Has anyone every actually built a high speed unfixed "floating" sled before? Something that can run on salt flats or lakebeds? Say a hovercraft, or a ground effect PAR-WIG? Some old concept designs for an SSTO had a "flying" jet sled/platform that resembled a WIG.
All of the hypersonic (and for that matter supersonic) sleds have been on tracks. The only reason they're 'sleds' on 'tracks' and not on 'rails' is because they slide on bearing surfaces rather than roll on wheels.
All time record for kerolox specific impulse: must be RD-0124, 362 seconds - in vaccuum. All time record propellant mass fraction: 0.962 - Titan 2 stage 1. Expendable, zero payload without a second stage.
The margins are razor slim or non existing.
I did a quick straw pole of empty/gross weights for some combat aircraft and it came up as around 38-45%, even for ones as "sporty" as the Talon T-38, a supersonic trainer with pretty much no payload but the instructor and trainee. The Virgin Global Challenger aircraft (an engine, air bubble and wings wrapped round a humongous fuel tank) gave a design of 17% structure, so better is possible if you sacrifice other things. Rockets can give you 10x the T/W ratio of SoA turbofans, OTOH they also give you roughly 1/10 the Isp So can you wrap 413247Kg of propellant in 32252Kg of structural mass (including escape module) to hold 4500 of payload? that's about 2.37x better than the Virgin Global Flyer managed.
negatives-internal fuel tank that needs to be covered in greater amount of TPS for re-entry.
Figure 2 in the report reckons Wings need 38.2%, body is 28.4% and tail takes 3.6% of the dry weight of the vehicle. "Propulsion" (SSME, OMS/RCS, pressurization and prop feed system) is 18.8%
What is really crazy is that the percentages, above, are "slices" or "bits" of... 5% of the GLOW (Gross Lift Off Weight) since 95% else is the propellant in the tanks.
I'm the proud owner of a Fiat Grande Punto since 2011. A very nice car. It weights 1000 kg. And since the tank hold 45 L of gasoline, that must be 40 kg in mass. 40 kg out of 1000 kg. Leaves plenty of margin. Now If I applied "all-rocket SSTO" numbers to it... the gasoline in the tank would represents 950 kg. No kidding.
Brilliant and interesting posts there. Couldn't remember if I had downloaded that RASV report or not, so downloaded it again.
"Takes the SSTO, standing still on the pad, ready to launch. The propellant tanks are full. Well - 95% of the mass must be raw propellants. Otherwise, kiss Earth orbit goodbye ! Sooo...- the tanks around the propellants, - and the SSTO around the tanks, - with the payload (obviously !) ...are allocated 5%. Five percent. And if you miss, and get 7% or even 6%, the SSTO falls short of orbit.
That isn't necessarily true. I graphed both 9000 m/s and 9300 m/s and as long as you can get average ISP of around ~330, dry mass around ~6% is sufficient if 9300 m/s is requires and ~7% is sufficient if 9000 m/s is required. Merlin 1D sea level performance is 282 seconds and vacuum performance is 311, but the vacuum variant gets about 348. RD-180 is between 311(sea level) and 338 (vacuum), so getting 330 average shouldn't be impossible. Even at 320, you still can go to 5.5-6% (of course, lower is better).
edit: average isp isn't really the best way to describe the above. Two systems that have the same average isp might perform somewhat differently from a change in velocity perspective. But starting at the RD-180 sea level isp of 311 and linearly moving to 338 over the burn, a vehicle that is 6% dry weight and 94% propellant gets about 9100 m/s. Which, adding 300 mph or 135 m/s for the rocket sled, could probably get you to orbit (dependent on gravity/drag loses and latitude).
What was the Delta II "recipe" to get that low - "only" 1150 m/s of gravity losses ?
Quote from: libra on 12/09/2020 03:10 pmWhat was the Delta II "recipe" to get that low - "only" 1150 m/s of gravity losses ? Super high TWR early in flight, with lots of SRBs (including air-lit ones in some configurations.)Having wings (even with a mediocre TWR) should actually help gravity losses significantly as the lift offsets gravity early in flight when losses would otherwise be the highest. Recall that the wings on the Pegasus rocket actually increased the payload capacity, despite the added mass.
Thanks you all for these numbers. No dogma for me, really - 6% , 7%, no problem. Still a daunting mass fraction. Although JS19 contenair and soda can examples are interestings. What was the Delta II "recipe" to get that low - "only" 1150 m/s of gravity losses ? With the rocket equation, even 200 m/s can make a difference... and there, we have 400 m/s variations, 8800 to 9200 m/s... Gravity losses - such a tricky thing.
Super high TWR early in flight, with lots of SRBs (including air-lit ones in some configurations.)Having wings (even with a mediocre TWR) should actually help gravity losses significantly as the lift offsets gravity early in flight when losses would otherwise be the highest. Recall that the wings on the Pegasus rocket actually increased the payload capacity, despite the added mass.
Quote from: libra on 12/09/2020 03:10 pmThanks you all for these numbers. No dogma for me, really - 6% , 7%, no problem. Still a daunting mass fraction. Although JS19 contenair and soda can examples are interestings. What was the Delta II "recipe" to get that low - "only" 1150 m/s of gravity losses ? With the rocket equation, even 200 m/s can make a difference... and there, we have 400 m/s variations, 8800 to 9200 m/s... Gravity losses - such a tricky thing.You'll note that it has the 2nd highest drag losses of the vehicles, suggesting it spends an extended amount of time in the atmosphere, but its steering losses are very low.
given the displacement formulas= ut + (1/2)a*t*tu or initial velocity is zero for a VTO rocket and a * t is velocity. So, we can simplify to s= (1/2) *v * t or v = 2s/t with s being displacement (in this case the altitude of the rocket). So, we know that for a certain altitude, the velocity for a linearly accelerating vehicle is inverse to the time it takes to get there (half the time, double the velocity). We also know that drag scales to the square of the velocity, so if you half the time, you are actually increasing the instantaneous drag at a certain altitude by 4x. Total drag would be average drag * time over the course of flight, so halving time as a result of increasing thrust isn't going to be enough to overcome 4x the instantaneous drag as a result of halving the amount of time. So, there is a trade off somewhat between aerodynamic losses and gravity losses. Your aerodynamic losses will increase if you increase your thrust as gravity losses decrease. This explains why the low gravity loses are correlated with higher aerodynamic losses. But the higher aerodynamic losses isn't associated with more time in the atmosphere, the higher thrust rocket spends less time in the atmosphere.At least, that is what I could come up with. Feel free to point out any logical/math/physics errors.