The Purpose of Scramjets

[Kinetics]

I interpret the above to suggest we are now in agreement regarding the validity of Nasa's ramjet figures and the finding that "ramdrag increases at a greater rate than thrust". :thumbup:

 

[Urwumpe]

I interpret the above to suggest now in agreement with regard to the original issue; and that the relationship of ramdrag to gross thrust is not linear with respect to aircraft speed. :thumbup:

PS - the main concept is that ram drag incrementally erodes net thrust at increasing speeds - I recommend you download the simulator

Not really needed, since I actually know this as well. At least for Ramjets it is pretty easily visible, less so for Scramjets, which don't need to end their compression with an oblique shock.

Also, as you might find out, that quite a lot of Mach numbers can pass for a Scramjet, until you reach the point at which vehicle drag force and engine thrust will neutralize each other.

Still the problem why I complained wasn't about the net thrust. It was about the change in inlet geometry that was evident in the ram drag data, which suggests a simple user error in the simulator.

The engine simulator is, BTW, public source and can be modified for direct Scram simulation, if needed. Also you can check there, which assumptions had been made for the component simulation.

 

[Kinetics]

... it is pretty easily visible....

:yes::banana:

 

[Urwumpe]

And stop wasting your time comparing subsonic ramjets with supersonic ones (No need to be aggressive, if you are not understanding what I mean. You could also just ask "WTF?".).

You will have, by design, a change in inlet geometry between those two types. Mentioned it already earlier, maybe you didn't notice it. Thus my recommendation to do the tests with Mach 2 and Mach 4 - both numbers that can be achieved troublefree by the same inlet.

Subsonic inlets look like that, completely different to supersonic ramjet inlets.

(Subsonic ramjets are pretty inefficient, but popular among hobbyists)

 

[Kinetics]

We agree that your original statement

Actually both thrust and inlet drag increase at the same rate, if you look at the terms. Both are velocity * mass flow, and mass flow is density * velocity.

has been shown to be incorrect and misleading for other readers.

However, thanks anyway for your other contribution to the discussion which I read with interest.

 

[TG]

At Mach numbers below Mach 5, you can compress air with only small increases in temperature. Past Mach 5, any small increase in air pressure comes with a strong increase in temperature (See spacecraft reentry as example there).

This gives me an opportunity to ask a question that's bothered me for awhile. Does anyone know the physics reason for this? I always wondered why max deceleration came after max heating during reentry, for example.

Why would a small increase in air pressure lead to higher and higher rates of increases in temperature when you approach larger speeds?

 

[Urwumpe]

Why would a small increase in air pressure lead to higher and higher rates of increases in temperature when you approach larger speeds?

This is because the effects of viscosity of the air becomes important at such speeds. Usually, you don't need to bother much about the viscosity of air, but at higher speeds, it starts to have effect. The viscosity of air is important for the transition of the conditions near the spacecraft walls to free stream conditions: The higher the viscosity, the larger this transition area (boundary layer) has to get, because it takes more force to deform the air(shear stress)

In simpler words: You get not only friction between your vehicle and the air, but also more friction between the molecules of the air near your vehicle.

 

[statickid]

I downloaded that NASA EngineSim before but never really used it much until today. It's interesting looking at the Ramjet data - shame they don't have scramjets on it.

For ramjets, obviously, net thrust is gross thrust less ram drag.

The applet shows that ramdrag increases with the square of speed, whereas gross thrust increases at a much lower rate compared to speed. It follows that at high speeds (or high power outputs) ramdrag will gradually reduce net thrust to zero.

It shows that overheating is not a risk factor at high altitudes - the reduced air intake limits the burn rate and therefore potential heat generation. Additionally, the speed required to maintain the air supply at altitude causes excessive ramdrag. This reduces net thrust to zero before overheating can occur. Note that temperature reduces with altitude - further reducing overheating risks.


So, why would the code cause the DG-S scramjets to overheat at increasing altitudes?

It doesn't seem to make sense because this uses a ram jet simulator to answer questions about a scram jet.

Temperature does not simply reduce with altitude, it varies with altitude, velocity, and angle of attack

It also seems like these observations dont take into account the significant difference between calibrated and indicated air speed. In practical use of a scram jet one of the primary objectives is to maintain certain conditions within the engine. This is done by flying a very specific ascent profile, maintaining a dynamic pressure to drag ratio that gives optimum engine performance while accelerating and gaining altitude.

---------- Post added at 08:36 AM ---------- Previous post was at 07:49 AM ----------

Also remember two of the headlining additions to orbiter 2010 were a new, complex atmosphere model for earth, and a scripting language. IIRC one of the reasons for the changes made to the DG 2-D panel were to facilitate the operation of the scram jet. The new instruments were a vertical acceleration tape and AOA tapes, and the scram status display.

Is it a coincidence that Martin mentions starting your testing of the scram at the tropopause and "testing its performance at different speeds and altitudes?". Or that we all can't seem to operate the scram any higher than the stratopause where there is a dramatic change in atmospheric density and temperature?

What better way to showcase a brand new shiny atmosphere model than with a scram engine that is sensitive to it. perhaps we've all just been overlooking one of the best new features of orbiter by blasting through it with rockets. Perhaps we can write a lua auto pilot that can execute a scram ascent in the the DG-S

 

[Kinetics]

:bananadance::bananadance::hotcool:

 

[V8Li]

Remember the DG-S is 2 tons heavier (I'm talking about the stock DG)

I did alot of tests and tried to make good economic sense by using all kind of tweaks like a higher weight or less ISP that would make both DG and DG-S stay longer in low atmosphere (10-33Km) so that the DG-S can have an advantage by using the more efficient scramjets. Simply put, once in orbit the DG has more DV available. Also, I use about 40% scram to get the DG-S from 12Km and 3 mach to 33Km and 6.9 machand transfer the remaining fuel to main tank.

So no, for orbit use the DG-S as it is set up won't be produced/used. It's just good for atmospheric flight but while ascending you get pretty fast to mach 7 where the scramjet engines can't be used. And then you're stuck with 2 extra tons worth of scramjet engines

 

[Erkha]

while you guys are on the subject of the delta glider, why can't I do vtol?
hover engines are at maximum yet it wont lift off, is it too heavy?

 

[ADSWNJ]

while you guys are on the subject of the delta glider, why can't I do vtol?
hover engines are at maximum yet it wont lift off, is it too heavy?

You can .... you just need to be on the right planet or moon. (Earth is too massive.)

 

[Andy44]

while you guys are on the subject of the delta glider, why can't I do vtol?
hover engines are at maximum yet it wont lift off, is it too heavy?

Too heavy for Earth, yes. VTOL works fine on Moon, Mars, and other low-G bodies.

To do VTOL you need thrust-to-weight ratio greater than 1, which would make the hoverjets absurdly powerful.

One cool thing you can do, though, is make a hoverjet-assisted short field landing, Harrier-style, using a combination of wing lift and hover thrust. This is tricky and actually harder than a standard wings-only landing, a good challenge.

You can also use the hover jets for a jet-assisted short roll takeoff.

These techniques could be used for carrier landings and takeoffs, although you'd damage the anti-skid coating on the deck pretty badly, I'd imagine!

 

[BruceJohnJennerLawso]

You can .... you just need to be on the right planet or moon. (Earth is too massive.)

Actually, I think you can, at least on the regular DG, and the tanks need to be drained quite a bit. I recall doing one near the pyramids early in my Orbiter travels.

 

[V8Li]

while you guys are on the subject of the delta glider, why can't I do vtol?
hover engines are at maximum yet it wont lift off, is it too heavy?

This is from the DG/DG-S manual: "Requires total mass < 16300kg to allow vertical liftoff from Earth's surface"

So it can, just not with full tanks

---------- Post added at 04:13 AM ---------- Previous post was at 03:24 AM ----------

... I use about 40% scram to get the DG-S from 12Km and 3 mach to 33Km and 6.9 mach ...

I was wrong actually, this is the consumption with a higher empty mass I was testing.

With the default DG-S settings (13 tons empty mass) I get to 12Km and 3 mach on rockets, start the scram and cut rockets at about 3.5 mach and climb to 33 Km and mach 6.9 on scram. The cost is only 25% scram fuel. Once in an orbit similar to a DG's (same Pe, Ap and inclination) I transfer the scram to main (might not be the same fuel in real-life but I'd really like the DG-S to win since it's so much cooler than the stock DG) I'm still left with less DV available (about 500Kg less fuel in the DG-S and the extra 2 tons).

So yea, it looks bad for the DG-S

 

[MaverickSawyer]

One cool thing you can do, though, is make a hoverjet-assisted short field landing, Harrier-style, using a combination of wing lift and hover thrust.
-snip-
These techniques could be used for carrier landings and takeoffs, although you'd damage the anti-skid coating on the deck pretty badly, I'd imagine!

Yup, done this a number of times. Most recently when I came in short at WIN and didn't want to light the mains, so I added a little hover, brought the nose up a little, and smacked it right down on the hash marks.

As for the anti-skid coating, I have heard from a former ordinance handler aboard the USS Nimitz that when the USMC deployed Harriers aboard the Nimitz, they could land and take off vertically at only 4 spots onthe entire flight deck: the jet blast deflectors fore each of the catapults. The JBDs are actively cooled, and the practice was to plate the aft nozzles directly above the center of the JBD, then throttle up. LHDs use 8 cooled landing zones instead of the deflectors.

 

[V8Li]

Well, I've tested some more settings: with the default weights, same orbits and by setting the scram propellant efficiency to 2 (default is 1) I managed to get the DG-S in the same orbit as DG with 100Kg more fuel (once in orbit I transfer the scram fuel to main tanks and compare). So even at twice the fuel efficiency with a climb from mach 3 to 6.9 (and keeping the scram engines on up to mach 8) the DG-S has less DV available once in space (again, the 2 extra tons woth of scram engines).

If you want to give the DG-S a chance set_propellantefficiency(tnkscrm,3) with LUA for the scenario



Note: during tests I'v set_propellantmaxmass for DG-S main tank to 12900 and it seems that the scales in the VC as well a on 2D panel are rendering wrong and off-scale (it seems that the process is as follow: the .dll is loaded for DG, the scenario is read and propelant is saved as fractions, not values in scenario files, the LUA is executed but the scales are generated from old max mass values (set by .dll) or are using absolute values (ie. 10500 is set as max scale). This is a bug and maybe some more experienced developer could have a look and report it.

Also, I'v set_emptymass for the DGs to higher values. Adding fuel with scenario editor works fine, however once a new passenger is added the scenario editor sets the empty mass to the default value. Also a bug.

Thanks!

Edit:
set_propellantefficiency(tnkscrm,3) is not good enough, the DG still has an extra 2000m/s DV in a 200x200 orbit

 

[Ripley]

You could report it in the bug section of OF.

 

[Kinetics]

Well, I've tested some more settings: with the default weights, same orbits and by setting the scram propellant efficiency to 2 (default is 1) I managed to get the DG-S in the same orbit as DG with 100Kg more fuel (once in orbit I transfer the scram fuel to main tanks and compare). So even at twice the fuel efficiency with a climb from mach 3 to 6.9 (and keeping the scram engines on up to mach 8) the DG-S has less DV available once in space (again, the 2 extra tons woth of scram engines).

If you want to give the DG-S a chance set_propellantefficiency(tnkscrm,3) with LUA for the scenario



Note: during tests I'v set_propellantmaxmass for DG-S main tank to 12900 and it seems that the scales in the VC as well a on 2D panel are rendering wrong and off-scale (it seems that the process is as follow: the .dll is loaded for DG, the scenario is read and propelant is saved as fractions, not values in scenario files, the LUA is executed but the scales are generated from old max mass values (set by .dll) or are using absolute values (ie. 10500 is set as max scale). This is a bug and maybe some more experienced developer could have a look and report it.

Also, I'v set_emptymass for the DGs to higher values. Adding fuel with scenario editor works fine, however once a new passenger is added the scenario editor sets the empty mass to the default value. Also a bug.

Thanks!

Edit:
set_propellantefficiency(tnkscrm,3) is not good enough, the DG still has an extra 2000m/s DV in a 200x200 orbit

Thanks for testing and confirming this. Another poster claimed my data was invalid when clearly it is not.

The main point is that due to air drag in the environment where the DG-S scrams operate, the scramjets are not worth using. It's more effective to climb to a zero drag altitude then use the main thrusters to accelerate. This is partly because of the ISP settings.