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Increasing Delta-V of Rocket SSTOs


SkyFall2489
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I'm trying to make a rocket-only SSTO spaceplane that can get to orbit and dock to a station. I have managed the first part, but only with like 30 m/s remaining - not enough to get to a station and return home.

My current design uses a few aerospikes, and has a vacuum delta v of around 4 KM/s.

Why I decided to use a rocket SSTO:

1. easier to fly than a jet SSTO

2. noting to have to balance or otherwise deal with, compared to a shuttle

3. cooler than a rocket

How can I increase the delta-V in orbit of my space plane to be able to dock to a station?

Of course, if all else fails, I might be able to add a few drop tanks or boosters, but I'd like to try and go fully reusable.

 

 

Edited by SkyFall2489
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I'm going to assume that when you say "SSTO", you actually mean "spaceplane" instead. Because it's trivially easy to make a standard rocket go single stage to orbit with plenty of dV left over.

Your plane has 4000m/s of vacuum dV, which should be plenty, given that it should only take about 3400m/s to reach low Kerbin orbit with a standard gravity turn trajectory. Hence, your choice of trajectory is to blame for losing about 600 m/s worth of dV along the way. Getting more TWR and climbing a little longer can certainly help improve this. But keep in mind that one major contributor is your launching off of the runway. That's just never going to be as fuel efficient as a start from the vertical pad.

Also, if getting more TWR means that you lose maximum dV in return, you may find that you gain little to nothing along the way. Your maximum possible upside from trajectory optimization is less than 600 m/s; if you stick to the runway start, it's probably in the realm of 400 at most. Switching from a Dart to a Swivel will drop your dV by about 300 m/s, depending on how much the extra weight is going to impact your plane. So you might gain about 100 m/s tops when reaching orbit. Workable, but not ideal.

(Of course, all of this is guesstimated, so your results may vary ;))

Another thing you can do is take a page out of the Space Shuttle's book, and make do with flying like a brick. As in: bring less wing, or bring more tank. This makes your landing approach harder, but it'll give you more dV to work with in orbit. You're at less than a 12x multiplier of your Isp in terms of dV, so you aren't that deep in the diminishing returns of your mass fraction just yet. If you can get back to like 4000m/s while mounting a Swivel, without increasing your wing surface, that should allow for more fuel leftover in orbit.

 

Edited by Streetwind
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drag may be the answer.

open aerodinamic window and check your drag. your ascent profile looks fine enough for a rocket with good aerodinamics, you should most definitely NOT spend 4000 m/s for it.

twr may be the other option; what's your twr? how long do you spend in the low atmosphere accelerating to 450 m/s?

overall, though, I think the most likely problem is aerodinamics. post some pictures of your vehicle to get a better diagnosis

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1 hour ago, SkyFall2489 said:

My TWR for the planes I'm building is a little low, around 1.0 to 1.2 at launch. That seems very low, so that is the reason for the 45 degree ascent - the wings can help lift the craft as well as the engines.

 

The wings can and do help with lift. but I have no idea if you are placing them properly. I also have no idea what's the effect of low thrust on a rocket plane, the general assumption is that you use jet engines and they use very little fuel anyway.

so higher thrust is certainly worth a try

Quote

Also, how much delta-v should I budget for docking? I want to go from a 72KM circular orbit to a 100KM station.

it really depends on how different the orbits of the two objects are. in this case you have to raise your orbit, so I'd factor at least 150 m/s for that

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After som research, I think having a lower efficiency, higher TWR engine might work out better as well as switching to prograde a little later. However, I'd like to see if anyone else has any good tips.

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Posted (edited)

My TWR for the planes I'm building is a little low, around 1.0 to 1.2 at launch. That seems very low, so that is the reason for the 45 degree ascent - the wings can help lift the craft as well as the engines. Also, how much delta-v should I budget for docking? I want to go from a 72KM circular orbit to a 100KM station.

I tried  using MechJeb to gravity-turn it, but that resulted in running out of fuel, likely due to gravity or drag losses. Also, I need the high wing area because my mission profile is to get to orbit, refuel at a refueling station,  fly all the way to Gilly to drop some crew off at a base, and return from interplanetary using an aerobrake. To survive, I will need a medium-high wing area. Gilly and back, once in orbit, is slightly less than the delta v needed to reach orbit, so if I can get to orbit I can get to Gilly. I figure, the plane is re-entering on an empty tank, though, so I may be able to get away with less.

 

I have several different prototypes all suffering from the same issue. I'm pretty good with SSTOs, and am confident that those designs, if the engines and fuel were swapped out, could function as jet SSTOs.

 

I was thinking, for a single aerospike design, I could swap it out for 2 Thuds and a Terrier, and the other, 2 spike one I could maybe swap for a Vector. I'm also considering the Swivel, Reliant, and Kodiak for future designs.

 

I'll try increasing the TWR and going for a vertical ascent later today.

Edited by SkyFall2489
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Posted (edited)
21 minutes ago, king of nowhere said:

The wings can and do help with lift. but I have no idea if you are placing them properly. I also have no idea what's the effect of low thrust on a rocket plane, the general assumption is that you use jet engines and they use very little fuel anyway.

so higher thrust is certainly worth a try

it really depends on how different the orbits of the two objects are. in this case you have to raise your orbit, so I'd factor at least 150 m/s for that

 I'd prefer at least 200 or 300 m/s for docking then.

As for low TWR, what it means is that:

1. if I try to launch vertically with a standard gravity turn, it takes a lot of time to build up vertical speed - during all that time gravity is pulling down and costing delta-v.

2. If I use my plane ascent profile, I lose slightly more delta-V to drag. However, I cans till get going quickly as the wings help provide lift to keep the SSTO in the air, while the engines provide half vertical and half horizontal thrust.

3. TWR is really only an issue during the earliest parts of the ascent, as when the craft burns off its fuel it becomes much lighter.

 

As for wing placement, the center of lift is where it should be on a good plane. It's slightly below the center of mass so the wings can sort of shield the fuselage, but the plane flies well. The plane is balanced so that the center of mass does not shift when fuel drains, like a jet SSTO, and there is enough wing area to not be too much of a brick. A fighter jet it is not, however.

Edited by SkyFall2489
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4 hours ago, SkyFall2489 said:

As for wing placement, the center of lift is where it should be on a good plane. It's slightly below the center of mass so the wings can sort of shield the fuselage, but the plane flies well. The plane is balanced so that the center of mass does not shift when fuel drains, like a jet SSTO, and there is enough wing area to not be too much of a brick. A fighter jet it is not, however.

can you post pictures? provide hard data?

how much drag do you have during ascent? lift-to-drag ratio? how much deltaV do you use in the various stages of ascent? what's the effect of using a bigger engine?

we can't troubleshoot without hard data, because every vehicle is different. it could even be a bug; I once had an aerodinamically-shaped vessel make a huge drag because it was united by a docking port and the game was modeling both as if they were exposed to full airflow. what I can say for sure is that there is some problem with your whole setup, because if you take a big fuel tank and you put an appropriately-sized engine underneath and a nose cone on top you can get to orbit with over 1000 m/s to spare. if your plane fails to match that performance, then it must have some problem, but we have no way of telling what it could be without specific information

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Posted (edited)
35 minutes ago, king of nowhere said:

can you post pictures? provide hard data?

how much drag do you have during ascent? lift-to-drag ratio? how much deltaV do you use in the various stages of ascent? what's the effect of using a bigger engine?

we can't troubleshoot without hard data, because every vehicle is different. it could even be a bug; I once had an aerodinamically-shaped vessel make a huge drag because it was united by a docking port and the game was modeling both as if they were exposed to full airflow. what I can say for sure is that there is some problem with your whole setup, because if you take a big fuel tank and you put an appropriately-sized engine underneath and a nose cone on top you can get to orbit with over 1000 m/s to spare. if your plane fails to match that performance, then it must have some problem, but we have no way of telling what it could be without specific information

The problem, as I said, is that this is an issue on several different ships I have designed. I'll be able to get you some pictures later today. I'm pretty sure the issue is either TWR or drag, though. Why else would a ship use 500 m/s more than expected on kerbin ascent?

Edited by SkyFall2489
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Posted (edited)
14 hours ago, Streetwind said:

You're at less than a 12x multiplier of your Isp in terms of dV, so you aren't that deep in the diminishing returns of your mass fraction just yet. If you can get back to like 4000m/s while mounting a Swivel,

I've tried adding more fuel, but I am still getting some diminishing returns. Also, more fuel tanks = more changing center of mass compared to the base design = bad plane. A Swivel is a horrible idea, they have not that much thrust at sea level, and in vacuum they aren't so good either. The Reliant and Kodiak are promising atmospheric engine candidates, and again a terrier or cheetah could be used in vacuum.

Edited by SkyFall2489
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Testing results:

MOAR POWER worked great - I doubled the thrust on each design and now they reach orbit with plenty to spare using a standard gravity turn via MechJeb. Thank you all for your help!

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