Simple Merlin-derived SSTO

[sevenperforce]

Specific impulse isn't nearly as important for SSTO designs as impulse density is...you can pretty much send the impulse density as high as you want as long as your engine T/W ratio is good enough. The Merlin 1D FT has T/W in spades, so why not build a really small SSTO powered by nothing more than a pair of Merlin 1Ds?

Basic HTOL design. The pair of fixed air augmentation ducts should increase the specific impulse enough to effectively zero out gravity drag and aerodynamic drag and give the Merlin 1Ds an average specific impulse equal to the Merlin 1D vacuum specific impulse. Kerolox is fairly dense so the craft is small, but heavy at 128 tonnes in only a 22-meter-long ship. HTOL requires pretty heavy landing gear, which increases dry mass, but thanks to the small overall size the payload fraction remains close to 1:1 with a total mass to LEO of 6 tonnes. Use VTOL with lighter landing gear and the payload mass jumps, but you lose the ability to take off and land from any large runway. You want to keep that in case you use this as an antipodal hypersonic suborbital transport.

Rolling takeoff is to gain airflow through the augmentation ducts, not build up speed for aerodynamic lift.

Payload looks like it should be just about enough for a 7-crew ISS ferry.

[todofwar]

What about coming back? Would it be able to glide to a runway with no jets? The shuttle could but it had pretty big wings.

[Nibb31]

I'm pretty sure Musk claimed that the Falcon 9 first stage could go SSTO.

[Spaceception]

@Nibb31

 

 

Elon Musk Verified account ‏@elonmusk

@TobiasVdb The F9 booster can reach low orbit as a single stage if not carrying the upper stage and a heavy satellite.

 

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4:37 AM - 24 Nov 2015

[sevenperforce]

20 minutes ago, todofwar said:

What about coming back? Would it be able to glide to a runway with no jets? The shuttle could but it had pretty big wings.

It would fly like a brick for sure, but the fuselage is a blended Sears-Haack/lifting body and most of the weight is toward the rear, so with a decent angle of attack it should be able to get a reasonable amount of lift. There would also probably be some degree of rocket-assisted braking during landing approach. In theory, you could even use SuperDracos to set it down vertically.

14 minutes ago, Nibb31 said:

I'm pretty sure Musk claimed that the Falcon 9 first stage could go SSTO.

It could, without payload. Way too much engine though. Drop four of them and you'd be in business...only 2 tonnes of payload, though, with a totally expendable rocket. 

This is more derived from the Falcon 9 FT second stage, actually. The fuel mass is only slightly greater than the F9FT second stage fuel supply.

Which, if you think about it, suggests that the Falcon 9 second stage is ALSO just about capable of SSTO.

[wumpus]

What part of SSTO is a dumb idea don't you get?  And just why did you draw a Merlin engine with inlets, when they are strictly LOX breathers?  And just how do you expect such a "heavy beast" to survive traveling at ~8000m/s in the atmosphere (which is what you would need if you are relying on lift to maintain altitude, otherwise you just rely on TWR).  The Blackbird went to heroic lengths to survive a "mere" 900 m/s and you want 7000 m/s *in the atmosphere*?

As noted above, space-x HAS an SSTO capable rocket.  They will never launch it as such because it would limit the whole point of the rocket (delivering payload to LEO and beyond) and be a general waste of money.  The entire reason the Merlin engine was designed was to build a "two stage to orbit" where the first stage would be reused.  The moment the first stage gets reused, it will utterly bury the idiotic idea of SSTO (for ISPs less than 800 or so).

[sevenperforce]

15 minutes ago, wumpus said:

What part of SSTO is a dumb idea don't you get?  And just why did you draw a Merlin engine with inlets, when they are strictly LOX breathers?  And just how do you expect such a "heavy beast" to survive traveling at ~8000m/s in the atmosphere (which is what you would need if you are relying on lift to maintain altitude, otherwise you just rely on TWR).  The Blackbird went to heroic lengths to survive a "mere" 900 m/s and you want 7000 m/s *in the atmosphere*?

As noted above, space-x HAS an SSTO capable rocket.  They will never launch it as such because it would limit the whole point of the rocket (delivering payload to LEO and beyond) and be a general waste of money.  The entire reason the Merlin engine was designed was to build a "two stage to orbit" where the first stage would be reused.  The moment the first stage gets reused, it will utterly bury the idiotic idea of SSTO (for ISPs less than 800 or so).

Think you missed a few points.

Those aren't inlets for the engine; they are inlets for ram compression and exhaust reheat. Wrapping even a very simple exhaust shroud around an engine results in a pressure-induced flow that increases thrust by 15%; at speed it goes up to 50% augmentation if your engine can vary its mixture ratio, which the Merlin 1D can. 

Takeoff is emphatically not relying on lift; it's relying on thrust. Rolling takeoff is to build up airflow through the ducts to increase the vehicle T/W ratio to 1.5-1.8, enough to point the nose (relatively) straight up. As I mentioned, it has enough static thrust (1.4:1) to take off vertically, but horizontal takeoff helps reduce gravity drag and allows it to fly out of any airport that can take a LOX depot, which is good if you want to use it for suborbital flights. 

This is similar in overall shape and re-entry profile to Skylon, though with a less depressed flight trajectory, a better T/W ratio, and a smaller overall size. 

And sure, SSTOs aren't generally a brilliant idea. But that is kind of the point of this. SSTOs aren't built because there isn't a reason to build them. And here's an example to show why building them is in many ways the easy part. 

[Streetwind]

14 hours ago, Nibb31 said:

I'm pretty sure Musk claimed that the Falcon 9 first stage could go SSTO.

On this topic:

Recently a fan performed some simulations and basically confirmed the claim. It's pretty tight though.

 

[A Fuzzy Velociraptor]

I think you have some misunderstandings about getting to orbit and the relevance of thrust to weight. The thrust produced is pretty much not relevant to the discussion provided that you are somewhere between a T/W of 1.2-1.6. What is more important is your total change in velocity and herein lies the difficulty of SSTO concepts.

Spoiler

ISPmax = 8000;
ISPmin = 2000;
stepsize = 50;
DeltaV = 9500;
numsteps = ceil((ISPmax-ISPmin)/stepsize+1);
X = zeros(numsteps, 2);
C = 1;
ISP = ISPmin;
for C=1:numsteps
    X(1,C) = ISP;
    X(2,C) = exp(-DeltaV/ISP);
    ISP = ISP + stepsize
end
plot(X(1,:),X(2,:))
title('Non-propellant mass fraction for SSTO vehicles');
xlabel('Effective Velocity');
ylabel('Non-propellant fraction')
grid on

From the graph you can see that the available non-propellant mass fraction increases almost linearly with the increased specific impulse. The Merline 1D engine on its own has an ISP over the course of a whole flight of probably about 3000m/s. This means you are looking at a 4% non-propellant fraction when generating 9500m/s delta V. Now with a 100000kg vehicle that results in only 4000kg to play around in between payload and structure which really isn't feasible for most systems where you might be able to maintain a 500-1000kg payload. For a hydrolox system which may have an average of 4200m/s or so effective velocity which would allow you 10%-11% non-propellant fraction. However, for a hydrolox system about 10% of your vehicle will be structural mass. The Aquarius launch vehicle was proposed for SSTO using hydrolox and would have had a wet mass of 130000kg, a dry mass of 10000kg and only maintained a payload of 1000kg. Of course you can use techniques to try to reduce the structural mass which would allow a higher payload mass like higher density propellant or composite materials, but there are many circumstances where that may not be available such as with the cryosystems used on many launch vehicles. You can of course try to increase the specific impulse which air augmentation like you are describing may be able to or through other means but ultimately you are limited. The large ducts for air augmentation are also quite heavy which will further ear into your payload fraction. At this point, your idea needs more development and numbers associated with it before any analysis of the system can really be done.

[Exoscientist]

On April 27, 2016 at 0:25 PM, sevenperforce said:

Specific impulse isn't nearly as important for SSTO designs as impulse density is...you can pretty much send the impulse density as high as you want as long as your engine T/W ratio is good enough. The Merlin 1D FT has T/W in spades, so why not build a really small SSTO powered by nothing more than a pair of Merlin 1Ds?

Basic HTOL design. The pair of fixed air augmentation ducts should increase the specific impulse enough to effectively zero out gravity drag and aerodynamic drag and give the Merlin 1Ds an average specific impulse equal to the Merlin 1D vacuum specific impulse. Kerolox is fairly dense so the craft is small, but heavy at 128 tonnes in only a 22-meter-long ship. HTOL requires pretty heavy landing gear, which increases dry mass, but thanks to the small overall size the payload fraction remains close to 1:1 with a total mass to LEO of 6 tonnes. Use VTOL with lighter landing gear and the payload mass jumps, but you lose the ability to take off and land from any large runway. You want to keep that in case you use this as an antipodal hypersonic suborbital transport.

Rolling takeoff is to gain airflow through the augmentation ducts, not build up speed for aerodynamic lift.

Payload looks like it should be just about enough for a 7-crew ISS ferry.

 

 Looks like it would be a kerosene-fueled analogue of the Skylon. But Skylon is a multi-billion dollar development. And getting the engines to operate in both an air breathing and rocket mode is on the ragged edge of technological feasibility. 

 If Skylon succeeds you might have a chance at this being developed, but not before.

 Actually SSTO using rockets only can be done with altitude compensating nozzles. Calculate the payload to LEO for the F9 first stage if the engines had the 340 s ISP of the Merlin Vacuum.

  Bob Clark

Edited April 29, 2016 by Exoscientist

[sevenperforce]

16 hours ago, A Fuzzy Velociraptor said:

I think you have some misunderstandings about getting to orbit and the relevance of thrust to weight. The thrust produced is pretty much not relevant to the discussion provided that you are somewhere between a T/W of 1.2-1.6. What is more important is your total change in velocity and herein lies the difficulty of SSTO concepts.

(snip)

From the graph you can see that the available non-propellant mass fraction increases almost linearly with the increased specific impulse. The Merline 1D engine on its own has an ISP over the course of a whole flight of probably about 3000m/s. This means you are looking at a 4% non-propellant fraction when generating 9500m/s delta V. Now with a 100000kg vehicle that results in only 4000kg to play around in between payload and structure which really isn't feasible for most systems where you might be able to maintain a 500-1000kg payload

You can of course try to increase the specific impulse which air augmentation like you are describing may be able to or through other means but ultimately you are limited. The large ducts for air augmentation are also quite heavy which will further ear into your payload fraction. At this point, your idea needs more development and numbers associated with it before any analysis of the system can really be done.

The high T/W ratio, coupled with dense fuel, greatly minimizes gravity drag losses. With a very dense fuel, you burn a larger proportion of your GLOW more quickly, reducing the total mass to orbit. A dense fuel also reduces overall size, cutting aerodynamic drag considerably in comparison to something like a hydrolox SSTO.

The ISP of the Merlin 1D is where you're getting hung up, I think. To start with, I did add a mass penalty for the air augmentation ducts: 25% of engine mass, which is fairly conservative given that they are part of the vehicle structure. Yet those simple fixed ducts add considerably to the specific impulse. At launch, they give the Merlin 1Ds an effective exhaust velocity of almost 3,200 m/s; from Mach 0.1 to Mach 2 they give an effective exhaust velocity starting at 4,150 m/s and climbing to 4,570 m/s; from Mach 2 to Mach 8 it slowly declines to around 3,300 m/s and maintains this to orbit.

The numbers I gave assumed that this range of specific impulses would effectively zero out gravity drag and atmospheric drag and result in a net average 348 seconds, but I can go back and run the numbers for each flight segment a little more tightly.

4 hours ago, Exoscientist said:

 Looks like it would be a kerosene-fueled analogue of the Skylon. But Skylon is a multi-billion dollar development. And getting the engines to operate in both an air breathing and rocket mode is on the ragged edge of technological feasibility. 

 If Skylon succeeds you might have a chance at this being developed, but not before.

It borrows slightly from Skylon, but with a vehicle TWR that starts at 1.4 and increases to well over 2 soon after launch, it's quite different. It's also not an airbreather; it's an AAR without the technological hurdles of airbreathing or rocket mode changes.

[hypervelocity]

I want to see this one in KSP with RSS and Realism Overhaul !!!!

[Thor Wotansen]

Dude, this looks great.  If you run the numbers as tight as you can, maybe you could pitch it to somebody as a crew transport.  (yes I know that's a big huge and morbidly obese maybe)  If all this cool stuff that's in the works like Biggelo hotels and interplanetary vehicles happens then they'll need a cheap way to get folks to space.

[wumpus]

On 4/27/2016 at 1:09 PM, sevenperforce said:

Which, if you think about it, suggests that the Falcon 9 second stage is ALSO just about capable of SSTO.

All it needs are 2km/s (of actual speed provided by the first stage) and vacuum and it handles the rest.

I'd be worried about all the extraneous size.  I doubt that this entire structure could be anywhere near F9's second stage dry mass.

[Elukka]

While I agree SSTO doesn't generally make a lot of sense it's an interesting design.

I wonder if the ducts could really be as light as you expect though. 25% of two Merlins is only 235 kilograms. Surely they'd be rather heavier than that. I tried a quick google for existing design but the best I got was an uncited claim on Wikipedia that you'd expect a duct-augmented rocket engine to be five to ten times heavier. I can't judge the accuracy of that number but if you made them five times heavier you would still have a payload.if your isp expectations hold.

Edited May 1, 2016 by Elukka

[MatterBeam]

How is the air-augmented Merlin engine better than an aerospike?

[sevenperforce]

2 hours ago, MatterBeam said:

How is the air-augmented Merlin engine better than an aerospike?

Every possible way. 

An aerospike is an altitude-compensating nozzle. It allows you to use a greater expansion ratio without risking flow separation, but it does nothing else. (Also, a Merlin can't be easily mated to an aerospike.)

An AAR, on the other hand, serves to collect and compress and airflow, then combine it with the exhaust flow from the ordinary engine in the center. The net exhaust velocity drops, but the thrust increases because you are using the same kinetic energy with a greater mass flow. Most importantly, this new mass flow doesn't have to be carried by the rocket, so the thrust-specific fuel consumption drops, causing specific impulse to skyrocket. Even with just a simple tapering cylindrical stainless steel shroud, static thrust goes up by 15% and thrust at speed goes up by 50%, without requiring any additional fuel. 

15 hours ago, wumpus said:

All it needs are 2km/s (of actual speed provided by the first stage) and vacuum and it handles the rest.

I'd be worried about all the extraneous size.  I doubt that this entire structure could be anywhere near F9's second stage dry mass.

I could probably streamline it a little better; integrate the ducting into the body a bit. 

Also, I think I would cut landing gear down considerably since there really is no need for horizontal takeoff. This increases payload fraction noticeably. 

3 hours ago, Elukka said:

While I agree SSTO doesn't generally make a lot of sense it's an interesting design.

I wonder if the ducts could really be as light as you expect though. 25% of two Merlins is only 235 kilograms. Surely they'd be rather heavier than that. I tried a quick google for existing design but the best I got was an uncited claim on Wikipedia that you'd expect a duct-augmented rocket engine to be five to ten times heavier. I can't judge the accuracy of that number but if you made them five times heavier you would still have a payload.if your isp expectations hold.

A stainless steel duct can't be THAT heavy.

[todofwar]

What about strapping two of these to a slightly larger version, to make a TSTO with reusable boosters capable of gliding down instead of needing a powered descent? The center ship would be similar in that it can be a lifting body and glide back down from orbit. 

[cantab]

4 hours ago, sevenperforce said:

A stainless steel duct can't be THAT heavy.

It has to deal with the aerodynamic forces involved in super and hypersonic flight.

[wumpus]

50 minutes ago, todofwar said:

What about strapping two of these to a slightly larger version, to make a TSTO with reusable boosters capable of gliding down instead of needing a powered descent? The center ship would be similar in that it can be a lifting body and glide back down from orbit. 

I'd expect the "two boosters" to be a single unit to reduce costs (largely cutting the ground/control teams by a third or so).  The big question then would be how you would handle re-entry and landing (as well as how much delta-v the lower end supplies).

Falcon 9 supplies 2km/s delta-v on the first stage, then burns around 1km/s delta-v to stop itself (and some relatively small amount on a precise suicide burn at the end).

In general, anything a SSTO can do a TSTO can do better.  One of the biggest advantages in this case would be the re-entry (having the two merlin engine lower stage land from 2km/s would be vastly easier than from 7km/s).  While it isn't clear how you would get the upper stage down, it also isn't clear how the SSTO could safely return as well.  To really have an advantage over falcon 9 you would have to separate at >2km/s (note that the greater dry mass means that reserve fuel "costs" relatively less in terms of dry mass).

Comparing the three ideas:

Falcon 9: relatively simple design (similar to all existing rockets, all engines roughly the same).  Relatively simple landing procedure (roughly the same up as down).  Big advantage: ACTUALLY WORKS.

Merlin derived SSTO: slightly more complicated.  Much more simple logistics.  Landing is a complete unknown (compare landing scheme to Shuttle and expect all simplicity to be long out the window before it can safely land).

Merlin derived TSTO: even more complicated.  Biggest advantage: uses less fuel (wait, wasn't there supposed to be a *real* advantage here?).  Might even land (compare to Falcon which has been landing (if falling over afterwards) fairly well the last few tries).  I'm not saying that Space X wouldn't consider such a craft, but that they wouldn't consider such a craft until they can put a manifest on every week (or more) and need to find ways to cut costs in a way that larger rockets can't cover.

[todofwar]

34 minutes ago, wumpus said:

I'd expect the "two boosters" to be a single unit to reduce costs (largely cutting the ground/control teams by a third or so).  The big question then would be how you would handle re-entry and landing (as well as how much delta-v the lower end supplies).

Falcon 9 supplies 2km/s delta-v on the first stage, then burns around 1km/s delta-v to stop itself (and some relatively small amount on a precise suicide burn at the end).

In general, anything a SSTO can do a TSTO can do better.  One of the biggest advantages in this case would be the re-entry (having the two merlin engine lower stage land from 2km/s would be vastly easier than from 7km/s).  While it isn't clear how you would get the upper stage down, it also isn't clear how the SSTO could safely return as well.  To really have an advantage over falcon 9 you would have to separate at >2km/s (note that the greater dry mass means that reserve fuel "costs" relatively less in terms of dry mass).

Comparing the three ideas:

Falcon 9: relatively simple design (similar to all existing rockets, all engines roughly the same).  Relatively simple landing procedure (roughly the same up as down).  Big advantage: ACTUALLY WORKS.

Merlin derived SSTO: slightly more complicated.  Much more simple logistics.  Landing is a complete unknown (compare landing scheme to Shuttle and expect all simplicity to be long out the window before it can safely land).

Merlin derived TSTO: even more complicated.  Biggest advantage: uses less fuel (wait, wasn't there supposed to be a *real* advantage here?).  Might even land (compare to Falcon which has been landing (if falling over afterwards) fairly well the last few tries).  I'm not saying that Space X wouldn't consider such a craft, but that they wouldn't consider such a craft until they can put a manifest on every week (or more) and need to find ways to cut costs in a way that larger rockets can't cover.

The reason for having two is to have a symmetric craft, rather than a shuttle style system. The advantage is having a passive landing so all your fuel goes towards getting to orbit rather than saving a third for the landing. And horizontal landing seems more reliable than verticle. Assuming this design allows your ship to be the lifting body without excess wings, you don't have to sacrifice much in the way of dry mass.

[PB666]

13 hours ago, wumpus said:

I'd expect the "two boosters" to be a single unit to reduce costs (largely cutting the ground/control teams by a third or so).  The big question then would be how you would handle re-entry and landing (as well as how much delta-v the lower end supplies).

Falcon 9 supplies 2km/s delta-v on the first stage, then burns around 1km/s delta-v to stop itself (and some relatively small amount on a precise suicide burn at the end).

In general, anything a SSTO can do a TSTO can do better.  One of the biggest advantages in this case would be the re-entry (having the two merlin engine lower stage land from 2km/s would be vastly easier than from 7km/s).  While it isn't clear how you would get the upper stage down, it also isn't clear how the SSTO could safely return as well.  To really have an advantage over falcon 9 you would have to separate at >2km/s (note that the greater dry mass means that reserve fuel "costs" relatively less in terms of dry mass).

Comparing the three ideas:

Falcon 9: relatively simple design (similar to all existing rockets, all engines roughly the same).  Relatively simple landing procedure (roughly the same up as down).  Big advantage: ACTUALLY WORKS.

Merlin derived SSTO: slightly more complicated.  Much more simple logistics.  Landing is a complete unknown (compare landing scheme to Shuttle and expect all simplicity to be long out the window before it can safely land).

Merlin derived TSTO: even more complicated.  Biggest advantage: uses less fuel (wait, wasn't there supposed to be a *real* advantage here?).  Might even land (compare to Falcon which has been landing (if falling over afterwards) fairly well the last few tries).  I'm not saying that Space X wouldn't consider such a craft, but that they wouldn't consider such a craft until they can put a manifest on every week (or more) and need to find ways to cut costs in a way that larger rockets can't cover.

I don't know how much it burns to land itself, it certainly burns more than 2km to grt to1856. For 100 seconds is loosing more than half its thrust to g, if the TWR is on average 1.5 then rougly 1000 dV right there. myguews is 3200 to MECO. 

Also we don't know how much RP-1 is left when it lands, could be more than zero. 

[sevenperforce]

With the newly-released FT numbers for the Merlin 1D, these margins end up looking quite a bit better. I'll run the numbers again shortly. 

One advantage of this design would be a very simple biamese setup. Connect two of these belly-to-belly on launch with crossfeed and you can carry something to GTO with ease. 

[John JACK]

16 hours ago, todofwar said:

all your fuel goes towards getting to orbit rather than saving a third for the landing.

Accelerating extra dead weight of tankage, ducts and redundant engines with payload 7000 m/v to orbit needs much more fuel than decelerating near empty first stage (without payload) less than 1000 m/s. SSTOs may be wicked cool, but they are ineffective.

1 hour ago, sevenperforce said:

Connect two of these belly-to-belly on launch with crossfeed and you can carry something to GTO with ease. 

Wouldn't it be Liquid Flyback Booster?

[sevenperforce]

2 minutes ago, John JACK said:

Accelerating extra dead weight of tankage, ducts and redundant engines with payload 7000 m/v to orbit needs much more fuel than decelerating near empty first stage (without payload) less than 1000 m/s. SSTOs may be wicked cool, but they are ineffective.

Wouldn't it be Liquid Flyback Booster?

Carrying that additional weight only makes sense if you can maintain a high payload fraction (actual payload / total mass to orbit). Adding an airbreathing engine to an SSTO is useless if you end up with only 10-20% of your total orbital mass as payload...but if you're looking at 40-50% of your orbital mass being payload, then you don't care so much about the extra weight.

And yeah, the biamese approach would be the equivalent of a parallel liquid flyback booster.