Silavite

The second question is probably more open-and-shut. A single P-51 took fewer resources to produce than a single P-38, and the P-51 could do the job of bomber escort just as well as the P-38. That still begets the question of why the P-38's configuration didn't last. To answer that, something of a dive into supercharging is required. (Note: if I say "gear driven supercharger" I mean a centrifugal supercharger driven by the engine driveshaft through gearing.) The reason that the P-38's twin boom design came about in the first place (and why the P-47 is such a beast of an airplane) has to do with the fact that the USAAF bet on turbo-supercharging rather than two-stage supercharging for engine compression. A quick summary of the difference between the two: a turbocharger and supercharger both compress the charge entering the engine, but a turbocharger is driven by an exhaust gas turbine, whereas a supercharger is driven by the engine's driveshaft through gearing. The term "two-stage" simply means that the supercharger has two compressors. The first stage is used for low altitude, and the second stage is engaged at higher altitude. There are also two-speed superchargers, which have a single compressor which can rotate at two different speeds via a change in gear ratio. The two-stage design is more potent, however, since the compression ratio between compressors in series is multiplicative (two compressors in series with a compression ratio of 5 give you a compression ratio of 52, or 25). The term turbo-supercharger is used because the system used by USAAF aircraft has both a driveshaft-driven supercharger and a turbocharger. Here's a schematic of the P-47's system (the gear driven supercharger is 3, and the turbocharger is 13): The gear-driven supercharger is designed to do all of the compression at sea level, while the turbocharger adds its assistance as altitude increases. This retains the advantages of multiplicative compression ratio in a two-stage system without requiring the gear-driven stage to be throttled (more on throttling losses later). This is a NACA report from 1932 which probably influenced the USAAF's decision: The Comparative Performance of Superchargers. In this context, a turbo-supercharger has two advantages over a two-stage centrifugal supercharger: efficiency and throttling. (This report also includes a vane-type and roots-type geared supercharger, but these aren't relevant.) The turbo-supercharger uses exhaust gasses to to drive a turbine which drives the compressor, which is (nearly) a free lunch, energy-wise. There are some losses from increased back pressure on the engine's exhaust (the exhaust gas turbine prevents gasses from flowing smoothly to the free atmospheric pressure), but these losses are smaller than what a gear-driven supercharger takes directly from the engine. Figure 4 shows the comparative power produced from each type for an engine which has 100 horsepower at sea level (not including throttling losses, which we'll get to soon): The turbo has another ace in its hole: it doesn't suffer from throttling losses. In the words of the report, This means that an engine with a gear-driven supercharger will suffer a loss in power below its critical altitude (the critical altitude occurs when the compressor can no longer keep up with the decrease in atmospheric density, so the density of the charge entering the engine begins to fall and engine power begins to drop off). The effects of throttling losses are quite significant, as figure 7 shows: A two-stage geared centrifugal supercharger averts the worst of throttling losses via effectively increasing its critical altitude when the second stage is engaged, but there are still losses, especially in the area between the critical altitude of each stage. The altitude range situated near the midpoint of the critical altitudes of a two-stage or two-speed system is known as the "supercharger gap". A turbocharger (or turbo-supercharger) suffers from no such gap because the turbocharger's compression can be smoothly varied by changing the amount of exhaust which passes through the exhaust gas turbine. (Bonus content: Throttling losses can be averted with a continuously variable supercharger gear ratio. Such a system was used on the German DB 605 via a fluid coupling.) Things are currently sounding pretty grim for the gear-driven supercharger, but the gear-driven system has one gigantic (or, should I say, tiny?) advantage: its size. The gear-driven system is much lighter and requires much less space than an exhaust-gas driven turbocharger. Here's another illustration of the P-47's system: As you can see, the ducting for cooling air (the exhaust gas turbine gets extremely hot, so getting enough air to cool it is quite the engineering challenge on its own) and sundry other equipment required by the turbo-supercharger makes it very difficult to fit into a single-engine fighter. Some attempts are... well, they resulted in airplanes with appearances that only their mothers could love: The impetus behind the P-38's twin-boom design was so that the whole turbo-supercharger assembly could fit in a streamlined package. Even with its twin-boom design, the P-38 still ended up leaving the exhaust gas turbine exposed to the airstream for cooling purposes. The fact that the P-47 managed to have an exhaust gas turbine which is adequately cooled while also being fully enclosed within the fuselage was one of the triumphs of its design. Contrastingly, the compact gear-driven supercharger on the Merlin made it a great fit for the (relatively) small and excellently streamlined design of the P-51. To conclude: The twin-boom design was born out of necessity from the turbo-supercharger system's large size. The twin-boom design wasn't revisited because the P-51 performed well with its gear-driven system and the P-47 performed well with its turbo-supercharger system enclosed within a single fuselage.

I promised a post about the fundamental conservation equations, so here it is. I originally wanted to do derivations, but the amount work and number of images I'd need to attach would be... well... more than my limited patience allows (though writing this whole thing ended up taking about 3 hours anyway... I have a newfound respect for textbook writers and my professors). Instead, I've decided to present the finished product and explain what each term represents. Notes: Variables in bold font are vectors, while scalars use regular font. The equations come from Fundamentals of Aerodynamics, Sixth Edition by John D. Anderson. A control volume is basically a box (or any closed shape one desires, really) in space. Fluid can flow through it, surface forces (like pressure) can act on its walls, and body forces (like gravity) can act on everything inside of it. Internal energy (e) is all of a system's energy which is not macro scale kinetic or potential energy. For example, internal energy includes the energy present from random molecular and atomic motion. Mass Conservation (or Continuity) This equation is represents the conservation of mass, otherwise known as continuity. Each term has the dimensions of mass over time, or MT-1. Going from left to right, each term represents: The rate at which the mass within the control volume is increasing. Density (ρ) is mass per unit volume, the triple integral across a space results in the volume of a space (V), and the time derivative (d/dt) shows how the value (mass) is changing with respect to time. The flow of mass across the boundary (or surface) of the control volume. More specifically, the mass flow rate exiting the control volume. Density (ρ) times velocity (V) times the area of the surface (S) is equal to the mass flow rate across that surface. That said, this relation only holds if the velocity is exactly normal to the surface in question. For this reason, the area is "oriented" in the sense that the actual scalar area is multiplied by the unit vector which is normal to and points outwards from the surface. The dot product between the velocity (V) and the surface (S) ensures that only the velocity normal to the surface (or, alternatively but equivalently, the surface normal to the velocity) is accounted for when the two quantities are multiplied. The fact that the surface normal unit vector points outward has an important consequence on the sign of the term. This means that a velocity vector going into the control volume would be pointing in the opposite direction of the surface normal unit vector, which would result in a negative sign for the term when the control volume's mass is actually increasing. Mass is conserved, so any increase in mass inside the control volume must be exactly equal to the mass which exits that volume across its boundary. Thus, the equation equals zero. It is frequently practical (and convenient) to assume that the control volume in question is at a steady state, such that no quantities vary with respect to time. By definition, the time derivative (d/dt) of a quantity which does not vary with time will be zero. Thus, for a steady state, the equation for mass conservation is: Momentum Conservation This equation represents the conservation of momentum. It's important to note that momentum is a vector quantity, so this equation may be divided into three scalar components. In addition, from Newton's second law, force is equal to the change in momentum over time. Each term has the dimension of force times mass over time, or MLT-2. Going from left to right, each term represents: The rate at which the net momentum of the control volume is increasing. This term looks quite similar to the first term of the mass conservation equation, but since momentum is mass times velocity, it is multiplied by the velocity vector (V). The flow of momentum across the boundary (or surface) of the control volume. More specifically, the momentum flow rate exiting the control volume. Again, this term looks quite similar to the second term of the mass conservation equation, but since momentum is mass times velocity, it is multiplied by the velocity vector (V). The rate at which pressure adds momentum to the control volume. Pressure is force per unit area, hence why it is multiplied by the area and surface normal unit vector (S) of the surface to find the force applied. The rate at which body forces (such as gravity) add momentum to the control volume. The f represents the body force per unit mass. Field forces like gravity and magnetism have the ability to act on the entire control volume body, hence the triple (volume) integral. Since f is per unit mass, it is multiplied by density (ρ) and volume (V). Any viscous forces which may act on the control volume. Viscous forces are... complicated, so any possible viscous forces are represented by a simple (if rather nebulous) Fviscous. For most aerodynamic applications, the effects of gravity are small (and it is unlikely that other body forces will be acting on the control volume. I suppose plasma could have electromagnetic interactions, but that's getting a little ahead of things), so term 4 can usually be dropped. Viscous forces are vital within the boundary layer, but can be neglected in the free stream, so term 5 can be dropped unless inside the boundary layer. Finally, as with mass conservation, it is frequently practical (and convenient) to assume that the control volume in question is at a steady state, so term 1 may be dropped. Thus, for a situation with no body forces, inviscid flow, and at steady state, the equation for momentum conservation is: Additionally, for low-speeds wherein density is constant (usually considered to be speeds below Mach 0.3), it is possible to fully analyze flows using only mass conservation and momentum conservation. Density cannot be considered to be constant once speeds increase beyond that point, so we must turn to... Energy Conservation This equation represents the conservation of energy (equivalently, the first law of thermodynamics). This one looks rather imposing, but it's really not that bad. Each term has the dimensions of energy over time, or ML2T-3. Going from left to right, each term represents: The rate at which energy is added to the control volume as a result of heat from a body source. This could be from radiation or a chemical reaction within the control volume. Mathematically, q_dot is heat addition per unit mass per unit time, which then becomes heat addition per unit time after being multiplied by the density (ρ) and volume (V). The rate at which energy is added to the control volume due to heat from viscous effects. Viscous effects are, again, complicated, so it's convenient just to leave contributions by viscous effects in this simple state. The rate at which pressure adds energy to the control volume. Pressure (p) times area (S) is force, and force times velocity (V) is power, or energy per unit time. The dot product ensures that the pressure gradient force is actually acting to accelerate the flow, rather than simply changing its direction. The rate at which energy is added to the control volume due to body forces. Similar to term 4 of the momentum equation, but multiplied by velocity (V) since force times velocity is power, or energy per unit time. The rate at which energy is added to the control volume due to work done by viscous effects. Viscous effects are, once again, complicated, so it's convenient just to leave contributions by viscous effects in this simple state. (This term seems a bit... strange to me, personally, since viscous effects act to turn organized motion into random motion over small scales. There's likely something here that I'm not understanding, or perhaps it's present simply for completeness.) The rate at which the net energy of the control volume is increasing. Conceptually, this is similar to term 1 of the mass and momentum equations. Within the parenthesis are internal energy per unit mass (e) and the macroscopic/organized kinetic energy per unit mass (V^2 / 2). The net energy per unit mass is then multiplied by density (ρ) and volume (V) to obtain the net change in energy per unit time of the control volume. The flow of energy across the boundary (or surface) of the control volume. More specifically, the energy flow rate exiting the control volume. Conceptually, this is similar to term 2 of the mass and momentum equations. Within the parenthesis are internal energy per unit mass (e) and the macroscopic/organized kinetic energy per unit mass (V^2 / 2). For most aerodynamic applications (outside of a combustion chamber, of course), body heating is negligible, so the flow may be assumed to be adiabatic and term 1 can be dropped. As was said with momentum: viscous effects are important within the boundary layer, but can be neglected in the free stream, so terms 2 and 5 can be dropped unless inside the boundary layer. For most aerodynamic applications, the effects of body forces are typically small, so term 4 can usually be dropped. Finally, as with mass and momentum conservation, it is frequently practical (and convenient) to assume that the control volume in question is at a steady state, so term 6 may be dropped. Thus, for a situation which is adiabatic, inviscid, without body forces, and at steady state, the equation for energy conservation is: (After waking up this morning, I realized that I should write a little more about the simplified versions of these equations which result from applying them to streamtubes. Stay tuned!)

A little extra about the geometry of oblique shocks from NACA Report 1135: Wherein, P is pressure ρ is density T is temperature a is the local speed of sound M is the local Mach number s is the mass-specific entropy V is the resultant/total velocity u is the component of velocity normal to the shock v is the component of velocity parallel to the shock You can see from (115) that (117) reduces to v1 = v2. As the v-component of velocity is conserved, its energy is conserved as well. (I'm thinking that I should probably write a post later going over the basic equations of mass, momentum, and energy conservation for fluid flows, since I basically just dropped these equations out of thin air, if you'll pardon the pun )

I'd imagine that laminar flow might be a little easier for this project since: It's a small aircraft The wing looks like it has a pretty high aspect ratio (so low chord) The lack of sweep also keeps the effective chord seen by the relative wind short All of these keep the length scale, and thus Reynolds number, down across the fuselage and wing chord, respectively... so perhaps it's possible? Still, the boundary layer is a fickle beast, and easily tripped by even small imperfections (icing conditions would likely be problematic). The manufacturer is claiming, "cruise speeds of up to 460 mph," (which corresponds to Mach 0.67 assuming an outside air temperature of -40 deg C at altitude) on an engine which puts out 500 - 600 bhp with turbocharging (so I'm assuming that the engine power is flat rated up to cruising altitude). Even with beautiful aerodynamics, 460 mph seems difficult to believe with those power numbers... maybe I'm just too used to WWII fighter aircraft behemoths which need ~1,800 bhp to reach such a speed.

Rockets already have some trouble making use of hydrogen's high energy density due to the reduced mass ratios inherent in hydrogen tankage. One could argue that volumetric density is even more important for aircraft than spacecraft because aircraft spend all their time fighting drag, and such large fuel tanks would add substantially to an aircraft's wetted area (hydrogen would lower both mass ratio and L/D). Hydrogen also leaks profusely and can cause embrittlement in certain metals. I'm certainly no expert in this area, but I think that a hydrocarbon based biofuel is a better bet than hydrogen for aircraft... though I wouldn't mind being proven wrong! Flying on LH2 sounds very cool (pun intended ).

Another question, since I seem to be in that kind of mood tonight. The vast majority of aircraft with wing mounted engines have the engines mounted below the wing. Why not mount engines above the wing? It would reduce noise, reduce the risk of FOD, and allow for a smaller undercarriage. The biggest disadvantage to me seems like increased difficulty of maintenance, but the advantages could be worthwhile for an aircraft operating out of minimally prepared areas (such that undercarriage sturdiness and resistance to FOD would be at a premium). Some examples of designs with an above wing engine mount: HA-420 HondaJet VFW-Fokker 614 I'm sure I missed some factors here, so have at it.

Ah, that's a good point. What about the fan at the front? I'd imagine that air bypassing the core would benefit from a variable pitch fan.

I understand that the vast majority (all?) of gas turbine engines today use fixed pitch compressor/turbine blades. Would it be practical to make a variable pitch system?

You'd be surprised at the lack of such information in the basic aerodynamic textbook my university uses (Fundamentals of Aerodynamics by Anderson), though my understanding of what a "basic aerodynamic textbook" entails may be off. What other aerodynamics books would you recommend? Thanks for the suggestion on NACA (I never cease to be amazed by the breadth and depth of research they did). This looks like a good start.

What literature and rules-of-thumb are available for fuselage shapes, wing fairings, and general streamlining?

Recording of the webinar is available:

There's probably been confirmation of this elsewhere, but this was confirmed in a webinar I attended. The other highlights included: Vulcan launches need to be at most 14 - 20 days apart to mitigate boiloff on the Centaur The unmanned test flight will demonstrate in-orbit refueling That said, I'm disappointed that something like a third of the webinar was composed of recordings, and another third was composed of not-terribly-interesting discussion (I swear that they talked about the importance of internationalism in space at least 3 times). Edit: Ninja'd by @tater, it seems! Yeah, there were a bunch of greyed out questions, unfortunately, and a lot of the questions which the moderator did choose to ask weren't very interesting. (My own was about the potential of using the lander with ACES down the road.) There was one question which asked, "Could a full mission (Orion + Lander) be launched on an SLS 1B plus a single Vulcan launch?" The moderator basically shrugged and nobody in the panel replied.

The NAM should never be used to forecast tropical systems. It's ill-suited to the task.

Perhaps I'm misunderstanding the question, but I think that the dynamics of a world wherein all bodies are cylinders (whose the length approaches infinity) would simply collapse into the dynamics of a 2-dimensional system wherein all objects are circular.

I wonder if it's still power limited at 12 light hours (86.6 AU)? The Earth's rotation would return it to roughly the same point in the sky. I dunno what you'd want to look at so far away, though... trans-Neptunian objects, maybe?

Mike Griffin joins board of Rocket Lab. https://spacenews.com/mike-griffin-joins-board-of-rocket-lab/

This is just my opinion, but it seems to me that all the discussion about the AN being wet, decayed, partially stolen, or not detonated seems to be rather beside the point right now. There is some circumstantial evidence for these, but trying to pin down the energy released in the blast by these (circumstantial) factors seems to be of questionable utility. The only piece of direct prior evidence is that the freighter offloaded 2,750 metric tons of AN. On the other hand, we have sundry imagery of the blast itself, the crater, and damage the blast caused. We have seismic data which may be compared to other explosions. Rather than trying to estimate the energy by factors from prior to the blast, it seems better to look at factors which came about during/after the blast.

I've said it before, and I'll say it again: SpaceX needs to take the threat of tropical cyclones more seriously in Boca Chica. There is certainly precedent for a severe hurricane directly impacting the area. I don't know the exact ASL elevation of the Boca Chica facility, but it seems likely to me that a storm surge of 18 - 20 ft would totally inundate the facility. And that's without getting into wind damage. Major hurricanes don't happen often, certainly. The hurricane return period for the area within a 50 nmi radius of Boca Chica is 13 years, and the major hurricane return period is 30 years. Those are averages, however, and there is a very real risk that a storm could strike the area in the near future. How long will it be until the area receives another hit? 5, 20, 50 years? Later this season? SpaceX may get lucky, but relying on luck is a dangerous game to play.

D'oh! Looks like I totally misread that. Apologies to all.

This may have been posted already, but these are the amounts awarded to total values of each HLS bid contract: SpaceX: $2.25B Dynetics: $5.27B National Team: $10.18B

If Glushko were still alive he'd probably say something like, "Only MMH and NTO? No pentaborane? And what's this; the booster uses kerolox?? This is far too pedestrian..."

All it needs is a fuel shift from methane/LOX to ethanol/LOX.

(Yes, yes... I know that Tater mentions asteroid mining at the end, but pretend you didn't see that!)