely such a contraption would be very expensive and difficult to build.
Maybe I’m overestimating the amount of fuel burned before clearing the tower and underestimating the velocity gained. The fact that I have never heard of this being done is evidence enough of it not being very practical.
As I understand it, it wasn’t entirely an optical illusion. At the moment of take off the rocket was as heavy as it was going to be, because it was loaded up with fuel (which was the bulk of its mass). The thrust was only something like 25% more than required to overcome the weight of the rocket, so the take off acceleration was not that large – you experience larger g forces than that while driving.
As more of the fuel mass got burned off, the thrust was a lot larger compared to the remaining mass of the rocket, so the acceleration really picked up.
