First off, I don’t mean to imply that this would be practical let alone possible.
This is merely a question of physics:
Let’s say you took the space shuttle complete with its booster rockets and hooked it up to a giant helium balloon.
The ballon carries the shuttle to about 30 or 40k feet into the air. At this point the rockets are ignited to carry the shuttle the rest of the way into orbit.
My question is: Did this balloon save any amount of fuel at all?
Putting aside the idea of the helium balloon, what you’re talking about is called an air launch, and like everything else, there’s a Wikipedia article about it.
If you disregard the energy spent gathering the helium and then compressing it into tanks (for transport to the launch site and subsequent release into the balloon, at which point the energy that went into compressing it is pissed away), then yes, you do save fuel. The energy to hoist the payload to 40K feet comes from the reduction of gravitational potential energy of the atmosphere as the heavier air is lowered when the helium-filled balloon rises. If you insist on winching the gas-filled balloon back down to earth after launch, then your winch will have to provide exactly that much mechanical work to make it happen.
For an orbital vehicle like the space shuttle, an air launch doesn’t save a whole lot. You can start at 50K feet (10 miles), but you still have to add at least a couple hundred miles of altitude; likewise, even if your mothership is an airplane trucking along at Mach 0.85 (500+ MPH), you need to add another ~17,000 MPH to achieve even low orbit. However, for suborbital vehicles - high-altitude/supersonic research vehicles, or things like Spaceship One - it makes a substantial difference.
If all you want to do is go up and then come back down then it saves quite a bit of fuel.
If you want to go up and stay in orbit though it’s a bit trickier. You don’t go into orbit simply by going up. You have to go fast. Your balloon trick doesn’t help you here. All it does is help you go up. It doesn’t help you go fast at all.
How you orbit depends a lot on your speed. If you aren’t going fast enough, you’ll just drop back to earth. If you go too fast you won’t stay in your current orbit. You’ll climb higher (possibly following an elliptical path). If you want to stay in a perfectly circular orbit, you have to match the orbital velocity for that height. Want to go higher? You need to go faster. Want to go lower? You need to go slower. Want to land? Slow down even more.
What the space shuttle does is it first goes pretty much straight up to get out of the thick part of the atmosphere. Then it slowly turns and goes sideways (tangential to the earth’s surface) while going faster and faster until it reaches orbital velocity. When it wants to land it simply slows down a bit and drops into the atmosphere, and then it uses the friction of the atmosphere to slow itself down the rest of the way.
Your balloon trick saves you a bit of fuel going through the thick atmosphere but it doesn’t help at all at achieving orbital velocity. The extra complexity of an air launch like this usually outweighs the fuel savings, which is why it isn’t done very often for orbital type space launches. For test launches like the X-15 where you just want to get into space and come right back down, an air launch makes a lot more sense.
“Rockoon” is such a cool word!
Even if you take all of this into account, the balloon can still in principle save you energy. Yes, you need the same net amount of useful energy to get the rocket and its payload to that height, no matter how you do it. But rockets are a very inefficient way of using energy. With the winch and balloon, most of the energy you put into the system is going to be doing useful work, but with the rocket, most of it is just making a lot of sound and fury, but signifying nothing.
Balloons can also help with the problem of getting up to orbital speed. Rockets can be made significantly more efficient in terms of energy, but the usual tradeoff is that they get much lower thrusts. This is no good for launching directly from the surface of the Earth, since you absolutely must have more thrust than the weight of your payload or you’re not going anywhere, and so there’s a hard limit on how low a thrust you can accept. If you’ve got something else supporting all or most of the weight of your payload, though, you can get away with using lower-thrust (but higher efficiency) engines.
Below is a comparison from an older post between a comparable ground launch vehicle (the Taurus) and an air launch vehicle (Pegasus), both produced by the same contractor (Orbital Sciences) and using essentially the same upper stack. Basically, the advantage of air launch is in an optimal selection of launch azimuth versus ground launch, where your azimuth and trajectory are governed by the limited number of locations of launch facilities and downrange hazards.Orbital Sciences does this with their commercial Pegasus family of launch vehicles, which are launched from beneath an L-1011. The Pegasus air launch vehicle can be compared directly with a ground launch vehicle, as the OSC Taurus is a Pegasus stack on top of the 92 inch Castor 120 “Stage 0”. For instance, the Taurus 3210 launching from Kwajalein Atoll on the Reagan Test Site can achieve a 200 nmi circular orbit at an 11° inclination with a >1200 kg payload (and larger fairing), while the air launched Pegasus XL launched from RTS can only deliver a payload of about 430 kg to the same orbit. Part of the difference is the level of thrust available from the Castor 120, but the efficiencies gained by air launch from a relatively slow moving aircraft are just not that great. The real advantages are the ability to locate your point of launch to the most efficient azimuth (within the constraints of the range and ground hazards) and increased launch availability (you don’t have to worry about the impact of ground winds and weather conditions on launch constraints, as well as not having to cope with things like ground facility conflicts, environmental impacts (every government-owned space launch range in the continental United States is located in or near a nature preserve or controlled habitat) or protesters.
From a practical standpoint, a helium balloon with sufficient buoyancy to suspend a system the size and mass of the Space Transportation System (the Shuttle Orbiter plus the External Tank and Solid Rocket Boosters) would be logistically horrific to assemble and launch, would impose significant placard constraints on allowable launch conditions, and would require a balloon skin that would exceed the tensile strength of any commercially available textile material, plus the complexity of coming up with a workable lift and deployment rigging system to the balloon. Air launch just isn’t practical for launch vehicles that are significantly larger than something like the X-15 or the Scaled Composites SpaceShipTwo.
Some of the initial Spiral 1 developments for the Space Transportation System (STS, or colloquially “the Shuttle”) included basically paired shuttles, one of which would loft the other into a high trajectory and then return to landing site while the other would continue to orbit. This was eventually deemed to be too complex and costly to even simulate accurately, much less develop and maintain, and NASA eventually went to a separate twin SRM booster system in parallel configuration. Even this turned out to be more complex, aerodynamically, than anticipated and resulted in negative structural margins on the Orbiter aeroloading that required trajectory modification to ameliorate. NASA revisited the concept of completely reusable liquid flyback boosters throughout the late 'Eighties and early 'Nineties, but the development cost and reliability issues associated with liquid boosters turned out to be prohibitive in every study performed versus the relatively simple SRBs, even though they were technically “refurbishable” rather than “reusable”.
Stranger
However, once your thrust exceeds the remaining “buoyant weight” of the vehicle and the speed at which the balloons will naturally rise in the atmosphere, you now have to lift the entire mass of the vehicle and cope with the additional (and very significant) drag of the balloons as well. So really, balloons will only help you for the first few seconds of action time, and shortly after liftoff become more of a hindrance than a help. You also have to make sure that your vehicle can separate from the balloons and clear them so that it doesn’t become entangled.
The amount of gravitational potential energy saved by launching from, say, 30k feet altitude versus average mean sea level (ASML) is almost negligible. The difference in acceleration due to gravity is less than 0.5%. The more significant reduction is in the amount of atmosphere your vehicle has to fly through and a presumed reduction in aerodynamic drag at the point of maximum dynamic pressure (since you are starting higher, you are presumably moving slower at the same altitude where max-Q would occur at a ground level launch, with a corresponding reduction in drag.) Realistically, the tradeoff would not be worth the substantial complexity and cost of operating an airborne platform. Even launching small sounding rockets from large cargo aircraft is significantly more complicated than launching the same class of launch vehicle from a ground facility.
Stranger
How are you comparing thrust to speed?
And I’ve seen proposals where, as the vehicle gains speed, the balloon begins acting as a wing, to continue to provide lift. Supposedly (though I haven’t actually done the calculations myself), this can be carried through progressively higher speeds and altitudes until the atmosphere becomes negligible and you’re in orbit.
I should also emphasize, by the way, that I am not saying that such a system is currently remotely close to practical. I’m just speaking about the energy aspect of it, but there’s a lot more that goes into practicality than just energy.
As soon as the propulsion system develops sufficient thrust to overcome gravity “drag” and aerodynamic drag, the vehicle will start accelerating (typically rapidly, as you are wasting energy just keeping the propellant aloft until you reach orbital speed). So basically, as soon as you start accelerating faster than the buoyancy of the balloons will provide themselves, the balloons become parasitic mass and contribute additional drag.
Well, this would assume a wing-shaped “balloon”. It is certainly feasible to make a frame-supported buoyant cavity of virtually any arbitrary shape, but any deviation from a spheriod (or more typically, ellipsoid) shape is going to make the dead mass to lifting fluid ratio larger, which increases the size of the balloon, which then increases the amount of overall drag for the same form shape, ad nauseam. Aerodynamic lift only helps you if you are flying at some significant angle of attack to the atmosphere; typically with launch vehicles, because you need to achieve orbital speeds that would be hypersonic in the atmosphere, you want to go up as quickly as possible to get above the atmosphere, and then pitch over to get your vehicle to achieve a lateral velocity sufficient that it can remain in orbit (and not intersect the planet on the way back around). The only practical reason for a winged launch vehicle to develop lift during flight would be if it used an air-breathing or combined cycle engine that draws oxidizer from the atmosphere so that you don’t have to carry it along with you for atmospheric portion of the flight.
Unfortunately, the modest weight savings of reducing carried oxidizer is vastly overwhelmed by the additional weight of the lifting structure (at subsonic and transonic speeds) and of course the technological feasibility of a combined cycle engine that can go from turbfan or turbojet all the way through scramjet and pure rocket operation. The SABRE engine notwithstanding (having been in development for the better part of my lifetime with no actual flying model demonstrated) there is no extant or proposed propulsion technology that can really satisfy this requirement. The winged shape of the STS Orbiter was entirely devoted to providing sufficient cross-range during reentry during the few minutes of low sonic flight below 150kft. On the way up, and most of the way down, it was just a heavy, fragile, and problematic design feature that provided no practical benefit throughout the vast majority of the mission operation.
Stranger
Perhaps we could aid the helium and rocket thrust by also striking it in an upward direction in a similar way to lobbing a tennis ball.
In other words, a racket rockoon.
But it looked so COOL!
Seriously, I’ve learned to open any aeronautics or space thread, hoping to see your posts. I’ve learned a ton from you over the years and don’t think I’ve ever said thanks. So thanks! It’s damn nice to have “our” own resident rocket scientist.