A ULA slide shows a future LV with flyback engine pods attached to a disposal tank. A hybrid SSTO could use same approach, not a true SSTO but simpler than a TSTO.
Quote from: TrevorMonty on 04/05/2016 06:47 pmA ULA slide shows a future LV with flyback engine pods attached to a disposal tank. A hybrid SSTO could use same approach, not a true SSTO but simpler than a TSTO. There is NOTHING SSTO in that; if some engines are dropped it's parallel staging, which is still staging.
My favourite is tripropellant TAN;Keep the main chamber burning hydrogen, and inject additional kerosine or methane and oxydizer into the nozzle.
Aerospike ? Not exactly thrust augmentation, but an efficiency compensation
Quote from: hkultala on 04/05/2016 07:58 pmMy favourite is tripropellant TAN;Keep the main chamber burning hydrogen, and inject additional kerosine or methane and oxydizer into the nozzle.That is essentially what I called "auxiliary afterburning" in the OP.
Page 626In 1989, Energomash started the development of two tripropellant large booster engines. The RD-701 had two gimbaled TCs, and the RD-704 had a single TC. Both used a staged combustion engine cycle. The aim was to drive a single-stage-to-orbit launch vehicle using a combined first- and second-stage engine....For the initial period of the flight (boost phase or ascent through atmosphere the engine burns both kerosene and LH2 with LOX at a high thrust level and a high chamber pressure. For the remainder of the flight (which is a sustalner phase) it burns only LH2 fuel with LOX at a much lower thrust and chamber pressure. The advantage of this dual fuel concept is a somewhat higher average fuel density, which allows a smaller total propellant tank volume, a slightly lower vehicle structure mass, a single TC for two propellant combinations, a lower drag. and a slightly improved vehicle performance.
But what about TAN with solids?Make a hydrogen staged cycle engine with a huge nozzle sized for good vacuum efficiency.Then put some solid fuel inside the nozzle so that the hydrogen ignites it, it burns for about first minute or two giving some extra thrust(and preventing flow separation) and after that it is gone and the nozzle acts as ordinary vacuum nozzle. No need for additional pumps or piping for the TAN.How is the burn speed controlled in solids? How easy or difficult would it be to make it burn at correct speed in this?
Staging works really well.[...]A reusable VTVL SSTO, then, has a major challenge.
Quote from: sevenperforce on 04/04/2016 03:18 pmStaging works really well.[...]A reusable VTVL SSTO, then, has a major challenge.Well, the obvious conclusion is that we should keep using staging.VTVL and SSTO should not be goals. They should be among the possible choices for achieving the real goal: cheap, reliable access to space.
Why derail a perfectly good tech thread into another reusability economics borefest ? Here is another thrust augmentation idea:http://www.gizmag.com/laser-propelled-ablation-space-rockets/34505/
Why derail a perfectly good tech thread into another reusability economics borefest ?
A reusable VTVL SSTO, then, has a major challenge. It must not only carry its entire dry mass to orbit and back while compensating for expansion ratios, but it must also find a way to deliver high takeoff thrust (to reduce drag losses) despite needing highly efficient fuel to keep the overall fuel fraction low.
Tripropellant engines. Design the same engine to burn different kinds of propellant, so that you can use a dense, high-thrust fuel like RP-1 at launch while using a lightweight, lower-thrust fuel like LH2 at altitude while still maintaining the same volumetric fuel flow rate. You could also use two different oxidizers, like high-test peroxide at launch and LOX at altitude.
Highly-variable mixture ratios. Engines have an optimal oxidizer:fuel ratio for the best specific impulse, but you could increase the amount of oxidizer (LOX being far denser than, say, LH2) and thus increase thrust at the expense of specific impulse. This requires your oxy turbopump to have a highly variable output, though, which is difficult to design.
It is also less efficient than some other options and only moderately augments thrust, and depending on your flow characteristics you may have trouble with altitude compensation.
Auxiliary afterburning. Extend your expansion bell and add auxiliary injectors, and you can inject auxiliary propellant (fuel or oxidizer) to boost thrust. You'll want to vary your engine's mixture ratio to match the injection, of course; if you're injecting a dense oxidizer then burn fuel-rich; if you're injecting a dense fuel than burn oxygen-rich.
One major advantage here is that you can use a vacuum-optimized expansion bell at sea level because you will be injecting enough auxiliary propellant at launch to bring your exhaust pressure up above atmospheric pressure and thus prevent flow separation. Your need for augmented thrust will drop off as atmospheric pressure does, so you can reduce this gradually. This also allows your combustion chamber to be optimized for your high-efficiency primary fuel without needing to worry about mixing or injecting a third propellant as in a tripropellant design.However, afterburning will occur at lower pressure and have lower efficiency than a tripropellant combustion chamber, increasing thrust-specific fuel consumption. The injectors also add an additional weight cost, and the exhaust bell must be quite large in order to have space for mixing, combustion, and expansion. So this will have a higher dry mass than a comparable tripropellant system. Other than that, the same factors apply with dual-use of auxiliary tankage and so forth.
Inert reaction mass injection. Rather than injecting auxiliary fuel, you can merely inject an inert working fluid into your exhaust stream, usually water. Although you are now carrying a propellant which does not contribute any chemical energy, you also don't have to worry about varying the mixture ratio of your actual engine (although the same is true of a hydrolox engine with HTP injection afterburning). You can inject quite a bit of water to raise thrust extremely high if you need to, and your dry mass is better because you only need space for mixing and expansion, rather than for combustion as with afterburning. Thrust and specific energy are now tied directly together; you can trade one for the other directly. Water injection will reduce the cooling requirements on your exhaust bell as well.
So those are the basic options. Are there any I missed? Which ones do you like, and which ones do you dislike? All factors being considered, which option (or combination of options) would be best for enabling a low-to-medium-payload VTVL reusable SSTO?
Launch costs can be roughly divided into three categories: vehicle costs, fuel costs, and ground support/infrastructure costs. Fuel cost is minimal compared to everything else, so that can be ignored. Reuse is aimed to reduce vehicle costs. Ground support and infrastructure, on the other hand, multiplies with the number of components you have in your launch vehicle.
Quote from: savuporo on 04/06/2016 04:00 pmWhy derail a perfectly good tech thread into another reusability economics borefest ?Because economics are a big part of what drives the tech? More to the point the OP seemed to be directed at a single-point-design, (VTVL and SSTO) rather than general tech but sevenperforce cleared that up later.
Quote from: sevenperforce on 04/04/2016 03:18 pmTripropellant engines. Design the same engine to burn different kinds of propellant, so that you can use a dense, high-thrust fuel like RP-1 at launch while using a lightweight, lower-thrust fuel like LH2 at altitude while still maintaining the same volumetric fuel flow rate. You could also use two different oxidizers, like high-test peroxide at launch and LOX at altitude.So far only the dual Oxidizer is anywhere close to going to a full size test bed engine.
Quote from: sevenperforce on 04/04/2016 03:18 pmAuxiliary afterburning. Extend your expansion bell and add auxiliary injectors, and you can inject auxiliary propellant (fuel or oxidizer) to boost thrust. You'll want to vary your engine's mixture ratio to match the injection, of course; if you're injecting a dense oxidizer then burn fuel-rich; if you're injecting a dense fuel than burn oxygen-rich.Actually most nozzles are designed for maximum efficiency at altitude, so are substantially over expanded at SL. Running fuel rich is standard for most rocket engines (and no flown engine AFAIK has run the MR through the stochiometric range from fuel rich to rich poor. You just need to add the Oxidizer injectors.QuoteOne major advantage here is that you can use a vacuum-optimized expansion bell at sea level because you will be injecting enough auxiliary propellant at launch to bring your exhaust pressure up above atmospheric pressure and thus prevent flow separation. Your need for augmented thrust will drop off as atmospheric pressure does, so you can reduce this gradually. This also allows your combustion chamber to be optimized for your high-efficiency primary fuel without needing to worry about mixing or injecting a third propellant as in a tripropellant design.However, afterburning will occur at lower pressure and have lower efficiency than a tripropellant combustion chamber, increasing thrust-specific fuel consumption. The injectors also add an additional weight cost, and the exhaust bell must be quite large in order to have space for mixing, combustion, and expansion. So this will have a higher dry mass than a comparable tripropellant system. Other than that, the same factors apply with dual-use of auxiliary tankage and so forth.Depends. In effect you're filling a vacuum space where most of the chamber pressure has already been lost in expansion. The injectors don't need that high a pressure. You seem to be looking to justify tripropellant as a design choice.
QuoteSo those are the basic options. Are there any I missed? Which ones do you like, and which ones do you dislike? All factors being considered, which option (or combination of options) would be best for enabling a low-to-medium-payload VTVL reusable SSTO?You missed the idea of ejectors around the rocket engines. Note this tactic improves when you have more perimeter between the nozzle and the airflow you're dragging in, so many small nozzles (driven by a single set of turbo machinery) should substantially improve this idea. However AFAIK the most this done with are the Russian 4 chamber, 1 pump set design. My instinct is a lot of nozzles on a plate, with essentially a pressed metal sheet duct plate in from fed from closeable side inlets.
SpaceX seems to have gotten "traditional" TSTO (big booster, small second stage) down to a science, but I'm interested in a reversed arrangement where you have a large SSTO-capable spacecraft with comparably smaller parallel-staged boosters to give it high payload capacity.
What are the pros and cons of using dual oxidizers vs dual fuels? Obviously LOX is king if you use dual fuels, but if you're using dual oxidizers, what's the best fuel? LH2 is good; liquid methane might be better.
One major advantage here is that you can use a vacuum-optimized expansion bell at sea level because you will be injecting enough auxiliary propellant at launch to bring your exhaust pressure up above atmospheric pressure and thus prevent flow separation. Your need for augmented thrust will drop off as atmospheric pressure does, so you can reduce this gradually. This also allows your combustion chamber to be optimized for your high-efficiency primary fuel without needing to worry about mixing or injecting a third propellant as in a tripropellant design.
However, afterburning will occur at lower pressure and have lower efficiency than a tripropellant combustion chamber, increasing thrust-specific fuel consumption. The injectors also add an additional weight cost, and the exhaust bell must be quite large in order to have space for mixing, combustion, and expansion. So this will have a higher dry mass than a comparable tripropellant system. Other than that, the same factors apply with dual-use of auxiliary tankage and so forth.