Back in the 90's, there were just as many people who thought we could pull it off, if not more. REL, for example, were pitching the same ideas for Skylon in the 90's. And big bucks were being invested in NASP and X-33/VentureStar.
I like this approach a lot, except that you run into extreme flow separation if you try to use that same engine for vertical landing.
Only SABRESkylon has survived a full external audit of the engine and structural plans by ESA and scrutiny at an international colloquim of launch vehicle and hypersonics researchers.
Quote from: nec207 on 04/20/2016 12:21 amQuote from: Jim on 04/20/2016 12:14 amQuote from: nec207 on 04/19/2016 11:57 pm Well exactly the 90's SSTO engines where not powerful enough to take it up 10 feet off the ground that alone take it up into space!!There was big holes with all the 90's SSTO programs. Not one of them shown that an engines could take it up into space. They put way too much money into 90's SSTO programs and the technology was just not there.Wrong. It had nothing to with engines. There is no difference between engines then, now or even in the 60's/70's. The issue is not the engines. There were many engines that could get into space much more than 10 feet The issues was getting into orbit with payload.Can you elaborate on this? More so the one in bold?If you want to learn the why's involved, I suggest you take a look at Kirk Sorensen's excellent series of articles here: http://selenianboondocks.com/category/rocket-design-theory/page/2/ , starting with this one: http://selenianboondocks.com/2010/02/rocket-equation-mod-1/The relevant part for "getting into orbit" would be this one: http://selenianboondocks.com/2010/03/payload-fraction/and this for an example of how more Isp is not necessarily better for getting into orbit if T/W is sacrificed: http://selenianboondocks.com/2010/06/ssto-ntr-bad/
Quote from: Jim on 04/20/2016 12:14 amQuote from: nec207 on 04/19/2016 11:57 pm Well exactly the 90's SSTO engines where not powerful enough to take it up 10 feet off the ground that alone take it up into space!!There was big holes with all the 90's SSTO programs. Not one of them shown that an engines could take it up into space. They put way too much money into 90's SSTO programs and the technology was just not there.Wrong. It had nothing to with engines. There is no difference between engines then, now or even in the 60's/70's. The issue is not the engines. There were many engines that could get into space much more than 10 feet The issues was getting into orbit with payload.Can you elaborate on this? More so the one in bold?
Quote from: nec207 on 04/19/2016 11:57 pm Well exactly the 90's SSTO engines where not powerful enough to take it up 10 feet off the ground that alone take it up into space!!There was big holes with all the 90's SSTO programs. Not one of them shown that an engines could take it up into space. They put way too much money into 90's SSTO programs and the technology was just not there.Wrong. It had nothing to with engines. There is no difference between engines then, now or even in the 60's/70's. The issue is not the engines. There were many engines that could get into space much more than 10 feet The issues was getting into orbit with payload.
Well exactly the 90's SSTO engines where not powerful enough to take it up 10 feet off the ground that alone take it up into space!!There was big holes with all the 90's SSTO programs. Not one of them shown that an engines could take it up into space. They put way too much money into 90's SSTO programs and the technology was just not there.
The problem with the X-33 and lot of other 90's SSTO and space planes was heat tolerance in the aerospike engines. Composite and ceramic materials weren't far enough along to stand up to the higher than standard temperatures. Ironically a few months after Bush slashed NASA's budget down to almost nothing, new materials solved the heat problem!!!
Quote from: Nilof on 04/20/2016 05:42 pmDry mass is the key source of payload uncertainty, which has been a much bigger obstacle for SSTO project funding than payload (i.e. independent engineers saying "this won't reach orbit with payload" andh aving a good argument).But no independent engineers are actually saying that in this case, at least not ones that have verifiably looked at the design for more than five seconds. The pessimism tends to exhibit a distinct lack of specifics, and to me it sounds more like a mixture of NIH and sour grapes left over from the '90s than any sort of actual well-grounded judgment.You're essentially dismissing a high-resolution preliminary design (Skylon D) built on an ESA-vetted detailed concept (Skylon C) with 30 years of risk reduction and design iteration behind it starting from an attempt to address the problems uncovered by a thorough, well-funded design study (HOTOL) on the basis of a crude personal rule of thumb.Some SSTO concepts amount to not much more than a couple of papers full of inconsistent, obviously fudged mass properties and engine performance parameters and handwaving about "next-generation TPS". Skylon is not one of those. REL have stated that the purpose of Skylon D is to anchor the engine design - the idea is to have enough confidence that the vehicle can be built to spec that the engine can be sized and developed first. This implies a level of understanding of the mass budget that is much better than you are apparently assuming, and suggests that requiring 100% mass growth margin at this stage is not reasonable.The mass estimates for Skylon supposedly have AIAA-standard mass growth margins built in, and that doesn't include the payload margin of slightly less than a tonne. Take out the margins, and you might have roughly a 23-tonne payload vs. about 46 tonnes of dry mass (not including ~5 tonnes of fluids at MECO), give or take a couple of tonnes (I'm not privy to the exact values carried on each subsystem, so I assumed a global 15%). That's about a 2:1 structure-to-payload ratio, contrasted with the usual 8:1 or worse for all-rocket hydrolox SSTO. They're still working on firming up their mass estimates, but then AFAIK they haven't frozen the engine parameters yet...
Dry mass is the key source of payload uncertainty, which has been a much bigger obstacle for SSTO project funding than payload (i.e. independent engineers saying "this won't reach orbit with payload" andh aving a good argument).
a multi-mode engine which makes the SSME look like simplicity incarnate
a completely new aeroshell construction method
an aerodynamic shape that has to balance subsonic performance, supersonic to low hypersonic flight in dense atmosphere, and upper atmosphere hypersonic reentry.
Meanwhile, much more "traditional" SSTO proposals can get similar or higher margins and less payload uncertainty with way fewer new technologies and a more reasonable development budget...
Quote from: Nilof on 04/20/2016 11:46 pmMeanwhile, much more "traditional" SSTO proposals can get similar or higher margins and less payload uncertainty with way fewer new technologies and a more reasonable development budget...Analysis or citation please. This statement is preposterous on its face.
The nuclear DC-X design research suggested that LOX-afterburning (technically TAN but I prefer to call it afterburning rather than augmentation because you're burning the oxygen and hydrogen together to increase exhaust energy, rather than just increasing mass flow) roughly triples the thrust while losing about 40% of your exhaust velocity. So for a standard solid-core NTR with 1000 s of specific impulse and a T/W ratio of around 3, you're looking at a launch T/W of 9 at 600 s of specific impulse.
Unfortunately, while a solid-core NTR doesn't emit fissile material, it does irradiate the H2 propellant stream enough to make it mildly radioactive.
REL has done a reasonably large number of tests and has matured some of the risk but it still introduces a lot of technology that has never been tested before.
Examples would include a multi-mode engine which makes the SSME look like simplicity incarnate,
a completely new aeroshell construction method,
and an aerodynamic shape that has to balance subsonic performance, supersonic to low hypersonic flight in dense atmosphere, and upper atmosphere hypersonic reentry.
Meanwhile, much more "traditional" SSTO proposals can get similar or higher margins and less payload uncertainty with way fewer new technologies
and a more reasonable development budget...
and of course for any TSTO RLV the margins go through the roof
and much of the risk can be retired by turning an existing expendable launcher into an RLV.
Using Skylon C1 as a baseline, that's 1105 tonnes of propellant, vs. about 217 in the base vehicle. To simplify the analysis, I will assume the engines are still mounted on the wingtips - ie: the vehicle still has wings. The tankage fraction will be better; probably not five times better, but let's assume that anyway. The truss will, however, have to take five times the load as a first approximation, so it might weigh 20 tonnes instead of 4. The thrust required to take off vertically should be perhaps 1400 tonnes or so, meaning a set of kerolox engines with a T/W of 180 will weigh close to 8 tonnes, down from just under 11 tonnes, and it may be possible to save a couple of tonnes on the nacelles too. The wings don't have to lift the loaded weight of the vehicle, but they do have to take 1400 tonnes of thrust instead of 300, which could increase their mass by up to five times, to roughly 24 tonnes from a little over 5 (I don't actually expect an increase that large, since the load is mostly along the long axis, but there will probably be some).A Merlin 1D might give an average Isp of about 302 seconds; with a total delta-V of 8.8 km/s, 1105 tonnes of propellant can put 65 tonnes in LEO. Skylon C1 less payload weighs about 45 tonnes at MECO. Accounting for the changes enumerated above results in an increase of between 11 and 32 tonnes, resulting in a payload between 9 and -12 tonnes in LEO. Increasing the delta-V to 9 km/s subtracts a little over 9 tonnes from the payload, leaving it negative in the best case.Using RD-180 instead of Merlin might average about 329 seconds, but the engines would weigh an extra 10 tonnes or so. With 77 tonnes in LEO, the payload is now between 11 and -10 tonnes. Going to 9 km/s takes ~5 tonnes off that, for a payload between 6 and -15 tonnes.Of course if you put the engines on the bottom without changing anything else, it would fold up like an accordion before you so much as lit them, so it's not trivial to remove the wings from the analysis...I'm sure I've missed something. 1% is pretty darn good for a kerolox SSTO...
You figure a wing that can take 1400 tonnes of thrust and 70 tonnes of lift would be lighter than one that can take 300 tonnes of thrust and 275 tonnes of lift? Okay... I haven't done any real analysis; as I said I'm handwaving.
pushing couple pressurized cylinders on top of each other from behind straight up sounds a lot more easier structurally than lifting two end-to-end connected horizontal cylinders from the middle while ensuring the whole thing doesn't sag and snap, even if latter cylinders are considerably lighter.
1. It isn't. In the second case you're only pushing half the length (and mass) and the rest just hangs down from the support. Max load is halved. 2And anyway compression is way gnarlier than tension because you can get buckling, and length makes it worse.
Err, the read end is not going to follow you just by asking nicely. So you push half and pull the other half.
Remember that you are dealing with large pressurized tanks. Longitudinal tension on the tank wall is huge, in regular LVs effectively always greater than compressive loads.