The picture worth 1000 words.
Yours truly is a moron…but I can read text and graphs. since I have not been trained in spacecraft engineering and have a bachelor degree in mechanical engineering I always reference my old and trusty Ninth Edition of Mark’s Standard Handbook for Mechanical Engineers. So armed with this information I am looking at the $SpaceX Starship..and I see..
Two sets of surfaces, aft ones which might be rotated around those red lines I had marked on the photo, and the bow surfaces which might have a dual purpose and possibly two axes of rotation. It is kind of obvious that the aft ones are for creating lift, which is force perpendicular to the direction of motion of the Starship in the atmosphere. The bow ones might serve the same purpose and additionally if they can be rotated about secondary axes like surfaces on modern fighter jets they can be used to maneuver the Starship (in the atmosphere upon reentry). It also is possible that the surface can be folded and will not generate lift at all, only some drag (force working against the motion of Starship in the atmosphere).
The angle at which a spacecraft enters the atmosphere is shallow and the drag force slows the craft against air at the same time gravity pulls it down toward the center of the planet but lift force pulls it up from the planet.
The speed with which craft moves through air dissipates the kinetic energy in air friction and backpressure (lower pressure behind your car sucking it back) ultimately resulting in heating air around it and then transferring that heat to the surface of the craft. Not all so dissipated energy goes into heating craft surface.
From the picture, I gather that $SpaceX intends to vary the lift drag ratio of the vehicle in reentry.
Russian Soyuz capsules are shaped almost like balls. These have 0 lift and some drag so lift drag ratio is 0 for Soyuz. What it means for reentry? High dynamic pressure working to decelerate the craft (due to drag) and high stagnation-heat rate compared to craft with lift equal to drag (lift/drag=1). Yet, it is stated on page 11-123;
Decelerations and temperatures are drastically reduced with increasing lift-drag ration (Fig. 11.6.13) and decreasing W/Cd*A (Fig. 11.6.14 and 11.6.15). This effect is particularly beneficial at steeper entry angles. The combination of longer flight times with lower heating rates may, however, result in larger total heat input into the vehicle.
The term W/Cd*A is referred to as ballistic ratio and the designers of Spaceship intend it to be as large as possible so that the kinetic energy to drag force ratio is large, kind of aerodynamic inertia.
The use of stainless steel is a telltale sign that the name of the game is to lower the heating rate. Stainless steel conducts heat at rate 3 to 4 times lower than the aluminum used in aircraft design. The other element is coming into the atmosphere at a steeper angle and deploying the surfaces at different stages of reentry. These are the strategies to lower the heating rate and the amount of heat conducted inside.
The question I have is whether the stainless steel surface is going to be covered in ceramic tiles? The information contained in the graphs is just to inform you about the shape of the solution and does not cover this particular case. One thing which I want to add here. It was reported that the surface of the stainless steel skin was polished after assembly. There is a graph on page 11-123 Fig. 11.6.11 representing the fraction of energy dissipated going into heating the craft on reentry. At about 100,000 ft to 0 ft and at high speeds of reentry, the surface of the craft is among others (convection and conduction) heated by infrared radiation of hot gas enveloping the craft. The polished surface would reflect the infrared radiation lowering the heating of aircraft. There is no other reason to rationalize polishing the surface (save esthetics) and yet Wikipedia article quotes using thermal glass tiles. LOL. Try to figure out what really is going on here!
So Starship weights 120 ton in 200 miles orbit on reentry; this is roughly at the least 3.6 x 10 to power 12 Joules of kinetic energy. To raise the temperature of ~10 tons (1mm thick cladding) of stainless steel skin to the limit of retaining full structural strength (870°C) is needed 3.9 x 10 to power 9 Joules, which is roughly 1000 times less than the energy to be dissipated. There is no way that “bare skin” can do the trick unless the energy will be dissipated by expanding rocket fuel (theoretically). Ergo tiles are a must. (just for laughs, imaging reentry angle of 90°) Nevertheless, a new idea of liquid cooling stainless skin was advanced. (Wikipedia). Let’s look into it. You need to remove halve of that 3.9×10 to power 9 Joules. How much coolant you need to lower skin temperature to 435 °C by rising coolant temperature 200 °C? You need 3800 kg of coolant (ethanol specific heat). That is probably a generational supply of Whiskey. Now add pumps, redundancy, power supply, and installation to direct flow. Meantime your temperature of cladding is 435 °C.
The most astounding thing is that aluminum alloy used in aviation (7075) has 4.5 times the strength to weight ratio of 316 stainless steel. I don’t know the alloy used by $SpaceX so I might be missing here something. This is astounding since the weight of the aircraft cladding could have been smaller by a factor of 4 at the same strength. This additional weight adds to the total energy to be dissipated. The question is: What advantage has stainless steel cladding in reentry at the cost of 4 times structural weight? I do not know how the structure’s strength requirements would change by the added weight of tiles, which anyway have to be added. Wikipedia lists all kinds of new developments put into the design. One of them is cryogenic treatment and cold rolling of the stainless cladding. I doubt that cold rolling alloy can increase its yield strength 4 times without making it brittle, even if it at all would be possible. This cryogenic treatment seems to be full baloney since the cladding is at least now welded together and you are as good as your welds; cryogenic treatment and cold rolling not there anymore in the welds.
In conclusion, this seems to be another $TSLA; somebody tries to be clever by half by making choices and selections of solutions rejected by others on the assumption that if he were “a contrarian engineer” he would beat the system (technological limitations) and win the crown of genius but what is defeating his intentions is lack of attention to details. At this point in the game Starship Mark 1 & 2 are testing beds for ideas that seem to be a gamble. This is not rigorous testing of complex systems but testing ideas whether they hold any water. The steeper reentry angle can only be gained at expanding rocket fuel, which had been previously brought into orbit. So everything comes with some cost, the question is what you buy with that expenditure? I once worked for a company where a strategic decision was made with one almost insignificant factor missing from consideration and vast investment had to be written off. The story was repeated to all newcomers as a lesson. The Starship is a list of revolutionary ideas never seen before but usually given a sentence or a paragraph in Science Fiction literature. Engineering is also about proving that something can not be done now. ;). The answer to that will be known after trials, either “nobody had tried that before for good reasons” or “why nobody had tried that”. As always in engineering, the devil is in the detail.
In the heyday of the Soviet Union, the railway designers faced pressure from management (activists of the communist party) to increase the size of locomotives because the bigger engines could pull larger loads. Yet, the length or weight of the engine was limited by the radius of the tracks on with the previous generation engines traveled without problems. The management deemed any resistance from engineers as a lack of faith in the Soviet Genius and forced its hand. End of story, the engineers were never heard of again.
The calculations I presented are so-called “back of an envelope” numbers setting physical limits on what is possible or required. On Wikipedia, the article on the Starship lists Elon Musk as the designer of the craft. Since that time radical changes were made to the rather conservative design of the Starship. As one writer (The Noble prize winner) was fond of saying; intellectuals (engineers, too) are hired guns for the powerful and mostly doing their bidding (paraphrasing). Sometimes, rewards of a genius (a stickler for truth) can be penury or prison cell. Those who have enough resources (money, political power) can enjoy doing what they want to do and ignore the rest of us.
What is in this for $TSLAQ; Elon does the $SpaceX too.
