The Purpose of Scramjets

[Kinetics]

Here is a question - what is the benefit of using scram jets on the Delta Glider DG-S.

Scramjet optimum fuel efficiency is achieved at an altitude of approximately 25,000 metres and a speed of Mach 5.

Burning fuel at the optimum altitude and speed scramjets could travel 2,800 km using 1,600kg fuel (Mach 5 at 25km altitude).

However, main thrusters could use the same 1,600kg of fuel to simply to climb to 50km altitude and then benefit from the lower drag.

The scramjets are 2.5 times more efficient than the main engines only when in their peak operating range. However this range is in a draggy environment.

Scramjets can maintain this speed burning fuel at a rate of 1 kg for every 1,750m.

While this is highly efficient it is not comparable to the efficiency of the Delta Glider at an altitude of 50,000 metres or higher.

Using main engines it takes 1,600kg engine fuel to climb to an altitude of 50 km where drag and fuel usage are negligible.

So what is the purpose of scramjets?

 

[Urwumpe]

Think in the other direction.

First of all, you can extend your cross range with the Scramjets. Next you could use the Scramjets for saving fuel during ascent, by gaining altitude AND horizontal speed at higher efficiency. It isn't alone about altitude.

 

[BruceJohnJennerLawso]

Think in the other direction.

First of all, you can extend your cross range with the Scramjets. Next you could use the Scramjets for saving fuel during ascent, by gaining altitude AND horizontal speed at higher efficiency. It isn't alone about altitude.

Not to mention that a real scramjet should be slightly easier to build than a gas core NTR. The DeltaGlider is built around propulsion concepts that probably wont be available for a very long time.

One other factor could be the presence of some aerodynamic lift at lower altitudes. That would allow for a lower AoA & consequently less fuel lost fighting against gravity.

 

[Cras]

I never use the scrams anyway. I ripped them out of my XR-2s and sold them to the local pawn shop.

For gas money.


and booze.

 

[worir1]

If you assend using SCAM engines you use less main fuel for the assent and then have more main fuel once in orbit. The Scram engines are more efficient at gaining horizontal speed which is neccicery to acheve orbit where as the main engines are more efficient at gaining height.

 

[Kinetics]

Thanks and this may be obvious to answer but why not just climb to 50km altitude using main engines. It would take only 200 seconds and 1.6 tons of fuel.

Consider this hyothetical scenario - over the Pacific at an altitude of 25km located some 8,000 km from the nearest base and carrying 3,200 kg fuel.

In that situation wouldn't distance be maximised by climbing to a low drag environment (1,600 kg fuel) before accelerating horizontally at altitude >50km?

This is because, at 50km altitude main engines travel 7,500m per kg fuel consumed, at a speed of Mach 12;

whereas, SCRAM engines will only produce 1,750m per kg and at a much lower speed of Mach 6.

 

[BruceJohnJennerLawso]

Thanks and this may be obvious to answer but why not just climb to 50km altitude using main engines. It would take only 200 seconds and 1.6 tons of fuel.

Consider this hyothetical scenario - over the Pacific at an altitude of 25km located some 8,000 km from the nearest base and carrying 3,200 kg fuel.

In that situation wouldn't distance be maximised by climbing to a low drag environment (1,600 kg fuel) before accelerating horizontally at altitude >50km?

This is because, at 50km altitude main engines travel 7,500m per kg fuel consumed, at a speed of Mach 12;

whereas, SCRAM engines will only produce 1,750m per kg and at a much lower speed of Mach 6.

7500m what? If youre referring to thrust force, scramjets apparently get better at higher mach numbers, so the value you listed there might not be accurate. The key to why scramjets can be better in some cases than normal rocket propulsion is that they dont carry all of the reaction mass with them, mixing onboard fuel with atmospheric gases & expelling them to create thrust. This allows for a much lower fuel mass to be carried, and the reason that modern jetliners can operate within reasonable cost ranges. If some of your reaction mass is obtained from the environment, actual hardware demands will decrease.

 

[Cras]

Thanks and this may be obvious to answer but why not just climb to 50km altitude using main engines. It would take only 200 seconds and 1.6 tons of fuel.

Consider this hyothetical scenario - over the Pacific at an altitude of 25km located some 8,000 km from the nearest base and carrying 3,200 kg fuel.

In that situation wouldn't distance be maximised by climbing to a low drag environment (1,600 kg fuel) before accelerating horizontally at altitude >50km?

This is because, at 50km altitude main engines travel 7,500m per kg fuel consumed, at a speed of Mach 12;

whereas, SCRAM engines will only produce 1,750m per kg and at a much lower speed of Mach 6.

The thing is, to get to orbit, you need a lot of horizontal speed, as you call it. So you need to start building that up as soon as possible.

Now if we are just talking about ships in orbiter.....you have SCRAMS for the XR-2 because they produce more thrust. It generates more energy at less fuel and it helps a great deal if you are going for SSTO. Like I said, I dont bother with them, I just carry a bit external tank full of main engine fuel up with me and use the mains the entire time. Because I like to keep things simple. Switching back and forth between engines is not appealing to me unless down really well (see the G42-200 for an example of how it is down REALLY well).

But I cant stand even taking off the XR-2 from a runway with rocket engines which is what initially led me down this dark rabbit hole of vertical launches and no scrams and velcro launchers.....

 

[Kinetics]

re the point about lift by flying at Mach 10 it's not a huge factor

 

[statickid]

...But you are aware that SCRAMs only work at around 25,000m altitude. Lower than that and the ship will get damaged due to drag at speeds higher than Mach 6. Higher than that and the SCRAM will not funcition....

what are you talking about? Don't SCRAMs have a theoretical operation ceiling of about 75km?

Maybe you mean they start working around 25km. Then you continually gain altitude to keep the pressure in the engine at the optimum performance level. Soon you are going mach 17+ at 60km+

 

[Kinetics]

what are you talking about? Don't SCRAMs have a theoretical operation ceiling of about 75km?

Maybe you mean they start working around 25km. Then you continually gain altitude to keep the pressure in the engine at the optimum performance level. Soon you are going mach 17+ at 60km+

Sorry but you don't know what you're talking about. If you bothered to check the SCRAM on the DG-S has limits. It cannot go at Mach 17 and it certainly does not have a ceiling of 75km.

It barely functions above 30km altitude. Optimum altitude is around 25km.

And no I don't mean to say "they start working at 25km". They work at 200 metres, just not very well.

Ah - perhaps there's something wrong with the standard DG-S model. I attached the test results (sheet 2)

 

[Urwumpe]

Nope, the Scram on the DG-S certainly does not have a ceiling of 75km. It does not function above 30km altitude. Optimum altitude is around 25km.

It does function. It simply throttles down once temperature exceeds the red line.

 

[Kinetics]

It does function. It simply throttles down once temperature exceeds the red line.

If you look at the flow rate (and the thrust produced by the Scram) you will see that fuel flow to the DG-S Scram falls from 6kg per second at 20,000m to less than 1kg/sec by 30,000m.

This trend continues into higher altitudes.

 

[BruceJohnJennerLawso]

Sorry but you don't know what you're talking about. If you bothered to check the SCRAM on the DG-S has limits. It cannot go at Mach 17 and it certainly does not have a ceiling of 75km.

It barely functions above 30km altitude. Optimum altitude is around 25km.

And no I don't mean to say "they start working at 25km". They work at 200 metres, just not very well.

Ah - perhaps there's something wrong with the standard DG-S model. I attached the test results (sheet 2)

He may be thinking of the XR2. Its rather popular you know

Only limit I know of for scramjets is that they can only operate when above mach 1.0. Efficiency is related to how rich the airstream mixture is.

 

[TMac3000]

If you assend using SCAM engines

You can steal the DG-IV's identity

 

[Andy44]

I find that DG-S scramjets work best around 27 km altitude and cruise at about Mach 7 or so.

Using mains for takeoff at the Cape I can cruise to Vandenberg on scrams and it uses up pretty much all my fuel.

Making the same flight in a suborbital hop on main engines alone I get there quicker and use a lot less fuel.

I can fly to Mars and back on one tank of fuel in a standard DG; but a trip across North America takes a whole tank of gas using scrams...Maybe I'm doing it wrong?

I guess the DG-S could be thought of as an example of using a DG airframe as a testbed for scramjets.

 

[statickid]

Sorry but you don't know what you're talking about. If you bothered to check the SCRAM on the DG-S has limits. It cannot go at Mach 17 and it certainly does not have a ceiling of 75km.

It barely functions above 30km altitude. Optimum altitude is around 25km.

And no I don't mean to say "they start working at 25km". They work at 200 metres, just not very well.

Ah - perhaps there's something wrong with the standard DG-S model. I attached the test results (sheet 2)

I'm sorry i misread your specific statement about DG-S engines as a generalized statement about actual SCRAM jet theory. I was going to look and see if I misread it, but oddly i can't find that post anymore, but i swear it just said "SCRAMs" not "DG-S SCRAMs." hmmmm

There's nothing wrong with the DG-S. The DG pdf clearly outlines the limitations you are referring to. It says that the DG-S operates between Mach 6-8. With that in mind I decided to give it a test flight. My personal conclusion is since the DG-S has some nice engine gimbals and trim options, its SCRAMs aren't meant for orbital insertion. Since it has a designed mach 8 performance drop off, but a huge tank of fuel, I believe the idea was to use it for long duration SCRAM flights. In my test I flew around 30km altitude at mach 6.2. After dialing in the trim, the DG-S was stable enough to ignore. I left the flight running while I did other stuff and found that I was able to fly travel 5708km over the Atlantic in 55 minutes before the SCRAMs ran out of fuel. My guess is the SCRAMs are for flying the DG-S between bases or just trying out SCRAM engine that is close in performance to the X-43

 

[C3PO]

I can fly to Mars and back on one tank of fuel in a standard DG; but a trip across North America takes a whole tank of gas using scrams...Maybe I'm doing it wrong?

I think the main problem with the SSRAMs on the DG-S is that they have a slightly lower magic-factor than then main engines. :lol: With a more plausible ISP on the mains, the SSRAMs would make more sense.

:ninja:'d

 

[BruceJohnJennerLawso]

I think the main problem with the SSRAMs on the DG-S is that they have a slightly lower magic-factor than then main engines. :lol: With a more plausible ISP on the mains, the SSRAMs would make more sense.

:ninja:'d

I too suspect that Martins only really created the DG-S as an example of a scram-jet propulsion system. Demostrators are always good:thumbup:.

 

[Kinetics]

I think the main problem with the SSRAMs on the DG-S is that they have a slightly lower magic-factor than then main engines. :lol: With a more plausible ISP on the mains, the SSRAMs would make more sense.

:ninja:'d

Yes, but they do have some use. Except that you must only allocate them around 150 kg of fuel as they will be on and off pretty quickly if you climb efficiently.

But, there is absolutely no point bringing any more fuel than that for the Scramjets as you can only use them if you deliberately blast hypersonically through the atmosphere pushing huge volumes of air around at an altitude of 25-30km. Which would be pretty stupid given that you could easily climb another 20km to "clean air" in only 80 seconds.

The best way to use them is for a relatively brief burn as you pass up and through their operating altitude - so only bring about 75kg scramjet fuel.

Or even better - dont use them at all. You actually want to keep speed (and drag to a minimum) at low altitudes; and you wont be able to fully use them unless you pass Mach 3 which is a schoolboy error at that altitude.