diluther said:
I hope to provoke a little more evaluation before anyone nails down exact numbers. I hope that horizontal launch may remain in consideration, and offered one possible way to shave off some of the drag and mass. I expect to see more propulsion technologies coming on line, and welcome proposals that may use an in-line configuration. This initial representation suggests using rockets to pass the altitude limitations of air breathing systems alone. The upper stage may have a less demanding mission if we can optimize both propulsion methods in the booster. I hope our model will establish basic handling of the configuration so it may be reasonable to consider for future use.
I think that's one of the biggest missunderstandings here is that the project is very early in flight testing and you don't have the entire system/concept completely fleshed out and ready to roll yet
I would like to see the aircraft use a rail launch partly to avoid the issues Concorde suffered from rubber tires. Rails control the craft in cross winds and may lighten the pilot workload. Vehicles full of rocket fuel may need more control than present aircraft, and rails may provide a good way to brake in case of an abort decision. Perhaps they won’t need maglev or elaborate boost energy if they only offer some additional control and abort potential.
I'd suggest NOT having any "launch-assist" ground system if you can help it. They add complexity and cost which detracts for the operations-ability of the system. Vehicles "half-full" of rocket fuel* do thousands of take-offs and landings every day, I don't see "control" as an issue
*= Kerosene is "technically" the "fuel" half of a number of rocket launch vehicles today and thousands of aircraft take off and land with similar amounts of aviation kerosene on-board all the time
Stratolaunch and Xcor both indicate conventional aircraft as a first stage, and I expect they have good reason for doing so. I would never propose using an airbreathing first stage. Air breathing stages also “almost never” pass trade studies. It sounds like they might have passed some trade studies in the past though. So I hope they remain an option for future studies, as new ideas may make them worthy in the future.
(You missed a couple as both Virgin Galactic and Boeing have both "suggested" air-breathing carrier vehicles for rocket orbital stages as well
)
May I suggest some research?
A LOT depends on what the assumptions and requirements are for any one design "trade" study. I'd suggest taking in as many various previous works as possible so you can evaluate the varius pros and cons along with the assumptions and requirements of those studies.
Unfortunatly a LOT of them, and most of the really good ones, were/are on the NASA Technical Report Server which is currently off-line for an unknown duration.
(This includes one of the "best," which if you can't find any other way let me know and I'll try and post my copy from NRTS here, is the "CROSSBOW Air Launch Trade Space" study. It is mentioned in passing in this article:
http://www.nasaspaceflight.com/2012/07/commercial-shows-reignited-interest-air-launch-system/ )
I can point to some sites for your information though,
A good basic referrence paper is this one; "A Study of Air Launch Methods for RLVs"
http://mae.ucdavis.edu/faculty/sarigul/aiaa2001-4619.pdf
AirLaunch LLc though they used a rather "normal" rocket dropped from the back of a C17 has some good papers on Air Launch and their studies here:
http://airlaunchllc.com/TechPapers.html
Searching the "Responsive Space" website for past conference papers is also helpful:
http://www.responsivespace.com/
I'd also suggest extensive web-searches for various "keyword" combinations, (such as "Air Launch Launch Vehicle Studies") for information, website, and reports.
Vertical launch is indeed a great way to get to orbit. At least you can deliver the upper stages to orbit, and even recover them. The booster is not often recovered since the shuttle was retired. Elon Musk and Buzz Aldrin have suggested that we need to bring these boosters back to use again. Our economy may be making the same suggestion now. Vertical landing is possible, but restarting 27 engines may be a statistical reliability challenge over many flights. Armadillo Aerospace reported a power loss as an “Interesting data point”. I don’t want to be data on a seismograph. If I lose propulsion I like having a wing and a prayer available. It worked for the shuttle and still does for the X-37.
To get several things "straight" from the above section, let me start with making sure we all understand that booster recovery, or non-recovery is more an economics issue than technical. With the current flight rate of launch vehicles the lower performance penalty has not favored booster recovery. It is cheaper to "expend" the rocket stage and build a new one than it is to recover and refurbish them. You also need to know that at no point has anyone suggested a "need" to restart more than one or two engines for a vertical landing rocket vehicle. What requires "27-engines" to lift off the pad comes back light enough to only need a single engine (per booster) for a fully propulsive landing.
As for an engine-out crash of a VTVL lander being "an interesting data point" and wanting "wings" to avoid this I'd point out that no rocket vertical landing vehicle was required to bring into general the use the terms "controlled-flight-into-terrain," "ditching" or "failure-to-achieve-adequate-flying-conditions"
Wings in fact are not needed as exampled by the HL-10, X-24, and X-24B among others
(My own "prefered" LV design is based on a Lenticular, or "saucer" shaped vehicle in fact
)
Something to keep in mind anyway
Orionblamblam said:
diluther said:
Orionblamblam:"Xcor proposes a single-stage pure rocket series of spaceplanes."
Current information about Xcor orbital vehicle design plans: "Orbital system would use an existing aircraft (not custom-built); both rocket-powered stages would be reusable."
http://www.parabolicarc.com/2013/04/23/jeff-greason-updates-lynx-status/
OK. Note that in using a conventional aircraft, the Xcor system would not be relying upon it for much in the way or either altitude or delta v; carrier aircraft are mostly a way to avoid ground launch infrastructure and to allow you to select launch latitude.
Actually the CROSSBOW study goes into more detail about what "advantages" you can gain from Air Launch. Technically there are three (3) types of "gain" that you can achieve from Air Launch though typically most systems only get a large percentage on one (1) of them.
Note these are NOT in "order" as far as I know,
1) Speed. You can gain speed with some air-launch concepts that directly relates to the LV total delta-V requirement to get into orbit. In most concepts where this is applicalbe the booster/carrier has to be capable of high-supersonic (Mach-4+) or hypersonic (Mach-5+) speeds in order for there to be appricable gain involved. Typically a 'booster' that is staging an orbital vehicle would be used to reach speeds between Mach-6 to Mach-10 or better. This is normally VERY expensive and requires a specifically designed and built airframe, along with a high speed propulsion system.
2) Altitude. Rocket engines can gain efficiency when they are "optimized" for specific altitudes. A nozzle used at sea-level is different than one that operates at high altitude and it saves complexity if a rocket motor does not have to operates at large differences in pressure. At around 35,000ft altitude a rocket nozzle designed to operate in vacuum has a lot more efficent operations than at sea-level. Typically "air-breathing engines depending on the type and how they operate can be utlized to a maxium altitude between 35,000ft and 70,000ft. Going higher requires extensive modifications and operating parameters which again leads to this being expensive. Also typically at very high altitude a rocket will supposedly be able to put more power into accellerating "side-ways" (building up delta-V velocity towards orbital speed) than in holding itself aloft.
3) Angle-Above-The-Horizon. The "typical" Air Launch carrier vehicle isn't fast enough nor does it fly high enough to really take advantage of those factors. In most cases as Orionblamblam mentiones, air launch is simply used to overcome other factors. So typically as in the case of Pegasus, StatoLaunch, or SpaceShip-2 the rocket vehices themselves are technically more "rocket-plane" than launch vehicle. This in that they have to "drop" from the carrier vehicle and then combine propulsive thrust and aerodynamic lift to achieve "flight" and then aerodynamically turn to begin the actual flight trajectory. This wastes a great deal of "energy" and detracts from the LV final delta-V total.
Typically an LV will waste less energy the closer to "vertical" (actual "optimum" is around 70 degrees) it is to the local "horizon" when it is launched. This is why T-Space and AirLaunch LLC developed such systems and the "T-LAD" harness and the "gravity-drop" method to align their launch vehicles. However since these methods pretty much preclude fireing the rocket engines until the LV has left the carrier aircraft and dropped some distance, the LV has to make up that alittude during its propulsive phase.
A "better" method would involve the LV being brought to the optimum angle while still attached to the carrier and/or being able to fire the LV engine prior to leaving the carrier aircraft. In many studies this would be achieved by adding rocket engines to the carrier aircraft to allow a high thrust-to-weight ratio to allow this pitch up. As you might imagine, the modifications and structural bracing involved in mounting a powerful rocket motor system to a "standard" airframe, (an AF study looked into installing a Space Shuttle Main Engine in the tail of a Boeing 747 and all the needed propellant tankage and equipment for example) would be expensive both in the conversion work and operations.
In some cases (the CROSSBOW concept itself for example) the LV rocket engine would be used to provide propulsive power to do the pitch up. Avoiding the need to install rockets on the carrier as well as allowing a check-out and testing of the LV rocket motor before launch. This again though falls back on needing either extensive modifications to an existing airframe or a custom airframe for the carrier aircraft, both of which can be highly expensive.
These are things that have to be considered in the design, and balanced with the various "trade-offs" involved with any systems approach. The biggest "downside" to air-launch is the payload is going to be restricted. StratoLaunch has built the worlds largest aircraft because they are "aiming" for around 10,000lbs to orbit with their system. XCOR will run into scaling issues with their system somewhat, though I'll point out that TSpace/AirLaunch LLC DID put forth a system with a similar payload that was compatable with a modified 747 air-freighter, (
http://www.airlaunchllc.com/AIAA-2008-7835-176.pdf) what will determine the amount of "actual" payload possible for the XCOR system is the "mass" penalty of the reusable first and second rocket stages.
I will add one more thing while I'm at it. As I noted in my earlier post "air-breathing" boosters do not HAVE to be horizontal take off and can in fact be vertical take off and/or landing. An early study by Dani Edar of Boeing showed some significant economic advantages to using Jet Engines to boost an LH2/LOX powered vehicle during the initial launch from start to around 50,000ft. (
http://yarchive.net/space/launchers/jet_first_stage.html)
Recently NASA-Dryden has put forth a variation where jet-engines are used for the first stage boost and then a ramjet for the second stage with an LH2/LOX stage to put the final payload into orbit.
(
http://www.nasa.gov/offices/ipp/centers/dfrc/technology/DRC-010-039-Ram-Booster.html)
Note that while the fist two "booster" stages are recovered it appears that the final stage is not.
Current jet engine technology means that the "choice" of VTVL (OR) HTHL isn't so cut and dried as one might think
Operations, flight rate capability, and other factors may mean different things and take on different priorities when different assumptions are made and requirements directed
Randy