Maybe it simply makes sense to start with lots of margin when you don't have the deep experience yet but all evidence I see is that people actually flying LVs find tweaking performance more cost effective than growing simpler boosters which makes me tend towards believing performance optimization probably _is_ cost effective.
One big topic will be reliability. We've seen with SpaceX and others that failures often happen in places you didn't expect them to so being conservative in your design doesn't necessarily prevent them.
No, SpaceX have started exactly where BO is now in terms of attitude towards performance optimization and they have completely turned around.
Quote from: pippin on 03/19/2017 03:29 amOne big topic will be reliability. We've seen with SpaceX and others that failures often happen in places you didn't expect them to so being conservative in your design doesn't necessarily prevent them.The reality does not meet your beliefs here.AMOS 6 failure was directly related to 1) using new materials(carbon fiber) in the helium tanks2) using subcooled propellant.Neither of these two things are conservative.Conservative design is a design which MINIMIZES those unexpected places.
The nice thing about everyone using different approaches is that we'll find out how well they work, or rather for what.
Trouble is it can be very hard to distinguish between the inherent potential of an approach from how well a particular organisation executes it. SpaceX have at least had years and dozens of launches to iterate on theirs. Could be a decade before Blue Origin have had similar time and experience.
Quote from: woods170 on 03/18/2017 08:51 amQuote from: Robotbeat on 03/18/2017 01:29 amQuote from: Bob Shaw on 03/18/2017 01:17 amIt seems strange that BO would consider ignoring SX's hypersonic retro-propulsion experience in even the short term; while not quite being a classic 1960s plug nozzle solution, it seems like a good - and cheap - engineering compromise. Also, I see no hint of SX attempting to patent their gradual, though ferocious, approach.... Because blue origin has been stuck with sub orbital Rockets for so long they don't truly understand the importance of mass fraction. Maybe. I dunno.You really should know better than that. Blue is hiring/has hired real professionals in the field of rocketry. They very much understand the importance of mass fraction.That is an unfair reading of my post. Someone offered a conundrum, and I posited a /possible/ answer. And it's not that they don't understand what mass fraction is. I think the overall industry (well, Boeing and NASA) underestimates the importance of mass fraction, IMHO.SpaceX has an almost obsessive grasp on mass fraction when compared to the rest of the industry. I think they're basically right. While adding more dry mass can reduce delta-V needed for things, you're just better off with very high mass fractions.This is not saying that Blue Origin is stupid, just that I think their prior experience could tend to bias them toward heavier designs. JMHO.
Quote from: Robotbeat on 03/18/2017 01:29 amQuote from: Bob Shaw on 03/18/2017 01:17 amIt seems strange that BO would consider ignoring SX's hypersonic retro-propulsion experience in even the short term; while not quite being a classic 1960s plug nozzle solution, it seems like a good - and cheap - engineering compromise. Also, I see no hint of SX attempting to patent their gradual, though ferocious, approach.... Because blue origin has been stuck with sub orbital Rockets for so long they don't truly understand the importance of mass fraction. Maybe. I dunno.You really should know better than that. Blue is hiring/has hired real professionals in the field of rocketry. They very much understand the importance of mass fraction.
Quote from: Bob Shaw on 03/18/2017 01:17 amIt seems strange that BO would consider ignoring SX's hypersonic retro-propulsion experience in even the short term; while not quite being a classic 1960s plug nozzle solution, it seems like a good - and cheap - engineering compromise. Also, I see no hint of SX attempting to patent their gradual, though ferocious, approach.... Because blue origin has been stuck with sub orbital Rockets for so long they don't truly understand the importance of mass fraction. Maybe. I dunno.
It seems strange that BO would consider ignoring SX's hypersonic retro-propulsion experience in even the short term; while not quite being a classic 1960s plug nozzle solution, it seems like a good - and cheap - engineering compromise. Also, I see no hint of SX attempting to patent their gradual, though ferocious, approach.
Been reading these posts and am not sure if Jim's comments were understood.Nor the basic differences BO/SX that involve them. Here's a try to get the point across.Most effective use of a LV's delta-v will be in staging chosen for the desired trajectory orbit uncompromised.You could lift the vehicle extending its downrange still further. The benefit is to have a longer, stretched out, coast where a large drag coefficient of a larger vehicle passively dissipates kinetic energy. Also, the EI is more gradual, and the structural/thermal design can be differently used in reuse. For different "heating damage".As opposed to a smaller, lighter vehicle limited to a lofted trajectory that buffers a more abrupt EI by use of retropropulsion. With different structural/thermal design and "heating damage" mitigation. Consistent with alternative use as an efficient ELV. Also, since the lofted trajectory doesn't go as far down range, selective RTLS reuse is just another added cost to minimize CONOPs complexity.Both vehicles are competently designed, just chose differently. NG's mass additions are for recovery and to insure stage integrity over reuse, and props/trajectory are to achieve vehicle performance with least given to recovery. F9's mass additions are for recovery, while props/trajectory are limited in addition for vehicle integrity / recovery / downrange....
Quote from: pippin on 03/19/2017 12:56 amNo, SpaceX have started exactly where BO is now in terms of attitude towards performance optimization and they have completely turned around.wrong. SpaceX started practically without ANY performance optimizations, they started with a very weak very low-isp gg engine that was just cheap. And they originally selected the simples possible upper stage engine, going to pressure-fed, only using gg upper stage engine in their second rocket.BO selecting to use staged combustion means they are MUCH HIGHER LEVEL of performance optimizations.
Well, given the size of the vehicle they could probably evaporate and vent some lox and fuel as a heat sink
The basic (X-15) structure is a conventional monocoque design, in which the primary loads are carried in the external skin of the fuselage and wing. The fuselage skin also forms the outer shell of the propellant tanks. Thus, it must withstand the stresses from propellant weight as well as from internal tank pressurization. To absorb heat input, skin thicknesses on the forward fuselage are about three times those near the tail section. Fifteen feet aft from the nose, skin thickness is sized by load, rather than by heating, and is comparable to that of aluminum structure.An important feature of the structural design is that only a small amount of the heat absorbed by the external skin is conducted, or radiated, to the internal structure. Consequently, much of the internal structure of the fuselage is of titanium and aluminum. Extensive use is made of corrugations and beading, to allow for uneven thermal expansion between external skin and internal structure.
A new element was also added to structural design, for with the heat-sink concept, the time of exposure became the critical parameter that established the amount of heat flow into the external structure when exposed to a 2500° F airflow. In areas that carry only small aerodynamic loads, Inconel X can withstand considerably more than 1200° F, perhaps 1600° F. The sharp leading edge on the vertical fin has withstood 1500° F, and one non-load-carrying section of the wing skin has successfully endured 1325° F. These temperatures are experienced for only brief periods of time, however. Prolonged exposure would eventually cause these temperatures to be conducted to load-carrying members, and thus impair the structural integrity of the X-15.The structural design requires a careful balancing between the amount of material required to carry the load and that needed to absorb the heat flow. On a typical flight, the structure near the nose experiences 20 times as much heat input as the aft end. In regions of high heat input -fuselage nose, wing leading edge, tail leading edge- solid bars of Inconel X are required to absorb the heat energy.