I think everyone who talks about reusable rockets is primarily inspired by airplanes. These winged machines fly actively, are very reliable and have a colossal resource. And unlike rockets, you can easily buy a ticket and fly wherever you go. Therefore, many science fiction writers, cinematographers and design engineers draw an airplane or a rocket with a wing as a reusable rocket, where at least the first stage lands on the cosmodrome along the airplane. This is a classic approach where everyone tries to build on what has been achieved to the maximum. Let's try to figure out what the missiles need to return safe and sound.
Entering the atmosphere
In order for the launch vehicle to put the satellite into orbit, the satellite (and the last stage) must be told a speed in the region of 7800 m / s. To understand this, the order of the numbers is needed, not the exact values. At the same time, the first stage, depending on the configuration of the launch vehicle, develops a speed in the region of 1600-3800 m / s. So, when returning to Earth, the rocket unit enters the atmosphere with almost the same speed with which it separated. This can be said to be our initial conditions. When entering the atmosphere, the rocket unit experiences atmospheric resistance, which causes mechanical stress and heating. Mechanical loads (high-speed head) are proportional to the square of the speed, and heating (heat flow) to the cube of the speed. In this case, both the velocity head and the heat flux are directly proportional to the density of the atmosphere. These are the most important relationshipswhich determine the way to enter the atmosphere and fly in it. And if a simple approach is used to compensate for the increasing loads - an increase in the mass of the structure, then the increasing heat fluxes cannot be compensated in this way. The heat fluxes that a structure can perceive per unit of time are uniquely determined by the used material of the structure or its outer surface. At high speeds, conventional materials of construction simply melt. But they found a way out of this situation. For example, ablation thermal protection is actively used for descent and re-entry spacecraft.are uniquely determined by the used material of the structure or its external surface. At high speeds, conventional materials of construction simply melt. But they found a way out of this situation. For example, ablation thermal protection is actively used for descent and re-entry spacecraft.are uniquely determined by the used material of the structure or its external surface. At high speeds, conventional materials of construction simply melt. But they found a way out of this situation. For example, ablation thermal protection is actively used for descent and re-entry spacecraft.
The Soyuz-type descent vehicle after landing
Apollo command module after landing
The pictures show that the heat shield is burnt and carried away. These are its main properties - to accumulate energy and be carried away. It is very similar to water, which, due to boiling and evaporation, maintains a strictly defined temperature. But this is not a reusable technology at all. Such heat-shielding materials are very expensive, weigh a lot and need to be restored or changed after each flight. VA TKS even worked out the technology of restoration of ablative thermal protection after its "evaporation". But this technology turned out to be quite expensive and for a number of reasons they did not go further.
In the USA, for the Space Shuttle, and later in the USSR, for the Buran spacecraft, carbon-carbon and silicon heat-shielding materials were developed, which were supposed to ensure low weight and reusability.
Heat-shielding coating of the "Buran" spacecraft
This became possible with the use of a glider shape for vehicles. Due to the large surface area per unit weight, the vehicle extinguished part of the velocity in the rarefied layers of the atmosphere and entered the dense layers at lower velocities. And using the aerodynamic quality when entering the atmosphere, the device converted its vertical speed into horizontal speed and due to this it gradually reduced its height. Thanks to these two factors, it was possible to reduce heat fluxes per unit surface, which, coupled with radiation heat radiation into the surrounding space, made it possible to use these materials. In addition to this, the gliding descent made it possible to reduce the mechanical loads and overloads experienced by the glider. In laboratories, carbon-carbon and quartz tiles have shown excellent results.They effectively contained the required heat fluxes and actively radiated heat back. Due to the low thermal conductivity coefficient, the spacecraft structure did not heat up above the permissible limits and retained its strength properties. But in reality, the materials used were very demanding to comply with the technological processes of their manufacture and application (gluing). The most important problem was the fragility of materials, which was not in any way evaluated in mathematical models during design. For example, quartz tiles were easily pushed through with a finger. Carbon-carbon tiles were easily chipped at the edges. Also, when flying in dense layers of the atmosphere, quartz tiles received a significant erosive effect from dust particles, which required subsequent restoration.Some of the tiles simply fell off during operation. All this has led to the fact that this heat-shielding coating in operation has become much more expensive than ablative types of heat shields. Well, everyone probably remembers the disaster of the Shuttle Columbia, which occurred on February 1, 2003 due to damage to the thermal protection. After the first (or last) flight, the spacecraft "Buran" also had a serious burnout of the heat-protective coating, which fortunately was not so critical.
How, then, to get around the problem of thermal heating? And here again it is necessary to remember that heat fluxes are proportional to the cube velocity. As I wrote above, the speed of the first stage can be three times less than that of the last stage. This means that the rocket block of the first stage accelerator upon entering the atmosphere can heat up 27 times less intensely than the block descending at orbital speed. That is, we need to reduce the speed of an object that enters the atmosphere. Unfortunately, due to the aerodynamic shape or aerodynamic effects, it will not be possible to reduce the speed so radically. It is necessary either to slow down, or simply not to gain as much speed as the first stage does. Calculations have shown that if the gliding vehicle develops a speed of up to 2500 m / s, then it does not experience that significant heating,which requires the use of special heat-shielding materials. In this case, titanium alloys must be used in the fairings of the wing, on the edges and in all heat-stressed places.
Subsequent blowing of the elaborated devices recommended to reduce the speed even more significantly or choose an aerodynamic shape that reduces the heat stress of edges, fairings and similar places. For classical rocket blocks, the value of this speed is even lower, since it plunges very intensively into the dense layers of the atmosphere. Based on the results of calculations and real flights, it turned out that the rocket unit does not require special protection at entry speeds in the region of 1200 m / s. At speeds in the region of 1400 m / s, local application of special refractory materials or thermal protection is required. Here we see that the required reduction in the speed of classic rocket units is very significant and it is extremely inefficient to separate the rocket units at such a flight speed. So what's the way out? And very simple - to brake the engines before entering the atmosphere,to provide an entry speed in the region of 1200-1400 m / s. The whole question is in the difference between the speeds of separation and entry into the atmosphere. The need for fuel for such deceleration can be quite accurately estimated using the Tsiolkovsky formula, adding gravitational losses for the deceleration time.
Soft landing
Here we have briefly reviewed the problem of re-entry into the atmosphere for reusable rocket units. And now, briefly about the issues of soft landing, which will keep the already not overheated structure intact and safe. Let's start with the winged structure again. There is probably no need to explain much. Everyone must have seen the planes land. Here is a similar scheme, but with one caveat. Since such vehicles are not airplanes, their horizontal landing speed is quite high, which requires long, high quality landing strips. On an ordinary lane, like in Sheremetyevo, such a device is likely to crash. I think we've sorted out the winged vehicles.
But what about classic rocket blocks? It is necessary to make sure that the structure is not damaged during landing. You can gently lower the rocket unit into the water, either with the help of parachutes, or by braking the engines.
Floating booster of the first stage Falcon-9
This option of landing in water seems to be good for everyone. But there are a couple of problems and practically unsolvable tasks. Not all launch vehicles have flight paths in the areas where the blocks fall above the water. For example, when launching from the Baikonur cosmodrome, such a thing cannot be done at all, from the Vostochny cosmodrome it is extremely problematic. When in contact with sea water, many alloys and materials begin to break down rather quickly. By itself, water can disrupt the functionality of many mechanical and electronic systems. There is a problem of block drying and cleaning of salt deposits. On contact with water, hot structural elements are susceptible to cracking and an overhardening effect. And in the end, pitching adds off-design loads. Considering all these factors, landing on water is usually not considered by specialists. And if it is considered,they quickly abandon this idea. It remains to land the rocket unit either on land or on an offshore platform.
The platform adds problems with pitching and drifting. But effective stabilization systems make the platform for the missile unit practically dry land. Although the development of such stabilization systems is an additional, but quite solvable task.
SpaceX Offshore Landing Platform
Next, you need to decide on the landing method. Usually the first thing offered is a parachute. He is familiar to everyone, kind of understandable and familiar. The parachute allows, with its acceptable areas and masses, to reduce the descent speed to about 8-12 m / s. But he will not be able to make a soft landing. This requires additional brake motors and shock absorbers. Only shock absorbers can be used. If we want to land a rocket unit with an overload of 2g only with the help of shock absorbers and at a speed of 8 m / s, then ideally a shock absorber stroke of 1.63 meters is required. The required shock travel is proportional to the square of the sink rate and is inversely proportional to the overload. By the way, the formula for the calculation is easily derived from the energy conservation law. It is only necessary to equate the kinetic energy with the potential. But let's continue with parachutes.The parachute has one bad property.
A classic canopy parachute will not provide accurate landing. The platform with it is useless, and on the ground the rocket will land on a hummock or in a forest. To maintain the integrity of the missile unit, it must land evenly on all supports, either vertically or sideways. After that, he should not roll, fall or roll. This will not work on an unprepared and unleveled site. Many remember how the imperfection of SpaceX's barge stabilization algorithms led to the subsequent fall of the rocket unit. It will be the same on a curved surface. Even when stacked sideways, the missile unit on a curved platform will simply break, as it was during tests of the side units of the Energia LV.
Scheme of the return of the side unit of the LV "Energia" (http://www.buran.ru)
Tests of dropping blocks showed that during landing they received damage that did not imply their subsequent use. It didn't even come to testing the rest of the flight stages.
Knowing this, the developers began to actively offer guided wing parachutes, which theoretically allow you to lower the load to the exact location. But such developments run up against the imperfection of control algorithms under rapidly changing environmental conditions (wind, gust, etc.). Now SpaceXare actively testing this technology for lowering the fairing flaps. In addition to a guided parachute, they use a ship with a huge net, which constantly moves in an attempt to catch the sash. Until recently, the results were not particularly positive, but not hopeless either. And more recently, the fairing flaps are increasingly caught in the net.
SpaceX ship for catching the fairing flaps
To solve the problem of soft landing of the parachuting rocket unit, my colleagues, S.V. Antonenko and S.A. Belavsky, a helicopter pickup of the parachuting rocket unit was proposed.
Scheme of helicopter pickup of the rocket unit
The advantage of this scheme is that you do not need to think about a prepared site and do not need to spend additional mass on landing devices (shock absorbers). In addition, the scheme for picking up parachuting objects in the world is well developed and does not raise any big questions. Offshore platforms can be used if pickup is required at sea. The limitation of this scheme is the mass of the rocket unit and the carrying capacity of the helicopter. Thus, the world's largest helicopter Mi-26 will be able to pick up no more than 16 tons. The missiles of the Angara family have a rocket unit weighing in the region of 11 tons, while the rocket unit of the Falcon-9 launch vehicle already weighs in the region of 23 tons.
I think we're done with parachutes. How can you do without parachutes? For this, engines can be used that will decelerate the rocket unit before landing to speeds of the order of 1-2 m / s. It is more difficult to land more precisely, but in the future I think we can talk about 0.5 m / s and below. The last crumbs should be damped by small shock absorbers. It should be borne in mind that this scheme requires landing on a prepared site and the correct orientation of the rocket unit when issuing a braking impulse. That is, we need controls and stabilization. At this stage in the development of technology, such control systems do not represent any particular problems. Algorithms for control, guidance and landing are also amenable to creation and development. And the controls in the form of gas jet engines and aerodynamic rudders are already becoming classics.Landing shock absorbers are also quite well developed today and worked out in at least two versions, fromSpaceX and Blue Origin . Also, with this method of landing, there are tasks of damping the horizontal components of velocity and angular velocities. But this is also all solvable and even worked out well.
Landing of Falcon Heavy LV side blocks
We see that such a landing (landing) scheme is already well developed and does not conceal unsolvable problems.
Not anywhere
It's probably all about landing methods. But how do you find yourself in a given area or on a prepared site? Gliding type aircraft with a wing, due to the aerodynamic quality, as I wrote earlier, convert the vertical speed into the horizontal one quite well. Therefore, they often reach the landing strip on their own. And if the flight range is not enough, then additional air-jet aircraft engines are used.
Rocket blocks of classical schemes have little opportunity to adjust the range by installing aerodynamic rudders. They can also make range adjustments when braking impulses are used to reduce heat fluxes. But often such ranges may not be enough. Let's look at the most logistically attractive scheme, when the rocket units return to the cosmodrome and they do not need to be additionally transported over significant distances. So, to implement the scheme with a return to the starting point, after the separation of the rocket unit, an additional activation of the rocket engine is used. In this case, the engine is oriented so as to simultaneously reduce the flight speed and set the return speed to the landing site.
The main advantage of such a corrective impulse is that after it, the missile unit makes the main range adjustment while moving practically in airless space. Such an impulse can be used not only for returning to the cosmodrome, but also for landing on almost any site.
Flight scheme of Falcon-9
For rocket units with parachutes, it is also possible to use combinations of corrective and braking impulses of rocket engines, among other things, as well as control of aerodynamic rudders. But it should be borne in mind that the parachute will still gain a random error up to several kilometers during its operation. I wrote about a controlled parachute wing.
Conclusion
So I examined all the stages of the flight of reusable rocket units and tried to explain in an accessible way what and why should be done at these stages so that the reusable rocket unit returned safe and sound. In reality, of course, there are several orders of magnitude more questions and nuances, but the questions I have considered are the main and decisive ones for the future scheme of a reusable rocket unit. Let's summarize the schemes for the implementation of reusable rocket blocks. The main ones in my opinion are:
- Winged block with horizontal aircraft landing.
- Rocket dynamic landing.
- Helicopter pickup of parachuting rocket units.
These are the most implemented and developed schemes, but you can combine your own scheme based on personal preference. But after that, the new scheme must be carefully calculated in order to be sure that it is realizable and you will not run into unsolvable problems. I'll make a reservation right away that each of the schemes has its own nuances and limits of realizability. Each has its own losses in payload mass, losses in cost and complexity of solving problems. But more on that another time.